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Last call for my last Rhode and Schwarz polyscop. Only ?20.
If no takers, I'll scrap it for the valves (EL84's) and metal for my friends machine shop

Collection Hampshire UK

73,
Martin
M0MGA

Hi,
A little more detail.
Using the variac to bring up the HT slowly is good. Probably best to put the system switch in Standby as this puts a fixed load on the HT line and isolates the rest of the set the rest in case of drama.
There wont be any HT voltage on the capacitors until there is enough voltage on the rectifier heaters for conduction to start. So good idea to monitor the HT voltage across the fixed resistor so that you can see what is actually happening to the electrolytics.
Assuming all goes well and you get to full HT without any snap, crackle or pop, then it is time to repeat the process but this time with the System switch in Operate, so that you give all the other electrolytics in the set the chance to reform. if all goes well you can then start looking for all the other faults which will have developed over 20 years of idleness.
Have fun
Peter G8BBZ

You have the best means of powering-up after a long period already. . .
Capacitor electrolyte in high capacity smoothing power supplies in
particular are notorious for being ineffective: possibly shorted: and even
blowing up the can with messy results.
Slowly increase voltage with the variac to reform electrolytic capacitors
with an ammeter in series with the mains output to the set: and if possible
another across the ht link to monitor current to ensure not excessive from
that normally expected.
Check voltages around the set afterwards.

Douglas Denny. G3ZQE.

On Sun, 9 Feb 2025, 22:37 Andrew Holme via groups.io, <andrew=
[email protected]> wrote:

My RA17 has been in storage unpowered since 2006. Prior to that it was in
tip top condition having been serviced by Rob Filby in December 1999. I
know it was unwise and neglectful of me not to power it for so long. What
should I do now to bring it up? I have a mains variac. Can I use that?
What is the procedure?

Thanks, Andrew.

My RA17 has been in storage unpowered since 2006. Prior to that it was in tip top condition having been serviced by Rob Filby in December 1999. I know it was unwise and neglectful of me not to power it for so long. What should I do now to bring it up? I have a mains variac. Can I use that? What is the procedure?

Thanks, Andrew.

Craig further to 40Mhz BPF:

I had a grest interest in filters many years ago and at that time used
Terman as the main reference source.

The most comprehensive full mathematical analysis of filters in print and
probably one of the earliest was "Transmission Networks and Wave Filters by
T. E Shea. My copy is 1929.

The modern resonator type with LC sections like the 40 Mhz BPF in RA17s
series is described in section 19 of "Radio-frequency Electronics" by Jon
B. Hagen on "Coupled-Resonator Filters". My copy is 1966.

In the 1980s I designed an aviation VOR receiver with one of the first
digital controlled rf synthesiser chips (for computer digital control) that
became available. The two sub-carrier signals after the detector for phase
comparison for determining azimuth from the VOR are at low frequencies,
need separation, and so I became highly interested in op-amp active
filters... superb for the task: as simple IC chips used whereas the
straight LC pot core filters as widely used in Racal equipment would be
needed - which are huge in comparison of size, heavy and expensive.

At that time in the 1980s Plessey produced a range of superb analogue chips
suitable for radio sections: rf amplifiers; detectors; ppl; etc in the
SL600 series. I used those extensively - which life much easier. Sadly
Plessey disappeared suddenly, (as did Ferranti; GEC and others) - as Tommy
Cooper used to say: "... Jus like that!"

On Sat, 12 Oct 2024, 10:32 Douglas Denny via groups.io, <ddennyoptom=
[email protected]> wrote:

Thanks Craig,

You must have a seriously detailed archive library on Racal ... :-)

The number of Racal equipments of that era that I have seen or worked on is
not a large number and not in any professional capacity - just as a radio
amateur so not a large sample.
That mod was dated 1971 so it must be getting on 15/20 years after the
first RA17s. Hence that explains why there will be a lot of the standard
spring finger type covers I've seen.
The mod says all _future_ equipment were to be using "knitmesh" covers. I
wonder if that was actually done? Costs always modify production lines; if
it was, they should be common. I'm not familiar, however, with the later
variants / developments of the RA17 types so wouldn't know that.

I have seen copper springing type cover edges for earthing on various other
equipment covers, but, curiously, not RA17s Ra117s or MA79s.

I also wonder at the _reason_ given for the change in cover type: "reducing
spurious osvilations" - I've never heard of any of that associated with the
37.5 and 40 Mhz filters... they are passive components of highly accurate
design and quality with very well known characteristics. I can't think of
any reason they'd cause spurii.
Also, If that was a known problem requiring a mod just of the filter covers
- why did it take nearly twenty years to do anything about it with such a
simple modification?
All very curious. . .

Douglas. G3ZQE.

=====================

On Sat, 12 Oct 2024, 07:17 Craig Sawyers via groups.io, <c.sawyers=
[email protected]> wrote:

Hi Douglas

A few posts ago, in a long meandering thread, I made an assertion that
Racal introduced a mod to replace the spring edged covers with knitted
mesh versions. You said you'd never seen any.

It took a while to scan through the various RA17 documents I have. But
here it is:

E729 Misc Instr No 9

...RA17L

...Change of filter covers

"In order to reduce spurious oscillation below specification
figures...change the covers of the 37.5MHz and 40MHz filters...new
covers contain "knitmesh" cushions, and will replace existing type
covers in future equipment manufacture."

There is then a list of existing spring finger part numbers and the
corresponding modified knitmesh part numbers.

Dated Jan 1971.

It may be possible to get the same effect by taping the filter seams
with adhesive copper EMC tape (once the filters are aligned of course!).

Craig

Hi folks -

The mesh covers appear to have been factory fitted from roughly mid-1969 in both RA17s and 117s. I currently own at least two RA17s with them (#12681 not dated but appears to be early 1971 and # 12829 dated 7309) and RA117 #2721 (date code 7121). If fitted, they may be recognised by the backing plates, which are more substantial than the usual aluminium finger-stock with a dark anodized finish. I have come across several more 17s with the mesh covers, all with 12000+ serial numbers, including if I remember rightly, the RA17C16/1 series for the Dutch Army.

Cheers everyone - Neil Clyne G8LIU

A sensible replacement would be a good thing. I've rivetted a few finger strips back onto their cover plates, but individual fingers are prone to snap off after a few cycles.
Colin mm1aps.

Thanks Craig,

You must have a seriously detailed archive library on Racal ... :-)

The number of Racal equipments of that era that I have seen or worked on is
not a large number and not in any professional capacity - just as a radio
amateur so not a large sample.
That mod was dated 1971 so it must be getting on 15/20 years after the
first RA17s. Hence that explains why there will be a lot of the standard
spring finger type covers I've seen.
The mod says all _future_ equipment were to be using "knitmesh" covers. I
wonder if that was actually done? Costs always modify production lines; if
it was, they should be common. I'm not familiar, however, with the later
variants / developments of the RA17 types so wouldn't know that.

I have seen copper springing type cover edges for earthing on various other
equipment covers, but, curiously, not RA17s Ra117s or MA79s.

I also wonder at the _reason_ given for the change in cover type: "reducing
spurious osvilations" - I've never heard of any of that associated with the
37.5 and 40 Mhz filters... they are passive components of highly accurate
design and quality with very well known characteristics. I can't think of
any reason they'd cause spurii.
Also, If that was a known problem requiring a mod just of the filter covers
- why did it take nearly twenty years to do anything about it with such a
simple modification?
All very curious. . .

Douglas. G3ZQE.

=====================

On Sat, 12 Oct 2024, 07:17 Craig Sawyers via groups.io, <c.sawyers=
[email protected]> wrote:

Hi Douglas

A few posts ago, in a long meandering thread, I made an assertion that
Racal introduced a mod to replace the spring edged covers with knitted
mesh versions. You said you'd never seen any.

It took a while to scan through the various RA17 documents I have. But
here it is:

E729 Misc Instr No 9

...RA17L

...Change of filter covers

"In order to reduce spurious oscillation below specification
figures...change the covers of the 37.5MHz and 40MHz filters...new
covers contain "knitmesh" cushions, and will replace existing type
covers in future equipment manufacture."

There is then a list of existing spring finger part numbers and the
corresponding modified knitmesh part numbers.

Dated Jan 1971.

It may be possible to get the same effect by taping the filter seams
with adhesive copper EMC tape (once the filters are aligned of course!).

Craig

Hi Douglas

A few posts ago, in a long meandering thread, I made an assertion that Racal introduced a mod to replace the spring edged covers with knitted mesh versions. You said you'd never seen any.

It took a while to scan through the various RA17 documents I have. But here it is:

E729 Misc Instr No 9

...RA17L

...Change of filter covers

"In order to reduce spurious oscillation below specification figures...change the covers of the 37.5MHz and 40MHz filters...new covers contain "knitmesh" cushions, and will replace existing type covers in future equipment manufacture."

There is then a list of existing spring finger part numbers and the corresponding modified knitmesh part numbers.

Dated Jan 1971.

It may be possible to get the same effect by taping the filter seams with adhesive copper EMC tape (once the filters are aligned of course!).

Craig

Its not hard ... as I said, i did it. You don't need to make special covers. I just used two pieces
of circuit board, positioned with a tiny slot, a bit away from the coil under test, with just enough space for two probes, one sig generator, one an oscilloscope or other suitable receiver with S meter. You do need heavy weights at several places
on the cover. I shorted the coils with a short wire with two of the tiniest alligator clips I own.

This is of course done with the set turned off. You do have to do tiny tweeks
of the two end coils using a sweep generator.

It works.

Doug McDonald

________________________________
From: [email protected] <[email protected]> on behalf of Douglas Denny <ddennyoptom@...>
Sent: Wednesday, October 9, 2024 5:55 PM
To: [email protected] <[email protected]>
Subject: Re: [RacalRA17forum] Marconi TF1168

Craig,
This Zverev reference is very interesting indeed. A completely new angle
on filters for me. I must look more into it.
I'm still dubious about the practical application for alignment with the
Racal 40 Mhz BPF however.
As I have mentioned before: just taking off the cover and replacing again
can drastically disturb the BPF characteristics and shape; so this
procedure would require the covers to be remaining in place. How is this
procedure of shorting either side to be done without removing the covers?
and how will one couple the Spec Analyser to the single resonator when
there is just a small hole in the top with the cap trimmer visible (which
is at HT incidentally so presumably to be done power off!).
Sorry, - just don't see it.

On Wed, 9 Oct 2024, 08:03 Craig Sawyers via groups.io, <c.sawyers=
[email protected]> wrote:

Douglas

This is a compedium of the mails on this topic from UK Vintage Radio (
;!!DZ3fjg!-GMqsIfqOIavANqwRc5tkLsfd6Udvab0X8q3AyRRYkBu88wRJPdf6EpBIuRa_B0ZpXrDJjtNcAMQ-EMFzZhDLCoj$ ), David is a list admin,
with the handle "Radio Wrangler"

Craig

"There is a dirty trick for aligning coupled resonator filters (whether
the coupling is spatial, L or C, top or bottom)

Each resonator should resonate at the geometric centre frequency when
the other resonators to each side of it are disabled (shorted)

You need to use very very light coupling from your tracking generator
and your analyser to do this, but it works. Spectrum analysers have tons
of sensitivity, which can be used for very light coupling. With the
floating resonators of the 40MHz filter a coupling loop on each of the
tracker and the analyser is needed.

There isn't just a mathematical basis to this method, it is actually
involved in the design of such filters.

Trying to just twiddle the trimmers on a high order filter is an
unstable process, and even with a display of the shape in front of you,
settings tend to diverge, not converge.

Racal used preset jigs and all sorts of custom hardware to make setting
easy, but the 3dB k and q method only really got going after Zverev in
1967. The older m-derived and image parameter methods were much more
tedious to work out and didn't give you an adjustment method as a
freebie. There was a bit of a quiet revolution in filter design methods
in the mid sixties that came out of Bell labs and Westinghouse. Filter
design was a big issue for telecomms firms of the day.

==================================
David,

Thank you for the swift reply, and for your "dirty trick"... Apologies
in advance for the tidal wave of questions...

I know very little about coupled resonators - does the 40MC/s filter
contain mutual-inductance coupled resonators?

When you say geometric centre frequency - do you mean sqrt(39.5 * 40.5)?

Also, I presume that since the filter bandwidth is relatively wide at
~1MHz, each of the resonators needs to be set for a different frequency.
Should I evenly space the resonators? Geometrically space them? Do they
need to be aligned in increasing or decreasing resonant frequency order?
Do you have a specification at all?

When you say "the other resonators to each side of it are disabled
(shorted)", do you mean ALL of the other resonators should be shorted?
Or just the immediate two resonators either side?

When you say "light coupling", I presume that a single turn for the
tracking generator and spec analyser input should be adequate.

How would you recommend achieving this, given that the proximity of the
shielding can will probably affect the resonant frequency of each
resonator?

I am not familiar with the work of Zverev - I will look for some online
reference material...

Many thanks again for your advice,

====================================================

The 40MHz filter is a constructed as a string of LC resonators relying
on magnetic field coupling between adjacent ones though there will be a
little E field as well.

You have the geometric centre right.

Each resonator is set in isolation to the geometric centre, and the
coupling to its neighbours pulls it off of the centre. In a neat feat of
mathematics the coupling factors not only move the resonances to the
right places, they also get the right Q for each resonator to complete
the pattern which synthesises the required shape.

The source and load impedances applied to the ends of the filter set the
Qs of the end resonators, and the coupling factors pass some of the
damping along to the inner resonators. The coupling just happens to get
the pattern of the Qs right. Handy Eh?

Zverev wrote the book which set out "Modern Filter Theory" it turned the
decision of where to place the roots of the differential equation
defining the transfer function of a filter into a science. It's an
approximation for most idealised filter shapes, but you can decide how
complex a filter you want to make and this defines how close an
approximation you'll get.

Zverev's Handbook of Filter Synthesis covers the theory and then gives
catalogues of achievable shapes, than gives tables of normalised values
for making these shapes.

ust about every filter book written since 1967 cites Zverev. Many books
have been written on how to understand Zverev, but they don't put it so
directly

Zverev scares a lot of people off. He writes equations of attenuation
when equations of transfer function are now the norm so his equations
are upside down. His poles are our zeroes, his zeroes our poles.

It's not a book to go into alone. You need a guide to show you what it
can do. Otherwise it will seem weird and you may not notice the valuable
bits.

A small single turn loop for each of tracker and analyser is right.
Ideally you want them on opposite sides of the resonator

David
==============================
Ah, I must learn to type faster and think faster a few posts happened
while I was writing.

Use the one resonator at a time process. You do NOT stagger them at all.
You tune them all to the geometric centre frequency. One stage at a time
with the neighbours each time shorted.

When you take the shorts off, the coupling moves them all onto the right
frequencies.

Staggering is automatic. it does it for you itself.

When the full filter is running you will see Chebyshev bumps in it, but
if you twiddle the trimmers you'll find that they all interact. It isn't
a case of trimmer X moves bump Y. You don't get anything so easy. That's
why most trimming attempts of people unfamiliar with a design go astray
even with wobbulators spectrum analysers etc.

Cryptography relies on mathematical one-way streets, functions which are
easy to do in one direction, but much harder to do in the reverse
direction. Filter twiddling is like that. You have to learn to cheat.

David
=======================================

The source and termination resistors set the Qs of the end stages of the
filter, and via the coupling factors set the Qs of all the other stages.

If the resistance at either end goes high, the Qs go too high for the
designed response and the ripple gets big.

Lower the resistances and the Qs fall too low for the designed shape and
the filter rolls off.

If you're filter-hacking on a simulator like Spice, you can vary width
by scaling the coupling factors, then fiddle with the terminating R at
each end to adjust to get the flatness/ripple right.

David
===================
There's a danger of confusion here with talk of zeroes and Zverev.

Zverev drew equations of attenuation but now we tend to use equations of
transfer function. These are reciprocals of each other, so no big deal.
But the zeroes of transfer function are the poles of attenuation and
vice versa. So Zverev and current usage have the words zreoes and poles
swapped.

The tune one stage at a time method works for "All-pole" filters (poles
of transfer function) realised as coupled resonators. The schematic
posted earlier of Racal filters as simple L-C tanks spatially separated
is such a filter... they have no notches.

This technique does NOT put the poles on the required frequencies. It
puts them all on the same frequency. Then it relies on the coupling
factors to pull the poles apart to shift them onto the required
frequencies. It is rather neat that the same coupling factor does the
right frequency pull and sets the right Q both at the same time.

The passband pole positions show up as blunt ripples in the passband
top, making them very hard to adjust if you try this way. Better, they
appear as much sharper nulls in the return loss. So a tracker and return
loss bridge on a spectrum analyser can be used this way.

Filters with real-axis zeroes give stopband nulls... EG Elliptic ==
'Complete Cauer' designs. these are usually designed by the transformed
lowpass technique and suffer from its limitations.

I've played around with narrow bandpass filters with notches achieved by
basing things on pseudo-crystals.

David "


------ Original Message ------
From "Douglas Denny" <ddennyoptom@...>
To [email protected]
Date 09/10/2024 00:56:03
Subject Re: [RacalRA17forum] Marconi TF1168

Craig,

Thanks for your interesting reply.
The method you described shorting of the 40Mhz resonators in turn

surprises

me that it is reported to work, but admit I fail to understand it at all.
If it does work - great !.. a new relatively simple new method to try.
They are relatively closely coupled however, and the image impedances
looking in and out at each resonator stage with the close coupling would

be

badly affected if shorted I would have thought - which in turn would badly
affect the bandwidth shape I would expect.
It is true the two end resonators are the most sensitive to bandwidth

shape

changing characteristics when adjusted, which is another indication of the
importance and sensitivity of the input and output impedence seen at the
filter by the pre and post circuitry. Interesting comments though...

thank

you for that.

I was always impressed at how acutely sensitive is the 40Mhz BPF for
alignment, especialy so are the caps when adjusted - for obtaining the
accuracy needed for a good passband shape. So much so that just removing
the cover and replacing it will change the passband shape - sometimes
drastically.

Douglas Denny G3ZQE

On Mon, 7 Oct 2024, 16:41 Craig Sawyers via groups.io, <c.sawyers=
[email protected]> wrote:

I'm lucky enough to have a R&S Swob II that I bought from Rob Filby
donkeys years ago (fully functional with all the probes) and a CT501
that I bought for a bewilderingly low ?40 maybe 10 years ago - clearly
no one knew what it was, and I was the only bid on eBay.

BTW, although I have never tried it, apparently a sneaky trick with

the

40MHz filter is this. Short circuit every resonator except one. Very
loosely couple a SA to the remaining resonator, and set that to to
40MHz. Do that for each resonator in turn. Every one in isolation set

to

40MHz. When all the shorts are taken off, the resonator couplings pull
each other to precisely the right frequencies to give the required
bandshape. Apparently the only ones that need care are the two end
resonators, because they have to be loaded correctly.

This is from a guy whose career was designing RF stuff (like spectrum
analysers) for HP. He tends to know every trick in the book regarding

RF

filter design and alignment. He is also an RA17 fan.

Yes the TF1186 was a strange and wonderful thing; I suspect that not
many were made.

I've just hauled my Tek 7L5 SA out; It is a while since I used it, and
quite forgot that it has a tracking generator option. 10Hz to 5MHz,

with

a resolution bandwidth down to 10Hz. That I believe is more is more

than

capable of aligning the IF chain filters ;-)

Incidentally Douglas, I was just reading an old post of yours

describing

the order of bandwidth filter alignment - IOW start with the 100Hz and
work up in frequency.

Craig


------ Original Message ------
From "Douglas Denny" <ddennyoptom@...>
To [email protected]
Date 07/10/2024 13:16:52
Subject Re: [RacalRA17forum] Marconi TF1168

>I still have two TF114H sig gens stored somewhere in the attic, and

have

>aligned RA117s with them in the 1970s (along with an early wobbulator

for

>the 40MHz bpf).
>They are a beautiful example of precision engineering in test

equipment of

>the time; if left on continuously were rock stable and very low

jitter and

>spectral purity compared to some modern sythesised equipments. I

think one

>amateur professed to use one as a vfo for vhf transmitter on 2 metres
>around that era.
>I have no knowledge about the TF1168.
>
>Douglas Denny. G3ZQE.
>
>
>
>On Mon, 7 Oct 2024, 11:58 Craig Sawyers via groups.io, <c.sawyers=
>[email protected]> wrote:
>
>> This is pretty much a historical curiosity. This was the "book"

method

>> of aligning the crystal and other bandpass filters on the IF

strip, and

>> IF transformers in the RA17 series of receivers.
>>
>> Basically described as a "High Discrimination Oscillator", it was
>> precision set to 100kHz against an internal crystal calibrator,

and had

>> three large tuning knobs covering 10kHz, 1kHz and 100Hz total

range,

>> selected by a switch.
>>
>> Nothing much on the web about this boatanchor (a hefty

40kg/88lbs), no

>> manuals anywhere. There are two references - one an old Marconi
>> catalogue that describes its characteristics, and a similarly old
>> Marconi price list that says it was ?328 in 1961 (around ?10k in
today's
>> money). However, as an example of the ravages of time and value,

the

>> TF144 signal generator was even more expensive - but last year I

was

>> given two. Both of which work bang on spec BTW, half a century on.
>>
>> Of course now a spectrum analyser is the way to go. However, for
>> curiosity's sake more than anything, does anyone know anything

about

>> this Marconi TF1168 beast?
>>
>> Craig
>>
>>
>>
>>
>>
>>
>
>
>
>
>

Many thanks for the very detailed post Douglas.

Although Zverev's seminal filters book, difficult to get a grip on, was only published in 1967 - and hence post-dates the RA17/117 etc series. Although Dishal's paper on shorting resonators from 1951 must have been know by the RA17 design team.

I did electronics at University (74 to 77) and at no point were we allowed to use calculators. So multi-section elliptic and m-derived filters were all calculated from normalised tables, and long division of polynomials. You could get most filter alignments this way. We were taught this by one of the co-founders of the electronics department in 1941 at Southampton U - SW Punnett. He was a choke and condenser man, but was a past master in filter synthesis.

There a number of text books by Zepler (the co-founder) and Punnet from 1943 through to 1963.

Looking at the preferred shape of the 40MHz filter, with pass-band ripples, it is an elliptic alignment, possibly Chebyshev . More than you need to know about all this is here . That is the sort of thing that I kind of did in my sleep in the early 70's. Whether it is low pass, high pass, bandpass or bandstop is done by taking the characteristic equation for whatever shape and attenuation (set by the order and shape of the filter), and carrying out a straightforward transformation to get low, high, pass or stop.

All this was well known back in the day the RA17 was designed. And yes, you needed a deep understanding of filter synthesis. But remember that the Sydney Harbour Bridge was designed in the early 20's with slide rules and log tables! You needed to be a real expert in whatever field back in the day.

I don't know what technique was used in the Racal factory to align the 40MHz filter. They made a lot of RA17's and derivatives, all of which have the 40MHz filter - so alignment had to be quick, accurate and repeatable.

Craig

Craig,

Re, 40Mhz BPF covers.

I have seen a number of Ra17 and Ra117 series, and the MA79 too based on
the same circuitry, but never come across filter covers with mesh. They all
possessed the spring-finger earthing method, and pushed into the chassis
cavity of the filter. These springs are in close proximity to the L/C
components within. The caps are all low pF values, and when tuned for
alignment, I note the tiniest movement twiddling, has a disproportionately
large effect of the BPF filter transfer function shape. In other words...
is exquisitely sensitive to the very smallest capacitance changes. So it
doesn't surprise me at all that those earthing fingers probably have a
large effect on the overall parasitic capacitance within each filter
section cavity; and further: as each section is closely coupled
electrically the effect is cumulative. Hence take cover off - put back on
and it's all slightly different capacitance wise!
Every Racal unit I've seen that has not been "breathed upon" by someone
and/or presumably untouched since leaving the factory, had black plastic
labels on the BPF covers with the warning: "Do not Remove".
It was Bob Finch who pointed this problem out to me in the 1970s when I
first obtained an RA117 and SSB unit and needed to align it (in those days
with a wobbulator and separate sig gen to produce the marker pips). I've
observed many times since he was quite right.

I do not think it is a problem with the quality of earthing of the fingers;
I'm satisfied that is more than reasonable; but is simply the variability
of capacitance with the close proximity of the earthing cover fingers
internally every time the cover is disturbed.

Thank you for the explanation of the "sniffer loop" idea. This introduces
considerable inconvenience for many reasons. It did occur to me thinking
about it earlier that one would have to drill a hole in each cover section
cover to the side of the existing central hole to introduce a fine wire
detector loop - but even _that_ probably will interfere with electrical
capacitance effects and BPF shape. You have now confirmed a sniffer loop
will have to be introduced inside each section.

It seems to me the 40 Mhz BPF is a crucial and highly sensitive part of the
receiver design - highly sensitive to component values and physical shape
in the design. It must have been a real headache when designed.
I think it is an Achlles Heel - a bottleneck - of the whole circuit
design, but imposed on Racal with not much in the way of alternatives
available because of the very stringent requirements of as close as
possible a perfect 'square edged' filter to give _exactly_ a 1Mhz
bandwidth, centred on 40 Mhz - definitely not easy in the 1950s !
Even when adjusted to as near "perfect" as possible it has limitations in
that the required bandwidth of exactly 1Mhz is only _just_ possible to meet
- but at the expense of allowing compromises: a large increase in overall
attenuation across the whole passband (ie. by widening it to the 1 Mhz);
... and rapid decrease in receiver sensitivity at each end of the Khz scale
due to the rapid (square) shape fall-off in passband at each end; and
finally, passband ripple, changing sensitivity of receiever across the Khz
scale.
Aligniment is tricky but doeable and though takes some fiddling is soon
learned.
I'm of the opinion that BPF needs to be isolated as much as possible for
alignment. Which is why I developed my method of using the input and
output valves as buffers to keep it as isolated as possible during
alignment. This method only needs a small cap from the BPF output valve
available for output to the Spec Analyser.

As a final note, the input and output image impedances presented to the
filter also seriously affects the performance (shape) of the filter, which
is why the two end sections are the most sensitive in twiddling to changing
passband shape; also:- even changing either of the E180F high gain valves
at input and output side can change performance of the filter and
performance of the whole receiver. I think it was recommended that
realigment of the BPF be done if these valves were changed.

Regards,
Douglas. G3ZQE.

On Thu, 10 Oct 2024, 00:23 Craig Sawyers via groups.io, <c.sawyers=
[email protected]> wrote:

Douglas

Quizzing David in more detail, it looks like a pair of sacrificial
covers is needed, with additional small holes through which a single
exciting loop from a sig gen and single spectrum analyzer loop to be
introduced.

A very similar approach was shown in a paper by Dishal "Alignment and
Adjustment of Synchronously Tuned Multiple-Resonant-Circuit Filters"
from 1951. Still copyright by the IEEE, and I've failed to find a
bootleg copy.

But here's a thread on this method


and doubtless others regarding Dishal; that link is just the first I
found.

It seems very strange that the 40MHz filter is so exquisitely sensitive
that removing the covers then screwing them back in messes the response
up. Racal cannot have let that one go. I realise that in very late
production they replaced the spring finger plate grounds with metal mesh
edged covers, so perhaps the effect you notice is down to imperfect
grounding of he spring finger plate grounds?

Craig




------ Original Message ------
From "Douglas Denny" <ddennyoptom@...>
To [email protected]
Date 09/10/2024 23:55:36
Subject Re: [RacalRA17forum] Marconi TF1168

Craig,
This Zverev reference is very interesting indeed. A completely new angle
on filters for me. I must look more into it.
I'm still dubious about the practical application for alignment with the
Racal 40 Mhz BPF however.
As I have mentioned before: just taking off the cover and replacing again
can drastically disturb the BPF characteristics and shape; so this
procedure would require the covers to be remaining in place. How is this
procedure of shorting either side to be done without removing the covers?
and how will one couple the Spec Analyser to the single resonator when
there is just a small hole in the top with the cap trimmer visible (which
is at HT incidentally so presumably to be done power off!).
Sorry, - just don't see it.

On Wed, 9 Oct 2024, 08:03 Craig Sawyers via groups.io, <c.sawyers=
[email protected]> wrote:

Douglas

This is a compedium of the mails on this topic from UK Vintage Radio (
), David is a list admin,
with the handle "Radio Wrangler"

Craig

"There is a dirty trick for aligning coupled resonator filters (whether
the coupling is spatial, L or C, top or bottom)

Each resonator should resonate at the geometric centre frequency when
the other resonators to each side of it are disabled (shorted)

You need to use very very light coupling from your tracking generator
and your analyser to do this, but it works. Spectrum analysers have

tons

of sensitivity, which can be used for very light coupling. With the
floating resonators of the 40MHz filter a coupling loop on each of the
tracker and the analyser is needed.

There isn't just a mathematical basis to this method, it is actually
involved in the design of such filters.

Trying to just twiddle the trimmers on a high order filter is an
unstable process, and even with a display of the shape in front of you,
settings tend to diverge, not converge.

Racal used preset jigs and all sorts of custom hardware to make setting
easy, but the 3dB k and q method only really got going after Zverev in
1967. The older m-derived and image parameter methods were much more
tedious to work out and didn't give you an adjustment method as a
freebie. There was a bit of a quiet revolution in filter design methods
in the mid sixties that came out of Bell labs and Westinghouse. Filter
design was a big issue for telecomms firms of the day.

==================================
David,

Thank you for the swift reply, and for your "dirty trick"... Apologies
in advance for the tidal wave of questions...

I know very little about coupled resonators - does the 40MC/s filter
contain mutual-inductance coupled resonators?

When you say geometric centre frequency - do you mean sqrt(39.5 *

40.5)?

Also, I presume that since the filter bandwidth is relatively wide at
~1MHz, each of the resonators needs to be set for a different

frequency.

Should I evenly space the resonators? Geometrically space them? Do they
need to be aligned in increasing or decreasing resonant frequency

order?

Do you have a specification at all?

When you say "the other resonators to each side of it are disabled
(shorted)", do you mean ALL of the other resonators should be shorted?
Or just the immediate two resonators either side?

When you say "light coupling", I presume that a single turn for the
tracking generator and spec analyser input should be adequate.

How would you recommend achieving this, given that the proximity of the
shielding can will probably affect the resonant frequency of each
resonator?

I am not familiar with the work of Zverev - I will look for some online
reference material...

Many thanks again for your advice,

====================================================

The 40MHz filter is a constructed as a string of LC resonators relying
on magnetic field coupling between adjacent ones though there will be a
little E field as well.

You have the geometric centre right.

Each resonator is set in isolation to the geometric centre, and the
coupling to its neighbours pulls it off of the centre. In a neat feat

of

mathematics the coupling factors not only move the resonances to the
right places, they also get the right Q for each resonator to complete
the pattern which synthesises the required shape.

The source and load impedances applied to the ends of the filter set

the

Qs of the end resonators, and the coupling factors pass some of the
damping along to the inner resonators. The coupling just happens to get
the pattern of the Qs right. Handy Eh?

Zverev wrote the book which set out "Modern Filter Theory" it turned

the

decision of where to place the roots of the differential equation
defining the transfer function of a filter into a science. It's an
approximation for most idealised filter shapes, but you can decide how
complex a filter you want to make and this defines how close an
approximation you'll get.

Zverev's Handbook of Filter Synthesis covers the theory and then gives
catalogues of achievable shapes, than gives tables of normalised values
for making these shapes.

ust about every filter book written since 1967 cites Zverev. Many books
have been written on how to understand Zverev, but they don't put it so
directly

Zverev scares a lot of people off. He writes equations of attenuation
when equations of transfer function are now the norm so his equations
are upside down. His poles are our zeroes, his zeroes our poles.

It's not a book to go into alone. You need a guide to show you what it
can do. Otherwise it will seem weird and you may not notice the

valuable

bits.

A small single turn loop for each of tracker and analyser is right.
Ideally you want them on opposite sides of the resonator

David
==============================
Ah, I must learn to type faster and think faster a few posts happened
while I was writing.

Use the one resonator at a time process. You do NOT stagger them at

all.

You tune them all to the geometric centre frequency. One stage at a

time

with the neighbours each time shorted.

When you take the shorts off, the coupling moves them all onto the

right

frequencies.

Staggering is automatic. it does it for you itself.

When the full filter is running you will see Chebyshev bumps in it, but
if you twiddle the trimmers you'll find that they all interact. It

isn't

a case of trimmer X moves bump Y. You don't get anything so easy.

That's

why most trimming attempts of people unfamiliar with a design go astray
even with wobbulators spectrum analysers etc.

Cryptography relies on mathematical one-way streets, functions which

are

easy to do in one direction, but much harder to do in the reverse
direction. Filter twiddling is like that. You have to learn to cheat.

David
=======================================

The source and termination resistors set the Qs of the end stages of

the

filter, and via the coupling factors set the Qs of all the other

stages.

If the resistance at either end goes high, the Qs go too high for the
designed response and the ripple gets big.

Lower the resistances and the Qs fall too low for the designed shape

and

the filter rolls off.

If you're filter-hacking on a simulator like Spice, you can vary width
by scaling the coupling factors, then fiddle with the terminating R at
each end to adjust to get the flatness/ripple right.

David
===================
There's a danger of confusion here with talk of zeroes and Zverev.

Zverev drew equations of attenuation but now we tend to use equations

of

transfer function. These are reciprocals of each other, so no big deal.
But the zeroes of transfer function are the poles of attenuation and
vice versa. So Zverev and current usage have the words zreoes and poles
swapped.

The tune one stage at a time method works for "All-pole" filters (poles
of transfer function) realised as coupled resonators. The schematic
posted earlier of Racal filters as simple L-C tanks spatially separated
is such a filter... they have no notches.

This technique does NOT put the poles on the required frequencies. It
puts them all on the same frequency. Then it relies on the coupling
factors to pull the poles apart to shift them onto the required
frequencies. It is rather neat that the same coupling factor does the
right frequency pull and sets the right Q both at the same time.

The passband pole positions show up as blunt ripples in the passband
top, making them very hard to adjust if you try this way. Better, they
appear as much sharper nulls in the return loss. So a tracker and

return

loss bridge on a spectrum analyser can be used this way.

Filters with real-axis zeroes give stopband nulls... EG Elliptic ==
'Complete Cauer' designs. these are usually designed by the transformed
lowpass technique and suffer from its limitations.

I've played around with narrow bandpass filters with notches achieved

by

basing things on pseudo-crystals.

David "


------ Original Message ------
From "Douglas Denny" <ddennyoptom@...>
To [email protected]
Date 09/10/2024 00:56:03
Subject Re: [RacalRA17forum] Marconi TF1168

>Craig,
>
>Thanks for your interesting reply.
>The method you described shorting of the 40Mhz resonators in turn
surprises
>me that it is reported to work, but admit I fail to understand it at

all.

>If it does work - great !.. a new relatively simple new method to

try.

>They are relatively closely coupled however, and the image impedances
>looking in and out at each resonator stage with the close coupling

would

be
>badly affected if shorted I would have thought - which in turn would

badly

>affect the bandwidth shape I would expect.
>It is true the two end resonators are the most sensitive to bandwidth
shape
>changing characteristics when adjusted, which is another indication

of the

>importance and sensitivity of the input and output impedence seen at

the

>filter by the pre and post circuitry. Interesting comments though...
thank
>you for that.
>
>I was always impressed at how acutely sensitive is the 40Mhz BPF for
>alignment, especialy so are the caps when adjusted - for obtaining the
>accuracy needed for a good passband shape. So much so that just

removing

>the cover and replacing it will change the passband shape - sometimes
>drastically.
>
>Douglas Denny G3ZQE
>
>On Mon, 7 Oct 2024, 16:41 Craig Sawyers via groups.io, <c.sawyers=
>[email protected]> wrote:
>
>> I'm lucky enough to have a R&S Swob II that I bought from Rob Filby
>> donkeys years ago (fully functional with all the probes) and a

CT501

>> that I bought for a bewilderingly low ?40 maybe 10 years ago -

clearly

>> no one knew what it was, and I was the only bid on eBay.
>>
>> BTW, although I have never tried it, apparently a sneaky trick

with

the
>> 40MHz filter is this. Short circuit every resonator except one.

Very

>> loosely couple a SA to the remaining resonator, and set that to to
>> 40MHz. Do that for each resonator in turn. Every one in isolation

set

to
>> 40MHz. When all the shorts are taken off, the resonator couplings

pull

>> each other to precisely the right frequencies to give the required
>> bandshape. Apparently the only ones that need care are the two end
>> resonators, because they have to be loaded correctly.
>>
>> This is from a guy whose career was designing RF stuff (like

spectrum

>> analysers) for HP. He tends to know every trick in the book

regarding

RF
>> filter design and alignment. He is also an RA17 fan.
>>
>> Yes the TF1186 was a strange and wonderful thing; I suspect that

not

>> many were made.
>>
>> I've just hauled my Tek 7L5 SA out; It is a while since I used it,

and

>> quite forgot that it has a tracking generator option. 10Hz to 5MHz,
with
>> a resolution bandwidth down to 10Hz. That I believe is more is more
than
>> capable of aligning the IF chain filters ;-)
>>
>> Incidentally Douglas, I was just reading an old post of yours
describing
>> the order of bandwidth filter alignment - IOW start with the 100Hz

and

>> work up in frequency.
>>
>> Craig
>>
>>
>> ------ Original Message ------
>> From "Douglas Denny" <ddennyoptom@...>
>> To [email protected]
>> Date 07/10/2024 13:16:52
>> Subject Re: [RacalRA17forum] Marconi TF1168
>>
>> >I still have two TF114H sig gens stored somewhere in the attic,

and

have
>> >aligned RA117s with them in the 1970s (along with an early

wobbulator

for
>> >the 40MHz bpf).
>> >They are a beautiful example of precision engineering in test
equipment of
>> >the time; if left on continuously were rock stable and very low
jitter and
>> >spectral purity compared to some modern sythesised equipments. I
think one
>> >amateur professed to use one as a vfo for vhf transmitter on 2

metres

>> >around that era.
>> >I have no knowledge about the TF1168.
>> >
>> >Douglas Denny. G3ZQE.
>> >
>> >
>> >
>> >On Mon, 7 Oct 2024, 11:58 Craig Sawyers via groups.io,

<c.sawyers=

>> >[email protected]> wrote:
>> >
>> >> This is pretty much a historical curiosity. This was the "book"
method
>> >> of aligning the crystal and other bandpass filters on the IF
strip, and
>> >> IF transformers in the RA17 series of receivers.
>> >>
>> >> Basically described as a "High Discrimination Oscillator", it

was

>> >> precision set to 100kHz against an internal crystal calibrator,
and had
>> >> three large tuning knobs covering 10kHz, 1kHz and 100Hz total
range,
>> >> selected by a switch.
>> >>
>> >> Nothing much on the web about this boatanchor (a hefty
40kg/88lbs), no
>> >> manuals anywhere. There are two references - one an old Marconi
>> >> catalogue that describes its characteristics, and a similarly

old

>> >> Marconi price list that says it was ?328 in 1961 (around ?10k

in

>> today's
>> >> money). However, as an example of the ravages of time and

value,

the
>> >> TF144 signal generator was even more expensive - but last year

I

was
>> >> given two. Both of which work bang on spec BTW, half a century

on.

>> >>
>> >> Of course now a spectrum analyser is the way to go. However,

for

>> >> curiosity's sake more than anything, does anyone know anything
about
>> >> this Marconi TF1168 beast?
>> >>
>> >> Craig
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >
>> >
>> >
>> >
>> >
>>
>>
>>
>>
>>
>>
>
>
>
>
>

Douglas

Quizzing David in more detail, it looks like a pair of sacrificial covers is needed, with additional small holes through which a single exciting loop from a sig gen and single spectrum analyzer loop to be introduced.

A very similar approach was shown in a paper by Dishal "Alignment and Adjustment of Synchronously Tuned Multiple-Resonant-Circuit Filters" from 1951. Still copyright by the IEEE, and I've failed to find a bootleg copy.

But here's a thread on this method and doubtless others regarding Dishal; that link is just the first I found.

It seems very strange that the 40MHz filter is so exquisitely sensitive that removing the covers then screwing them back in messes the response up. Racal cannot have let that one go. I realise that in very late production they replaced the spring finger plate grounds with metal mesh edged covers, so perhaps the effect you notice is down to imperfect grounding of he spring finger plate grounds?

Craig

Craig,
This Zverev reference is very interesting indeed. A completely new angle
on filters for me. I must look more into it.
I'm still dubious about the practical application for alignment with the
Racal 40 Mhz BPF however.
As I have mentioned before: just taking off the cover and replacing again
can drastically disturb the BPF characteristics and shape; so this
procedure would require the covers to be remaining in place. How is this
procedure of shorting either side to be done without removing the covers?
and how will one couple the Spec Analyser to the single resonator when
there is just a small hole in the top with the cap trimmer visible (which
is at HT incidentally so presumably to be done power off!).
Sorry, - just don't see it.

On Wed, 9 Oct 2024, 08:03 Craig Sawyers via groups.io, <c.sawyers=
[email protected]> wrote:

Douglas

This is a compedium of the mails on this topic from UK Vintage Radio (
), David is a list admin,
with the handle "Radio Wrangler"

Craig

"There is a dirty trick for aligning coupled resonator filters (whether
the coupling is spatial, L or C, top or bottom)

Each resonator should resonate at the geometric centre frequency when
the other resonators to each side of it are disabled (shorted)

You need to use very very light coupling from your tracking generator
and your analyser to do this, but it works. Spectrum analysers have tons
of sensitivity, which can be used for very light coupling. With the
floating resonators of the 40MHz filter a coupling loop on each of the
tracker and the analyser is needed.

There isn't just a mathematical basis to this method, it is actually
involved in the design of such filters.

Trying to just twiddle the trimmers on a high order filter is an
unstable process, and even with a display of the shape in front of you,
settings tend to diverge, not converge.

Racal used preset jigs and all sorts of custom hardware to make setting
easy, but the 3dB k and q method only really got going after Zverev in
1967. The older m-derived and image parameter methods were much more
tedious to work out and didn't give you an adjustment method as a
freebie. There was a bit of a quiet revolution in filter design methods
in the mid sixties that came out of Bell labs and Westinghouse. Filter
design was a big issue for telecomms firms of the day.

==================================
David,

Thank you for the swift reply, and for your "dirty trick"... Apologies
in advance for the tidal wave of questions...

I know very little about coupled resonators - does the 40MC/s filter
contain mutual-inductance coupled resonators?

When you say geometric centre frequency - do you mean sqrt(39.5 * 40.5)?

Also, I presume that since the filter bandwidth is relatively wide at
~1MHz, each of the resonators needs to be set for a different frequency.
Should I evenly space the resonators? Geometrically space them? Do they
need to be aligned in increasing or decreasing resonant frequency order?
Do you have a specification at all?

When you say "the other resonators to each side of it are disabled
(shorted)", do you mean ALL of the other resonators should be shorted?
Or just the immediate two resonators either side?

When you say "light coupling", I presume that a single turn for the
tracking generator and spec analyser input should be adequate.

How would you recommend achieving this, given that the proximity of the
shielding can will probably affect the resonant frequency of each
resonator?

I am not familiar with the work of Zverev - I will look for some online
reference material...

Many thanks again for your advice,

====================================================

The 40MHz filter is a constructed as a string of LC resonators relying
on magnetic field coupling between adjacent ones though there will be a
little E field as well.

You have the geometric centre right.

Each resonator is set in isolation to the geometric centre, and the
coupling to its neighbours pulls it off of the centre. In a neat feat of
mathematics the coupling factors not only move the resonances to the
right places, they also get the right Q for each resonator to complete
the pattern which synthesises the required shape.

The source and load impedances applied to the ends of the filter set the
Qs of the end resonators, and the coupling factors pass some of the
damping along to the inner resonators. The coupling just happens to get
the pattern of the Qs right. Handy Eh?

Zverev wrote the book which set out "Modern Filter Theory" it turned the
decision of where to place the roots of the differential equation
defining the transfer function of a filter into a science. It's an
approximation for most idealised filter shapes, but you can decide how
complex a filter you want to make and this defines how close an
approximation you'll get.

Zverev's Handbook of Filter Synthesis covers the theory and then gives
catalogues of achievable shapes, than gives tables of normalised values
for making these shapes.

ust about every filter book written since 1967 cites Zverev. Many books
have been written on how to understand Zverev, but they don't put it so
directly

Zverev scares a lot of people off. He writes equations of attenuation
when equations of transfer function are now the norm so his equations
are upside down. His poles are our zeroes, his zeroes our poles.

It's not a book to go into alone. You need a guide to show you what it
can do. Otherwise it will seem weird and you may not notice the valuable
bits.

A small single turn loop for each of tracker and analyser is right.
Ideally you want them on opposite sides of the resonator

David
==============================
Ah, I must learn to type faster and think faster a few posts happened
while I was writing.

Use the one resonator at a time process. You do NOT stagger them at all.
You tune them all to the geometric centre frequency. One stage at a time
with the neighbours each time shorted.

When you take the shorts off, the coupling moves them all onto the right
frequencies.

Staggering is automatic. it does it for you itself.

When the full filter is running you will see Chebyshev bumps in it, but
if you twiddle the trimmers you'll find that they all interact. It isn't
a case of trimmer X moves bump Y. You don't get anything so easy. That's
why most trimming attempts of people unfamiliar with a design go astray
even with wobbulators spectrum analysers etc.

Cryptography relies on mathematical one-way streets, functions which are
easy to do in one direction, but much harder to do in the reverse
direction. Filter twiddling is like that. You have to learn to cheat.

David
=======================================

The source and termination resistors set the Qs of the end stages of the
filter, and via the coupling factors set the Qs of all the other stages.

If the resistance at either end goes high, the Qs go too high for the
designed response and the ripple gets big.

Lower the resistances and the Qs fall too low for the designed shape and
the filter rolls off.

If you're filter-hacking on a simulator like Spice, you can vary width
by scaling the coupling factors, then fiddle with the terminating R at
each end to adjust to get the flatness/ripple right.

David
===================
There's a danger of confusion here with talk of zeroes and Zverev.

Zverev drew equations of attenuation but now we tend to use equations of
transfer function. These are reciprocals of each other, so no big deal.
But the zeroes of transfer function are the poles of attenuation and
vice versa. So Zverev and current usage have the words zreoes and poles
swapped.

The tune one stage at a time method works for "All-pole" filters (poles
of transfer function) realised as coupled resonators. The schematic
posted earlier of Racal filters as simple L-C tanks spatially separated
is such a filter... they have no notches.

This technique does NOT put the poles on the required frequencies. It
puts them all on the same frequency. Then it relies on the coupling
factors to pull the poles apart to shift them onto the required
frequencies. It is rather neat that the same coupling factor does the
right frequency pull and sets the right Q both at the same time.

The passband pole positions show up as blunt ripples in the passband
top, making them very hard to adjust if you try this way. Better, they
appear as much sharper nulls in the return loss. So a tracker and return
loss bridge on a spectrum analyser can be used this way.

Filters with real-axis zeroes give stopband nulls... EG Elliptic ==
'Complete Cauer' designs. these are usually designed by the transformed
lowpass technique and suffer from its limitations.

I've played around with narrow bandpass filters with notches achieved by
basing things on pseudo-crystals.

David "


------ Original Message ------
From "Douglas Denny" <ddennyoptom@...>
To [email protected]
Date 09/10/2024 00:56:03
Subject Re: [RacalRA17forum] Marconi TF1168

Craig,

Thanks for your interesting reply.
The method you described shorting of the 40Mhz resonators in turn

surprises

me that it is reported to work, but admit I fail to understand it at all.
If it does work - great !.. a new relatively simple new method to try.
They are relatively closely coupled however, and the image impedances
looking in and out at each resonator stage with the close coupling would

be

badly affected if shorted I would have thought - which in turn would badly
affect the bandwidth shape I would expect.
It is true the two end resonators are the most sensitive to bandwidth

shape

changing characteristics when adjusted, which is another indication of the
importance and sensitivity of the input and output impedence seen at the
filter by the pre and post circuitry. Interesting comments though...

thank

you for that.

I was always impressed at how acutely sensitive is the 40Mhz BPF for
alignment, especialy so are the caps when adjusted - for obtaining the
accuracy needed for a good passband shape. So much so that just removing
the cover and replacing it will change the passband shape - sometimes
drastically.

Douglas Denny G3ZQE

On Mon, 7 Oct 2024, 16:41 Craig Sawyers via groups.io, <c.sawyers=
[email protected]> wrote:

I'm lucky enough to have a R&S Swob II that I bought from Rob Filby
donkeys years ago (fully functional with all the probes) and a CT501
that I bought for a bewilderingly low ?40 maybe 10 years ago - clearly
no one knew what it was, and I was the only bid on eBay.

BTW, although I have never tried it, apparently a sneaky trick with

the

40MHz filter is this. Short circuit every resonator except one. Very
loosely couple a SA to the remaining resonator, and set that to to
40MHz. Do that for each resonator in turn. Every one in isolation set

to

40MHz. When all the shorts are taken off, the resonator couplings pull
each other to precisely the right frequencies to give the required
bandshape. Apparently the only ones that need care are the two end
resonators, because they have to be loaded correctly.

This is from a guy whose career was designing RF stuff (like spectrum
analysers) for HP. He tends to know every trick in the book regarding

RF

filter design and alignment. He is also an RA17 fan.

Yes the TF1186 was a strange and wonderful thing; I suspect that not
many were made.

I've just hauled my Tek 7L5 SA out; It is a while since I used it, and
quite forgot that it has a tracking generator option. 10Hz to 5MHz,

with

a resolution bandwidth down to 10Hz. That I believe is more is more

than

capable of aligning the IF chain filters ;-)

Incidentally Douglas, I was just reading an old post of yours

describing

the order of bandwidth filter alignment - IOW start with the 100Hz and
work up in frequency.

Craig


------ Original Message ------
From "Douglas Denny" <ddennyoptom@...>
To [email protected]
Date 07/10/2024 13:16:52
Subject Re: [RacalRA17forum] Marconi TF1168

>I still have two TF114H sig gens stored somewhere in the attic, and

have

>aligned RA117s with them in the 1970s (along with an early wobbulator

for

>the 40MHz bpf).
>They are a beautiful example of precision engineering in test

equipment of

>the time; if left on continuously were rock stable and very low

jitter and

>spectral purity compared to some modern sythesised equipments. I

think one

>amateur professed to use one as a vfo for vhf transmitter on 2 metres
>around that era.
>I have no knowledge about the TF1168.
>
>Douglas Denny. G3ZQE.
>
>
>
>On Mon, 7 Oct 2024, 11:58 Craig Sawyers via groups.io, <c.sawyers=
>[email protected]> wrote:
>
>> This is pretty much a historical curiosity. This was the "book"

method

>> of aligning the crystal and other bandpass filters on the IF

strip, and

>> IF transformers in the RA17 series of receivers.
>>
>> Basically described as a "High Discrimination Oscillator", it was
>> precision set to 100kHz against an internal crystal calibrator,

and had

>> three large tuning knobs covering 10kHz, 1kHz and 100Hz total

range,

>> selected by a switch.
>>
>> Nothing much on the web about this boatanchor (a hefty

40kg/88lbs), no

>> manuals anywhere. There are two references - one an old Marconi
>> catalogue that describes its characteristics, and a similarly old
>> Marconi price list that says it was ?328 in 1961 (around ?10k in
today's
>> money). However, as an example of the ravages of time and value,

the

>> TF144 signal generator was even more expensive - but last year I

was

>> given two. Both of which work bang on spec BTW, half a century on.
>>
>> Of course now a spectrum analyser is the way to go. However, for
>> curiosity's sake more than anything, does anyone know anything

about

>> this Marconi TF1168 beast?
>>
>> Craig
>>
>>
>>
>>
>>
>>
>
>
>
>
>

A similarly surprising to me filter (when I first saw it without the background of why it works) is the crystal ladder filter (e.g. ) which uses a number of (ideally) identical quartz crystal resonators. All resonators have identical frequency response _when in isolation_ but when correctly coupled, the resonances shift to give an overall shape very different from the individual.

I think we have a similar structure here, so the key to alignment is isolating each resonator, without disturbing the intrinsic elements of the resonator itself, including the strays. Shorting the resonators on either side will drastically (!) affect their resonance - hence they will no longer couple strongly to the one being aligned. If you open-circuited them, that would work too, but seems rather more destructive.


-Iain Lindsay..
GM8LYQ

Douglas

This is a compedium of the mails on this topic from UK Vintage Radio ( ), David is a list admin, with the handle "Radio Wrangler"

Craig

"There is a dirty trick for aligning coupled resonator filters (whether the coupling is spatial, L or C, top or bottom)

Each resonator should resonate at the geometric centre frequency when the other resonators to each side of it are disabled (shorted)

You need to use very very light coupling from your tracking generator and your analyser to do this, but it works. Spectrum analysers have tons of sensitivity, which can be used for very light coupling. With the floating resonators of the 40MHz filter a coupling loop on each of the tracker and the analyser is needed.

There isn't just a mathematical basis to this method, it is actually involved in the design of such filters.

Trying to just twiddle the trimmers on a high order filter is an unstable process, and even with a display of the shape in front of you, settings tend to diverge, not converge.

Racal used preset jigs and all sorts of custom hardware to make setting easy, but the 3dB k and q method only really got going after Zverev in 1967. The older m-derived and image parameter methods were much more tedious to work out and didn't give you an adjustment method as a freebie. There was a bit of a quiet revolution in filter design methods in the mid sixties that came out of Bell labs and Westinghouse. Filter design was a big issue for telecomms firms of the day.

==================================
David,

Thank you for the swift reply, and for your "dirty trick"... Apologies in advance for the tidal wave of questions...

I know very little about coupled resonators - does the 40MC/s filter contain mutual-inductance coupled resonators?

When you say geometric centre frequency - do you mean sqrt(39.5 * 40.5)?

Also, I presume that since the filter bandwidth is relatively wide at ~1MHz, each of the resonators needs to be set for a different frequency. Should I evenly space the resonators? Geometrically space them? Do they need to be aligned in increasing or decreasing resonant frequency order? Do you have a specification at all?

When you say "the other resonators to each side of it are disabled (shorted)", do you mean ALL of the other resonators should be shorted? Or just the immediate two resonators either side?

When you say "light coupling", I presume that a single turn for the tracking generator and spec analyser input should be adequate.

How would you recommend achieving this, given that the proximity of the shielding can will probably affect the resonant frequency of each resonator?

I am not familiar with the work of Zverev - I will look for some online reference material...

Many thanks again for your advice,

====================================================

The 40MHz filter is a constructed as a string of LC resonators relying on magnetic field coupling between adjacent ones though there will be a little E field as well.

You have the geometric centre right.

Each resonator is set in isolation to the geometric centre, and the coupling to its neighbours pulls it off of the centre. In a neat feat of mathematics the coupling factors not only move the resonances to the right places, they also get the right Q for each resonator to complete the pattern which synthesises the required shape.

The source and load impedances applied to the ends of the filter set the Qs of the end resonators, and the coupling factors pass some of the damping along to the inner resonators. The coupling just happens to get the pattern of the Qs right. Handy Eh?

Zverev wrote the book which set out "Modern Filter Theory" it turned the decision of where to place the roots of the differential equation defining the transfer function of a filter into a science. It's an approximation for most idealised filter shapes, but you can decide how complex a filter you want to make and this defines how close an approximation you'll get.

Zverev's Handbook of Filter Synthesis covers the theory and then gives catalogues of achievable shapes, than gives tables of normalised values for making these shapes.

ust about every filter book written since 1967 cites Zverev. Many books have been written on how to understand Zverev, but they don't put it so directly

Zverev scares a lot of people off. He writes equations of attenuation when equations of transfer function are now the norm so his equations are upside down. His poles are our zeroes, his zeroes our poles.

It's not a book to go into alone. You need a guide to show you what it can do. Otherwise it will seem weird and you may not notice the valuable bits.

A small single turn loop for each of tracker and analyser is right. Ideally you want them on opposite sides of the resonator

David
==============================
Ah, I must learn to type faster and think faster a few posts happened while I was writing.

Use the one resonator at a time process. You do NOT stagger them at all. You tune them all to the geometric centre frequency. One stage at a time with the neighbours each time shorted.

When you take the shorts off, the coupling moves them all onto the right frequencies.

Staggering is automatic. it does it for you itself.

When the full filter is running you will see Chebyshev bumps in it, but if you twiddle the trimmers you'll find that they all interact. It isn't a case of trimmer X moves bump Y. You don't get anything so easy. That's why most trimming attempts of people unfamiliar with a design go astray even with wobbulators spectrum analysers etc.

Cryptography relies on mathematical one-way streets, functions which are easy to do in one direction, but much harder to do in the reverse direction. Filter twiddling is like that. You have to learn to cheat.

David
=======================================

The source and termination resistors set the Qs of the end stages of the filter, and via the coupling factors set the Qs of all the other stages.

If the resistance at either end goes high, the Qs go too high for the designed response and the ripple gets big.

Lower the resistances and the Qs fall too low for the designed shape and the filter rolls off.

If you're filter-hacking on a simulator like Spice, you can vary width by scaling the coupling factors, then fiddle with the terminating R at each end to adjust to get the flatness/ripple right.

David
===================
There's a danger of confusion here with talk of zeroes and Zverev.

Zverev drew equations of attenuation but now we tend to use equations of transfer function. These are reciprocals of each other, so no big deal. But the zeroes of transfer function are the poles of attenuation and vice versa. So Zverev and current usage have the words zreoes and poles swapped.

The tune one stage at a time method works for "All-pole" filters (poles of transfer function) realised as coupled resonators. The schematic posted earlier of Racal filters as simple L-C tanks spatially separated is such a filter... they have no notches.

This technique does NOT put the poles on the required frequencies. It puts them all on the same frequency. Then it relies on the coupling factors to pull the poles apart to shift them onto the required frequencies. It is rather neat that the same coupling factor does the right frequency pull and sets the right Q both at the same time.

The passband pole positions show up as blunt ripples in the passband top, making them very hard to adjust if you try this way. Better, they appear as much sharper nulls in the return loss. So a tracker and return loss bridge on a spectrum analyser can be used this way.

Filters with real-axis zeroes give stopband nulls... EG Elliptic == 'Complete Cauer' designs. these are usually designed by the transformed lowpass technique and suffer from its limitations.

I've played around with narrow bandpass filters with notches achieved by basing things on pseudo-crystals.

David "

Craig,

Thanks for your interesting reply.
The method you described shorting of the 40Mhz resonators in turn surprises
me that it is reported to work, but admit I fail to understand it at all.
If it does work - great !.. a new relatively simple new method to try.
They are relatively closely coupled however, and the image impedances
looking in and out at each resonator stage with the close coupling would be
badly affected if shorted I would have thought - which in turn would badly
affect the bandwidth shape I would expect.
It is true the two end resonators are the most sensitive to bandwidth shape
changing characteristics when adjusted, which is another indication of the
importance and sensitivity of the input and output impedence seen at the
filter by the pre and post circuitry. Interesting comments though... thank
you for that.

I was always impressed at how acutely sensitive is the 40Mhz BPF for
alignment, especialy so are the caps when adjusted - for obtaining the
accuracy needed for a good passband shape. So much so that just removing
the cover and replacing it will change the passband shape - sometimes
drastically.

Douglas Denny G3ZQE

On Mon, 7 Oct 2024, 16:41 Craig Sawyers via groups.io, <c.sawyers=
[email protected]> wrote:

I'm lucky enough to have a R&S Swob II that I bought from Rob Filby
donkeys years ago (fully functional with all the probes) and a CT501
that I bought for a bewilderingly low ?40 maybe 10 years ago - clearly
no one knew what it was, and I was the only bid on eBay.

BTW, although I have never tried it, apparently a sneaky trick with the
40MHz filter is this. Short circuit every resonator except one. Very
loosely couple a SA to the remaining resonator, and set that to to
40MHz. Do that for each resonator in turn. Every one in isolation set to
40MHz. When all the shorts are taken off, the resonator couplings pull
each other to precisely the right frequencies to give the required
bandshape. Apparently the only ones that need care are the two end
resonators, because they have to be loaded correctly.

This is from a guy whose career was designing RF stuff (like spectrum
analysers) for HP. He tends to know every trick in the book regarding RF
filter design and alignment. He is also an RA17 fan.

Yes the TF1186 was a strange and wonderful thing; I suspect that not
many were made.

I've just hauled my Tek 7L5 SA out; It is a while since I used it, and
quite forgot that it has a tracking generator option. 10Hz to 5MHz, with
a resolution bandwidth down to 10Hz. That I believe is more is more than
capable of aligning the IF chain filters ;-)

Incidentally Douglas, I was just reading an old post of yours describing
the order of bandwidth filter alignment - IOW start with the 100Hz and
work up in frequency.

Craig


------ Original Message ------
From "Douglas Denny" <ddennyoptom@...>
To [email protected]
Date 07/10/2024 13:16:52
Subject Re: [RacalRA17forum] Marconi TF1168

I still have two TF114H sig gens stored somewhere in the attic, and have
aligned RA117s with them in the 1970s (along with an early wobbulator for
the 40MHz bpf).
They are a beautiful example of precision engineering in test equipment of
the time; if left on continuously were rock stable and very low jitter and
spectral purity compared to some modern sythesised equipments. I think one
amateur professed to use one as a vfo for vhf transmitter on 2 metres
around that era.
I have no knowledge about the TF1168.

Douglas Denny. G3ZQE.



On Mon, 7 Oct 2024, 11:58 Craig Sawyers via groups.io, <c.sawyers=
[email protected]> wrote:

This is pretty much a historical curiosity. This was the "book" method
of aligning the crystal and other bandpass filters on the IF strip, and
IF transformers in the RA17 series of receivers.

Basically described as a "High Discrimination Oscillator", it was
precision set to 100kHz against an internal crystal calibrator, and had
three large tuning knobs covering 10kHz, 1kHz and 100Hz total range,
selected by a switch.

Nothing much on the web about this boatanchor (a hefty 40kg/88lbs), no
manuals anywhere. There are two references - one an old Marconi
catalogue that describes its characteristics, and a similarly old
Marconi price list that says it was ?328 in 1961 (around ?10k in

today's

money). However, as an example of the ravages of time and value, the
TF144 signal generator was even more expensive - but last year I was
given two. Both of which work bang on spec BTW, half a century on.

Of course now a spectrum analyser is the way to go. However, for
curiosity's sake more than anything, does anyone know anything about
this Marconi TF1168 beast?

Craig

I tried that method as most of the caps had to be replaced.. Apparently only the two adjacent coils need be shorted.I did discover upon sweeping that the center was a bit off, so I redid it with the correct offset and that worked.

I also tried, carefully, tweeking it using a noise generator and an SDR waterfall. I was unable to improve it except for small tweeks of the two end parts. This works for both big filters.

Doug

________________________________
From: [email protected] <[email protected]> on behalf of Craig Sawyers <c.sawyers@...>
Sent: Monday, October 7, 2024 10:41 AM
To: [email protected] <[email protected]>
Subject: Re: [RacalRA17forum] Marconi TF1168

I'm lucky enough to have a R&S Swob II that I bought from Rob Filby
donkeys years ago (fully functional with all the probes) and a CT501
that I bought for a bewilderingly low ?40 maybe 10 years ago - clearly
no one knew what it was, and I was the only bid on eBay.

BTW, although I have never tried it, apparently a sneaky trick with the
40MHz filter is this. Short circuit every resonator except one. Very
loosely couple a SA to the remaining resonator, and set that to to
40MHz. Do that for each resonator in turn. Every one in isolation set to
40MHz. When all the shorts are taken off, the resonator couplings pull
each other to precisely the right frequencies to give the required
bandshape. Apparently the only ones that need care are the two end
resonators, because they have to be loaded correctly.

This is from a guy whose career was designing RF stuff (like spectrum
analysers) for HP. He tends to know every trick in the book regarding RF
filter design and alignment. He is also an RA17 fan.

Yes the TF1186 was a strange and wonderful thing; I suspect that not
many were made.

I've just hauled my Tek 7L5 SA out; It is a while since I used it, and
quite forgot that it has a tracking generator option. 10Hz to 5MHz, with
a resolution bandwidth down to 10Hz. That I believe is more is more than
capable of aligning the IF chain filters ;-)

Incidentally Douglas, I was just reading an old post of yours describing
the order of bandwidth filter alignment - IOW start with the 100Hz and
work up in frequency.

Craig


------ Original Message ------
From "Douglas Denny" <ddennyoptom@...>
To [email protected]
Date 07/10/2024 13:16:52
Subject Re: [RacalRA17forum] Marconi TF1168

I still have two TF114H sig gens stored somewhere in the attic, and have
aligned RA117s with them in the 1970s (along with an early wobbulator for
the 40MHz bpf).
They are a beautiful example of precision engineering in test equipment of
the time; if left on continuously were rock stable and very low jitter and
spectral purity compared to some modern sythesised equipments. I think one
amateur professed to use one as a vfo for vhf transmitter on 2 metres
around that era.
I have no knowledge about the TF1168.

Douglas Denny. G3ZQE.



On Mon, 7 Oct 2024, 11:58 Craig Sawyers via groups.io, <c.sawyers=
[email protected]> wrote:

This is pretty much a historical curiosity. This was the "book" method
of aligning the crystal and other bandpass filters on the IF strip, and
IF transformers in the RA17 series of receivers.

Basically described as a "High Discrimination Oscillator", it was
precision set to 100kHz against an internal crystal calibrator, and had
three large tuning knobs covering 10kHz, 1kHz and 100Hz total range,
selected by a switch.

Nothing much on the web about this boatanchor (a hefty 40kg/88lbs), no
manuals anywhere. There are two references - one an old Marconi
catalogue that describes its characteristics, and a similarly old
Marconi price list that says it was ?328 in 1961 (around ?10k in today's
money). However, as an example of the ravages of time and value, the
TF144 signal generator was even more expensive - but last year I was
given two. Both of which work bang on spec BTW, half a century on.

Of course now a spectrum analyser is the way to go. However, for
curiosity's sake more than anything, does anyone know anything about
this Marconi TF1168 beast?

Craig

I'm lucky enough to have a R&S Swob II that I bought from Rob Filby donkeys years ago (fully functional with all the probes) and a CT501 that I bought for a bewilderingly low ?40 maybe 10 years ago - clearly no one knew what it was, and I was the only bid on eBay.

BTW, although I have never tried it, apparently a sneaky trick with the 40MHz filter is this. Short circuit every resonator except one. Very loosely couple a SA to the remaining resonator, and set that to to 40MHz. Do that for each resonator in turn. Every one in isolation set to 40MHz. When all the shorts are taken off, the resonator couplings pull each other to precisely the right frequencies to give the required bandshape. Apparently the only ones that need care are the two end resonators, because they have to be loaded correctly.

This is from a guy whose career was designing RF stuff (like spectrum analysers) for HP. He tends to know every trick in the book regarding RF filter design and alignment. He is also an RA17 fan.

Yes the TF1186 was a strange and wonderful thing; I suspect that not many were made.

I've just hauled my Tek 7L5 SA out; It is a while since I used it, and quite forgot that it has a tracking generator option. 10Hz to 5MHz, with a resolution bandwidth down to 10Hz. That I believe is more is more than capable of aligning the IF chain filters ;-)

Incidentally Douglas, I was just reading an old post of yours describing the order of bandwidth filter alignment - IOW start with the 100Hz and work up in frequency.

Craig