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Hi guys, just happened upon this thread. Not sure if its applicable, but if you're measuring a scope probe it maybe more complex than you think (10x probe). See the attached patent on the scope probe. The cable from the tip to the instrument (scope) is actually a distributed resistor. If you have a cheap (or bad one) available you should tear it apart. Quite interesting.

Allen Hill
KI4QCK

On Nov 8, 2023, at 5:24 PM, Richard Jamsek <K8cyk56@...> wrote:

?File won't download. Flaged as corrupted.

Richard K8CYK

On Wed, Nov 8, 2023, 11:30 AM Allen Hill via groups.io <Allenanalog1=
[email protected]> wrote:

Hi guys, just happened upon this thread. Not sure if its applicable, but
if you're measuring a scope probe it maybe more complex than you think (10x
probe). See the attached patent on the scope probe. The cable from the tip
to the instrument (scope) is actually a distributed resistor. If you have a
cheap (or bad one) available you should tear it apart. Quite interesting.

Allen Hill
KI4QCK

On Nov 8, 2023, at 5:24 PM, Richard Jamsek <K8cyk56@...> wrote:

?File won't download. Flaged as corrupted.

Richard K8CYK

On Wed, Nov 8, 2023, 11:30 AM Allen Hill via groups.io <Allenanalog1=
[email protected]> wrote:

Hi guys, just happened upon this thread. Not sure if its applicable, but
if you're measuring a scope probe it maybe more complex than you think (10x
probe). See the attached patent on the scope probe. The cable from the tip
to the instrument (scope) is actually a distributed resistor. If you have a
cheap (or bad one) available you should tear it apart. Quite interesting.

Allen Hill
KI4QCK

1) Be sure your coax is less than 1/4-wavelength long in the coax
(considering Vp). Conversely, be sure your measurement frequency of
measurement is less than the electrical length of the cable.
2) Open terminate the far end of the cable.
3) On the Smith chart more-or-less in the center of the locus, note the
capacitance.
4) Short terminate the far end of the cable.
Do not change frequency or move your measurement frequency.
5) On the Smith Chart note the inductance.
6) Calculate the impedance using:
Zo = SQRT [ L / C ]
Dave - W?LEV
On Tue, Nov 7, 2023 at 8:58?PM Russell C. Nixon <rcnixon@...>
wrote:

Can someone point me to a reference in the Wiki for screen scaling? I’m
trying w1aew’s method for determining the characteristic impedance of an
unknown piece of coax as an exercise. I’m getting close but the Smith chart
trace is a tight little knot in the center. W1aew’s video shows a nice
“ram’s horn” trace on the Smith chart. I think the scaling will be useful
in other areas too. Thanks in advance.

-----Message d'origine-----

1) Be sure your coax is less than 1/4-wavelength long in the coax
(considering Vp). Conversely, be sure your measurement frequency of
measurement is less than the electrical length of the cable.
2) Open terminate the far end of the cable.
3) On the Smith chart more-or-less in the center of the locus, note the
capacitance.
4) Short terminate the far end of the cable.
Do not change frequency or move your measurement frequency.
5) On the Smith Chart note the inductance.
6) Calculate the impedance using:
Zo = SQRT [ L / C ]
Dave - W?LEV
On Tue, Nov 7, 2023 at 8:58?PM Russell C. Nixon <rcnixon@...>
wrote:

Can someone point me to a reference in the Wiki for screen scaling? I’m
trying w1aew’s method for determining the characteristic impedance of an
unknown piece of coax as an exercise. I’m getting close but the Smith chart
trace is a tight little knot in the center. W1aew’s video shows a nice
“ram’s horn” trace on the Smith chart. I think the scaling will be useful
in other areas too. Thanks in advance.

Can someone point me to a reference in the Wiki for screen scaling? I’m
trying w1aew’s method for determining the characteristic impedance of an
unknown piece of coax as an exercise. I’m getting close but the Smith
chart trace is a tight little knot in the center. W1aew’s video shows a
nice “ram’s horn” trace on the Smith chart. I think the scaling will be
useful in other areas too. Thanks in advance.

Gee, Dave . . . -632 ohms? If you parallel it with a reaonator, does it
oscillate? ;-)

I don't remember the 10430A exactly, and I'm too lazy to look it up, but
typically the cable is rather high impedance, so relatively low
capacitance. If it's one of the high frequency probes (e.g. 500MHz) the
compensation at the output is a bit more complicated, but lower
frequency probes are generally pretty simple.

The 1meg (rather than 10meg) input drives the cable with a lower
impedance and helps raise the frequency response. The probe end must be
(900k and a little capacitance in parallel) in series between the probe
tip and the cable. On most probes, that parallel capacitance is not
adjustable. There may also be a bit of intentional inductance in
series. And there needs to be 111k shunt to 'ground', so that and the
scope 1M parallel to 100k. To get a passive probe that works well to
~500MHz, the scope input capacitance has to match the design of the
probe, thus that 6-9pF spec for the probe.

I do have a 50 ohm probe; the kit includes a selection of coaxial series
resistors that yield higher resistance at the probe tip, but obviously
various attenuations that the user must take into account. As you've
said, the high impedance probes typically have several pF shunt
capacitance; 10pF at 200MHz is -j79.6 ohms! In other words, your "high
impedance probe" isn't exactly that at RF. My 50 ohm probe with a 10:1
450 ohm series resistor looks like 500 ohms with shunt capacitance in
the "under a pF" region, but of course that depends a lot on how you
connect it to the circuit.

I wonder if there were any scope probe articles in the Hewlett-Packard
Journal. If so, they might tell you a LOT more about scope probe design.

Cheers,
Tom
----------------
"Weird hou men maun aye be makin war insteid o
things they need." -- Tom Scott (1918-1995)

On 11/6/2023 4:37 PM, W0LEV wrote:

Not sure I follow you there Dave. I did an OSL before the measurement.

How

can any plot be beyond the perimeter given the right hand side perimeter
represents infinity?

Yes, the right hand side of the Smith Chart, especially on the central
horizontal line, represents extremely high impedance, and on the line,

high

resistance. Most o'scope probes are high-Z. 1 MegOhm with a small
capacitance. They should measure on the extreme fight side of the chart.
If you happen to have a relatively rare 50-ohm probe or using a 50-ohm
feedthrough connector, your probe will measure on the extreme right side

of

the Smith Chart.

Outside the chart simply indicates the VNA is incapable of calibrating or
measuring such a high impedance. An RF open, especially at the higher
frequencies, is far more difficult to make than a short or proper
termination.

BOTTOM LINE: Your VNA is incapable of measuring such a high impedance

(or

resistance). Even the very expensive VNAs are incapable of measuring 1
MegOhm with any accuracy.

Your o'scope probe SHOULD measure at the extreme right and "almost" on

the

central horizontal line depending on your measurement frequency. Yes,

but it isn't, it's outside the chart area, hence the question.

Your NANOVNA is incapable of calibrating and measuring such a high
impedance or resistance. I have an HP 8753C and I can assure you, even
that can not calibrate and accurately measure a 1 MegOhm carbon resistor

at

any reasonable frequency (bottom end is 300 kHz).


I wouldn't expect the nano to measure 1Meg R accurately. That's well

beyond

it's 50 Ohm reference. It's the reactive component I'm interested in and
that should be within range.

The reactive component should be somewhere between 5 and 40 pF. It is
adjustable and is there to compensate for the lead length between the
o'scope and the business end of the probe. I'd suggest attempting to
measure at a very low frequency like 500 kHz or 1 MHz.

Interesting: I just picked an HP 10430A 10:1 o'scope probe. On the
o'scope end of the probe, it is noted as 1 Meg, 6 to 9 pF input. I
calibrated my 8753C between 300 kHz and 30 MHz and made an S11

measurement

(actually Z11). At 300 kHz, it measures in excess of 5 kohms which is
unstable due to noise in the measurement and 4 pF. However, at 30 MHz

it's

in the 100's of ohms, but similar reactance for the frequency. I'm

puzzled.

In measuring a 4.7 kohm carbon resistor, the 300 kHz reading is nuts on,
but at 30 MHz, it's in the -632 ohms OUTSIDE the perimeter. Now I've got
to think about how these probes are designed. Mystery?

Dave - W?LEV

On Mon, Nov 6, 2023 at 9:52?PM KillingTime <bbdowns@...> wrote:

On Mon, Nov 6, 2023 at 08:55 PM, W0LEV wrote:

Of course the Smith Chart plot is outside the perimeter! A proper OSL
should show your o'scope probe right on the perimeter.

Not sure I follow you there Dave. I did an OSL before the measurement.

How

can any plot be beyond the perimeter given the right hand side perimeter
represents infinity?

Your o'scope probe SHOULD measure at the extreme right and "almost" on

the

central horizontal line depending on your measurement frequency.

Yes, but it isn't, it's outside the chart area, hence the question.

I wouldn't expect the nano to measure 1Meg R accurately. That's well
beyond it's 50 Ohm reference. It's the reactive component I'm

interested in

and that should be within range.