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Tony, I was looking at the TT1-6 which is also a 1:1 and it’s listed as 4kHz low frequency limit. But I agree with you below this would be difficult on the transformers. I do think capacitance coupling would work, but see my previous reply about the leakage issues that I think is more disturbing.
Brian

Gary, thanks for all the great feedback. I completely agree that your sound card approach is the cleanest solution if if want to do pure audio responses. In fact I used your description of 2 scope probes on the source injected signal and saw the approximate phase shift and gain on my oscilloscope. Since I am using a nanovna for higher frequency work I figured I would modify the input transformers and the bridge circuit to be more inline with lower frequency operation and higher impedances. Basically exactly what you do with lab view and a sound card, but I would use the n2pk with myVNA software to provide a generic tool for any tests between 4khz and a couple of MHz. I think if I was only doing audio then your method has the highest accuracy.

The good news is the mc1496 detector can run at lower lo frequencies and assume I can couple in without the transformers. Yes this limits my high frequency dynamic range, but l’m giving this up.

So based on all this good feedback, I still have a couple of questions.

1) without transformers and capacitive coupling or at least the 4kHz TT1-6 parts, I should be able to resolve some of the dynamic range issues at low frequency. However my newest concern (from Gary’s linked document) is that the LO signal leakage would be too large to resolve the dc output on the a/d converter because the 10k/.1uF anti-alias filters would just start rolling off at 1kHz. Completely destroying the desired dc output.

I’m not a mc1496 expert, and it might be possible to cancel the LO leakage with a potentiometer adjusted LO signal. hopefully some previous expert on the mc1496 can tell me if this is possible.

Brian

It could be possible to replace coupled transformers, but I doubt, that measurements can be done below 5kHz and also there is a calibration issue as well.
Using different transformers means also circuit tweaking to maintain input/output impedance of transformer, since they have impedance ratio of 1:1, but
it could be promising to extend lower frequency limit.

On Mon, Feb 15, 2021 at 09:12 AM, Brian k wrote:

Has anyone previously implemented (using myVNA) an active loop injection
measurements as described in the above link?

Yes, during development of an audio amplifier.

The VNA output was coupled to the summing node of the differential input stage, with suitable attenuation. The method described by Ridley is effectively the same thing. The key is to maintain normal closed-loop operation especially at low frequencies where loop gain is extremely high. Some math is required to extract the open-loop transfer function. The reference for that is included on a graph on the above webpage.

Gary NA6O

The main limitation with the N2PK VNA is that the inputs are transformer-coupled and response drops rapidly below the 10s of kHz, along with increasing distortion. Also the DDS outputs start rolling off around 3 kHz. The applications that I'm aware of don't have LF limits (MyVNA or the DG8SAQ VNWA software, which also supports N2PK, nor my own LabVIEW-based app). I did in fact get the think working down to 20 KHz for some audio testing but found that calibration was relatively unstable. By the way, the VNWA is not transformer-coupled and is rated to operate down to 1 kHz. The other hassle is that the VNA is designed around a 50-ohm environment, whereas most low-frequency audio and control systems are based on much higher impedances.

And yes your WB6DHW USB board will work...

There was an article in QEX about building a low-frequency adapter for VNAs (you need to be an ARRL member to download this):


What you really want for controls work is a dynamic signal analyzer (DSA). That typically would be an expensive commercial instrument from Keysight, B&K, SRS, etc. A basic DSA has an output (excitation) and two inputs. One input monitors the excitation signal, the other the response of the DUT. In the frequency domain, the ratio of output to input is the complex transfer function (magnitude and phase), and that will give you just about all you need to know for classical control system design.

A DSA can also be implemented on the cheap with a good sound card and the right software. Being a LabVIEW guy, I use the fact that sound I/O and DSA functionality are built-in (see Dual Channel Spectral Measurement). And there's now a free community version of LabVIEW :-) You could also acquire sound data using any app that saves standard audio files, and then use something like MatLab or Octave to do the math.

-Gary NA6O

After looking though the myVNA user documentation from Dave (g8kbb) it appears that he has implemented every possible option using the myVNA program. I see concurrent dual detector operation and which uses the 12 term correction. So the two detectors must be simultaneously. in addition the program appears to have programmable default limits of 0 Hz. So my new questions. (Note: I have not tried anything yet)

Has anyone previously implemented (using myVNA) an active loop injection measurements as described in the above link?

Has anyone modified the output transformers T4_6T or T1-6T for lower frequency operation? Mini Circuits has TT1-6 which appears to have decent performance down to 4kHz vs 15kHz?

Brian

Thanks Toni for the feedback. Did VNA4win allow you to enter a frequency lower than 50kHz? I understand the vna may not have performed well, but was the software a limitation? Also does it do simultaneous 2 detectors readings in one sweep. I would load the software and check it out, but I need to build the usb interface first.
Brian

I am also very interesting to do some HW modification to measure at low frequencies. I have no success so I was forced to use commercial instrument for this task.
I also searched for any new updated software running on win10. VNA4win is a good software, but I retired my old PC and on new one I constantly have a problems with it.

Thanks for all the help
Toni

Hopefully one of the experts can help me decide if I should dig farther. I built a dual (2 detector) version the the LTC2440 (faster) ADC converters at least a dozen years ago. I'm not really sure about the years because they just fly by. Well the VNA has been great for me for a number of projects (thanks Paul and others) and I have kept it alive my keeping an old win 2000 machine with the parallel port. In the last year with the combination of the more convenient nanoVNA and keeping a parallel port machine alive, I have retired my N2VNA to the ex-project pile.

Recently I wanted to make a some low frequency power supply and PID control loop measurements and thought that the N2PK VNA could be adopted for this purpose. With the non-integrated source and detector ports (2) (external bridge) , I thought I could modify the bridge and make a specialized bridge for these measurements. As a reference you can look at the RIdleyEngineering software and injection measurements he makes with his low frequency analyzer (AP310). In order to make these measurements I think I would need to make the following adjustments, hopefully someone can provide me some feedback if this is possible. I've been using exciter which can measure both S21 and S11 but not simultaneously. In addition I still use the parallel port, but I purchased the USB adapter years ago but never completed the build.

My question are:
1) Which of the more modern software interfaces supports measuring both S21 and S11 at the same time. This would be required to perform injection measurements.

2) Does any of the software support sweeping below 50kHz. This is the exciter low freq limitation and also the DDS transformer limit. I don't mind changing the hardware to make this a low freq only analyzer

3) Could I use my unfinished WB6DHW HSUSB_R1 interface to support the correct software?

4) Are there better/easier ways to make these low freq phase and amplitude measurements (100Hz - 2MHz) than the above concept.

Thanks in advance for any suggestions
Brian

Right you are and thanks for getting me un-stuck.
My mistake was concentrating on the Display Options tab.

Thanks,

Chuck

G'day Chuck.

You probably have the Lock boxes ticked.



I've been caught by this on a few occasions.

Kerry VK2TIL.

Group -

For many many years my N2PK/MyVNA combo has been great.
Recently I have gotten it into a state and not found an exit.

With a short cable from RF Out to Detector 1 and the M2PK ion transmission mode, the sweep looks perfect: flat with loss < .01 dB.

With no cable, the trace is very noisy and loss is just a little more than .01 dB.
The trace looks like the trace normally seen with no input, which normally is -85 to -90 dB.

So the trace seems to be correct but the vertical axis labels were not updated. I tried clicking on AutoScale many times, restting the N2PK, and restarting MyVNA.
I find nothing in Display Options nor in the window that appears after left clicking on the plot.

So what have I done?

Chuck

Please take a look at the files section for this group - I did my best to capture all of the content about one year ago; of course there has been message activity since, so this is a little dated. As others have posted, there are other archives as well.:

archive zips from N2PK-VNA at yahoogroups.com 4 files Update Move Delete Report File
archive zips from N2PK-VNA at yahoogroups.com Folder - 10/25/19
N2PKwebsite.zip Update Move Delete Report File
N2PK web site archive captured January 1, 2020 from URL: File David Feldman 33 MB Jan 1

To my knowledge Ivan Makarov has captured all too
Eric

I did an archive of the entire group on Oct. 26 of last year. Messages, files, photos, the whole enchilada captured with PGO
The message file is about 100 Mb and can be opened with PGO or SQLlite, PGO offers a host of options for searching and organizing the archive.

Ron K0IDT

Hello,

Has someone captured all of the messages from N2PK yahoo group?

Thanks,

Dave - wb9own

Hi all
in myVNA the calibration standards are defined in terms of fringe capacitance, resistance, inductance ( load )etc etc.
I purchase a calibration KIT with parts from Rosemberger , they define the standards in term of delay from the reference plan .
How is possible to transform the value parameters between the two definitions ?
The standard by default in myVNA are referred to which calibration KIT ?
Thanks to all
Regards
IZ2ZNC Gianfranco

As WFH and sorting out stuff, I have also managed to go through some of the older projects I needed to complete from 2006 and found I have a V4.1 dual detector N2PK and an original N2PK to make up. Got the dicast enclosures as well as all the parts (phew). Lacking PSU and sma/Ntype chassis connectors though.

Seem to have a USB to LPT control project as well for the N2PK control based on a Cyrus chipset?

Hope to pop back as I melt solder on these and to find more destructions and ideas. I see the N2PK site was archived so grabbed copies of that too now.

73

Dom
M1KTA

Hi Marc,

Thanks for the information! Now I understand how I should have laid it out.
For now I have added some copper shielding and the stop-band has improved to about 60 dB, which is still not great, but will do for now. I'm only using this filter as a crude way of performing phase noise measurements (by putting the center frequency of the oscillator-under-test to sit in the stop-band of the filter, and then observe the phase noise within the pass-band - means the spectrum analyzer isn't overloaded and therefore I can use a cheap spectrum analyzer for what would normally require an expensive spectrum analyzer.
For this, 60 dB is adequate for now, but I will try the better construction soon.

Many thanks,

Shabaz.

________________________________
From: [email protected] <[email protected]> on behalf of Marc olanie <Marc.olanie@...>
Sent: 01 September 2020 12:50
To: [email protected] <[email protected]>
Subject: Re: [N2PK-VNA] Matching for a Xtal filter

Hi Shabaz

I think your bench platform need some small improvement. Your coils seems to touch the aluminum shielding, causing an important parasitic capatitor between your inductance and ground, and crosstalk could be enhanced with a small separation wall between each filter port

My "ugly style" Xtal filter DUT holder looks like this

The upper side of the xtal filter don't need to be shielded, which is not exactly the case of the "lower" side.

Trying to solve the impedance adapter in reflection mode and duplicating the same L-C- adapter on both sides (with a 50 Ohms load on the "open" side I imagine, but you didn't mention it) seems too tricky for me. Even if most xtal filter are fully symetrical. Transmission mode is probably easier.

On my "Scotty's Modular Spectrum Analyzer", several RBW filters have been bought or built, with different BW, with the same progressive methodology .
A "quick and dirty" Z adaptation, after clean calibration for the choosen span, and a full S2P recording

the simulation using the vnwa matching tool, using the S2P datas

the "real thing", using the values given by the simulation tool (if you don't intend to use the VNWA software, any simulation software able to import S2P files will be ok, QUCS for example)

(with a "last minute" narrow band filter added)

Same operation with an 8 kHz HI-Q filter : raw tuning

after tuning

a far better dynamic, far less ripple. Tuning time is approximately 5 minutes, including simulation (with an integrated S-Param testhead)

In all cases, tests and measurements have been made with my N2PK VNA in transmission mode (S12/S21), never in reflection mode.

Cheers
Marc


-----Message d'origine-----
De : [email protected] <[email protected]> De la part de Shabaz Yousaf via groups.io
Envoyé : mardi 1 septembre 2020 02:45
? : [email protected]
Objet : Re: [N2PK-VNA] Matching for a Xtal filter

I should also mention, I did all measurements with the RF-IV sensor. It seems to work so well, I didn't use a directional coupler.

Thanks,

Shabaz.


________________________________
From: [email protected] <[email protected]> on behalf of Shabaz Yousaf <shabaz_yousaf@...>
Sent: 01 September 2020 01:34
To: [email protected] <[email protected]>
Subject: Re: [N2PK-VNA] Matching for a Xtal filter

Hi Marc,

Thanks for the information, it's great to hear about the procedures used. I've not used the touchstone format yet, that's something I'm looking forward to. I didn't realize VNWA had a matching tool to solve this.
I guess I just needed to persevere, I spent hours today on it, and finally have something I'm happy with for now.
Basically, after thinking about it last night, I figured I must have missed the matched spot due to too few points or something.
Also, my setup was crude. I re-did it all today, replacing the ferrite toroids with more stable T50-6 iron cores, and removing the trimmer capacitor (which was hard to trim) with fixed ceramic capacitors. Although the online calculators (based on the QMF filter datasheet value of 910 ohm//25 pF) had calculated 71 pF, I found that 100 pF worked better to shift some of the peaks more closer to the center of the Smith chart. Also, I had 23 turns of wire on the T50-6 cores for 2.4 uH, and I found I needed to squeeze them to occupy half of the toroid area, i.e. an inductance slightly higher than the calculated 2.4 uH was needed.

Now the filter result looks like this:

The passband peaks are -2.5 dB which is within spec, although the troughs are a little out of spec, but this was a filter bought from ebay so probably to be expected. The stop-band is only -50 dB, which is not good, but I'm guessing it is due to lack of shield between the input and output of the filter, although I did try to position the toroids far apart. A photograph of the construction is here:


I terminated the filter with 50 ohm and did a S11 measurement. The VSWR is 1.4 at the filter peaks, the graph is here:


The other port is only slightly worse, at VSWR of 1.6, graph here:


I placed a sketch of the design here:


Many thanks,

Shabaz.


________________________________
From: [email protected] <[email protected]> on behalf of Marc olanie <Marc.olanie@...>
Sent: 31 August 2020 09:03
To: [email protected] <[email protected]>
Subject: Re: [N2PK-VNA] Matching for a Xtal filter

Hi Shabaz

I'm a lazzy guy and definitely not a "tech pundit". But when I'm playing with xtal filters, I first run a transmission measure in both direction, store the datas in an S2P touchstone file format with at least 1000 to 2000 measurement points (due to the high Q of these devices)

Then I use the VNWA software, and more particularly it's "matching tool" to simulate the input and output impedance transformer, an eye on the ripple of the amplitude trace, an eye on the C// value.
It's a pure "visual" tuning approach and the final "real life result" is identical to the simulated Z match.


VNWA is fully compatible with the N2PK VNA and could be downloaded from the following url

I indifferently use vnwa and myvna, depending on the abilities and qualities of each software.

Cheers
Marc f6itu

-----Message d'origine-----
De : [email protected] <[email protected]> De la part de Shabaz Yousaf via groups.io Envoyé : lundi 31 ao?t 2020 05:20 ? : [email protected] Objet : [N2PK-VNA] Matching for a Xtal filter

Hi Everyone,

I hope you're well. Although I have some RF background, I don't do this for a day job, I'm very much a beginner so apologies if I'm asking something basic.

I've never had to match such a filter before, but long story short, I have a 8-pole filter model "QMF 10725" which according to the PDF datasheet<> looks like 910 ohm in parallel with 25 pF.
I wish to match to 50 ohms input and output.

According to calculators, that means that I can use a series capacitor of 72 pF and a parallel inductor of 2.4 uH.
I didn't get good results when I tried it (not sure how precisely I need to trim the capacitance and inductance - I would say I had the inductance and capacitance to within a few percent since I measured them at 10.7 MHz).
With the matching circuit attached to the QMF filter, on a Smith Chart I could not get the 10.7MHz point anywhere near the centre, I was just circling not far from the perimeter!

So, I decided to measure the filter on its own (i.e. without any matching) using the RF-IV sensor, to confirm if it is 910 ohm//25pF or not, and I see Rp and Xp as shown in this screenshot:


I'm fairly confident I'm using the RF-IV sensor and MyVNA correctly (I tested with known resistances to confirm the OSL calibration), so I have confidence in the measurements.

As can be seen, in the filter passband, Rp varies considerably. Xp also varies a lot, in some places inductive, and some capacitive.
How can I match it do you think? Should a series C and parallel L be fine, and if so, what values would you suggest?

The CSV file of the data is here:


The Extended CSV is here:


Many thanks,

Shabaz.

Hi Shabaz

I think your bench platform need some small improvement. Your coils seems to touch the aluminum shielding, causing an important parasitic capatitor between your inductance and ground, and crosstalk could be enhanced with a small separation wall between each filter port

My "ugly style" Xtal filter DUT holder looks like this

The upper side of the xtal filter don't need to be shielded, which is not exactly the case of the "lower" side.

Trying to solve the impedance adapter in reflection mode and duplicating the same L-C- adapter on both sides (with a 50 Ohms load on the "open" side I imagine, but you didn't mention it) seems too tricky for me. Even if most xtal filter are fully symetrical. Transmission mode is probably easier.

On my "Scotty's Modular Spectrum Analyzer", several RBW filters have been bought or built, with different BW, with the same progressive methodology .
A "quick and dirty" Z adaptation, after clean calibration for the choosen span, and a full S2P recording

the simulation using the vnwa matching tool, using the S2P datas

the "real thing", using the values given by the simulation tool (if you don't intend to use the VNWA software, any simulation software able to import S2P files will be ok, QUCS for example)

(with a "last minute" narrow band filter added)

Same operation with an 8 kHz HI-Q filter : raw tuning

after tuning

a far better dynamic, far less ripple. Tuning time is approximately 5 minutes, including simulation (with an integrated S-Param testhead)

In all cases, tests and measurements have been made with my N2PK VNA in transmission mode (S12/S21), never in reflection mode.

Cheers
Marc