Electronics > RF, Microwave, Ham Radio

Generic VNA software for a saved file of ADC samples

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RoGeorge:
I would like to use a DDS frequency sweep (Rigol DG4000 generator) and a DSO to record the ADC samples (Rigol DS1000z oscilloscope), then to look at the saved datafile presented as it would be the display of a VNA.

Is there any FOSS software tool to use or to modify for such a setup, software that can read some recorded ADC samples and present that data with the typical GUI charts and cursors of a VNA?

joeqsmith:
With the mention of vectors, I wonder if you know what you are asking.  Assuming you have a way to get the complex data, I would just create a Touchstone file.  You can then post process it with several free tools. 

RoGeorge:
I didn't know about Touchstone, thank you for the hint about that file format!   :-+

At a brief search about Touchstone and by looking at the menu fields of the program seen in this video https://youtu.be/ehmDhLn0Bm8, the file format seems easy to implement, and there are some Touchstone viewer programs already.  Some viewers are free, and I bet some of them might be also FOSS.  Will dig more about that later.



I never had a VNA, so I might be asking for weird things.  What I have in mind is to use a dual channel DDS generator and a 4 channel digital oscilloscope to cobble up together a VNA, for hobby projects around the lab.

The generator has 2 independent channels that can be set to play identical signals, so one DDS output goes straight to one oscilloscope channel, as a phase reference.  The other DDS output goes to the DUT.  The remaining oscilloscope channels can be used to probe the DUT. (having in mind a 2 port DUT)

For example:
- one channel of the oscilloscope is sampling the generated signal (as a phase reference)
- the second channel is sampling the voltage at the input port of the DUT
- the third channel is sampling the voltage of the output port of the DUT

These 3 signals seem to me enough to calculate the instantaneous V and I at the input and output of the DUT, and from there to calculate the S parameters or to draw a Smith chart, assuming the input and output impedance seen by the DUT is 50 ohms.



Please let me know if I left out any measurement that can not be deduced from these 3 sampled waveforms:  Vref, Vin and Vout.

After data acquisition, the ADC samples of Vref, Vin and Vout taken at each frequency needs to be converted into a datapoint to be displayed on the PC, with some nice formatted plots and sliders, and Smith charts, and engineering units on axis, and zoom, and click to show the values for a selected point on a plot, etc.

My hope was to find a free GUI for displaying and handling the plots, without having to program myself all the layout and the details for each chart and text box.  Seems like that's exactly what a Touchstone Viewer software can do.

The data acquisition part had to be done anyway, and converting data to a Touchstone format seems easier than writing a custom GUI viewer.  Either way, using an established intermediary file format can only be an advantage.

rf-messkopf:

--- Quote from: RoGeorge on October 17, 2021, 06:18:43 pm ---Please let me know if I left out any measurement that can not be deduced from these 3 sampled waveforms:  Vref, Vin and Vout..

--- End quote ---

In order to determine the S-matrix you need to know more. For example, to measure \$S_{11}\$ of your device, you need the instantaneous voltage and current into port 1, together with the phase angle between them. This assumes that the other port is terminated with the system impedance. Your circuit will not be able to measure that.

If you determine these three quantities, you can work out the impedance looking into port 1, or equivalently, the reflection factor, or equivalently, the incident and reflected waves at port 1. Notice, though, that the ware quantities are a more general concept than the instantaneous voltage and current on a transmission line. They are also meaningful for waves in waveguides or free space. Only on TEM lines these concepts coincide.

A real VNA measures them by separating the incident and the reflected wave by a directional coupler. In low frequency VNAs this is usually a resistive device, somewhat similar to a Wheatstone bridge.


--- Quote from: RoGeorge on October 17, 2021, 06:18:43 pm ---My hope was to find a free GUI for displaying and handling the plots, without having to program myself all the layout and the details for each chart and text box.  Seems like that's exactly what a Touchstone Viewer software can do.

--- End quote ---

Only plotting the raw data measured with a directional device is not enough, and will be rather useless. Much of the power of a VNA is achieved though vector error correction, more commonly called calibration. Your software must be able to take care of that as well. The METAS VNA tools can do off-line calibration with Touchstone data. Also, there is the http://scikit-rf.org/ package for Python which contains methods for the most common calibration types.

joeqsmith:
AppCAD is another free viewer.  METAS is nice but will not plot Smith charts like AppCAD can.

Depending one what your DUT is, will determine what you need for a setup.  For simple S11 or shunt impedance, you need the incident, reflected and phase between them.  Normally done with a a directional coupler.

The NanoVNAs are fairly low cost.  Software to control them and collect data is available for free.

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