How to build a do-it-yourself vector-network analyzer that’s actually useful

[Mechatrommer]

They use some 'tricks' only the ultimate creators know...

[xaxaxa]

There is a guy,which told me something like this : "to make an VNA you need another VNA ......."
At this point if this is true or was,then how the first VNA was born ?
LOL

I don't think that's really true; you can easily construct 1-port or 2-port devices with known characteristics, for example shorting out a board mount sma connector with a big blob of solder will give you almost ideal short up to 4GHz, and soldering 2 0603 100ohm resistors to the connector (after grinding off the pins) will get you a decent 50ohm reference with return loss >30dB. I didn't have access to a "real" VNA when I built mine.

[Mechatrommer]

for example shorting out a board mount sma connector with a big blob of solder will give you almost ideal short up to 4GHz, and soldering 2 0603 100ohm resistors to the connector (after grinding off the pins) will get you a decent 50ohm reference with return loss >30dB.

and how do you come to know all that? see below...

I didn't have access to a "real" VNA when I built mine.

then you must have the 'tricks' or at least one 'another tool', a spectrum analyzer maybe? btw, i can bode (respond) plot something out of FG and a DSO, no need an SA...

why it is said you need a VNA to build a VNA because thats the most convenient around today. its not necessary means you 'really' need the VNA first place. multi GHz VNA are in existence to build lower grade VNA (pcb characterization, sure you can do with a SA w tracking gen of higher grade), then why use archaic or physics or whatever troublesome method? do you consider a full solder blob on a SMA usefull or practical? good luck building a VNA pcb empty handed from scratch...

[hendorog]

It is useful because it is one of the three standards you need. The other two are an open and a load.
With those three characterised standards then you can build a vna.
With physics you can characterise three standards from first principles.
So yes, you can build a vna without another vna.

[Mechatrommer]

if first principles and SOL is all that required, the guy who made the diy VNA should already come out with something usefull by now on the pcb version 1. then again, how do you get a good SOL kit?

[TheUnnamedNewbie]

You don't need SO(T)L to do the calibration. In fact, SOTL is not really a good calibration method outside of lower-frequency (few GHz) applications. We almost always calibrate our systems with TRL, as this does not require perfectly known standards. (Don't ask me about the math though, as I'm not that familiar with it either). The issue with TRL at lower frequencies is that the line becomes large.

Right now, from what I have been told, the difficult part making a high-end VNA is not getting the performance in the first place - it is stability and speed. Keeping that performance right there within a fraction of a dB for many hours or days, and allowing fast measurements to be done to better characterize the system. And don't think this is just something needed for production test - colleagues of mine working on microwave for biomedical and spectroscopy applications sometimes do measurements that take 72 hours (as they need to determine how a parameter changes as a yeast culture grows, for example). As such, they need that stability over time to ensure that they are not measuring the VNA drift but actually measuring the DUT.

The beauty of a VNA calibration is that (to some extent at least) the performance of the VNAs components doesn't matter that much - you calibrate that out anyways. Of course, you run into limits of dynamic range and return loss - if your output connector on the VNA has an S11 of -15 dB, good luck measuring the S11 of a device that should have extremely low return loss...

[hendorog]

You don't need SO(T)L to do the calibration. In fact, SOTL is not really a good calibration method outside of lower-frequency (few GHz) applications. We almost always calibrate our systems with TRL, as this does not require perfectly known standards. (Don't ask me about the math though, as I'm not that familiar with it either). The issue with TRL at lower frequencies is that the line becomes large.

Right now, from what I have been told, the difficult part making a high-end VNA is not getting the performance in the first place - it is stability and speed. Keeping that performance right there within a fraction of a dB for many hours or days, and allowing fast measurements to be done to better characterize the system. And don't think this is just something needed for production test - colleagues of mine working on microwave for biomedical and spectroscopy applications sometimes do measurements that take 72 hours (as they need to determine how a parameter changes as a yeast culture grows, for example). As such, they need that stability over time to ensure that they are not measuring the VNA drift but actually measuring the DUT.

The beauty of a VNA calibration is that (to some extent at least) the performance of the VNAs components doesn't matter that much - you calibrate that out anyways. Of course, you run into limits of dynamic range and return loss - if your output connector on the VNA has an S11 of -15 dB, good luck measuring the S11 of a device that should have extremely low return loss...

I agree completely.

The point being that the VNA doesn't define any standards, it just needs to be stable enough that you can compare your reference standards - whatever they may be - with the thing you want to measure. There is no absolute accuracy in a VNA. This is different to a spectrum analyser.

This is why it I think is theoretically easier to build a VNA from scratch, than build a Spectrum Analyser.
The SA needs to be calibrated so that it knows where 0 dBm is with high accuracy.
The VNA just needs to be able to compare two things with high accuracy.

[Free_WiFi]

I want to make this question.

Is it possible to make an VNA from zero by having only a good Scope and wide knowledge of advanced math?

[tautech]

Swatting up some for the new Siglent SVA and a couple of docs that offer some VNA understanding:
https://www.keysight.com/upload/cmc_upload/All/BTB_Network_2005-1.pdf
http://download.ni.com/evaluation/rf/Introduction_to_Network_Analyzer_Measurements.pdf