Electronics > RF, Microwave, Ham Radio

VNA signal source SNR importance for S-parameter measurements?

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To what extent does the SNR of a VNA signal source matter for S-parameter measurements? To make this a bit more concrete, imagine I have a signal source with an SNR of 100dB and then degrade the SNR to 80dB. Would this affect the S-parameter results and, if so, how and why? This question is purely out of curiosity.

My expectation is that this metric isn't super important. In a super-heterdyne system such as in a typical VNA, noise far from the carrier should be strongly attenuated by the bandpass IF filters and therefore shouldn't have a large effect on the measured response. On the other hand, I would expect close-in phase noise to have a measurable effect since this could pass through the filters and effectively reduce the frequency selectivity of the measurement. Is this logic correct? Anything I'm not considering?

Your intuition is likely correct but the calculations get hairy.  S parameter measurements have all kinds of interacting error sources and uncertainties.  The biggest error source is the return loss of your test set and directional couplers.  A good test set might have a return loss of 60 db which sets a fundamental limit on everything else.  Google "mismatch uncertainty" or get a good microwave reference text- lots has been written about the topic.  Be prepared to do a lot of algebra with equations with upside L's in the- Gamma, a small joke.  HP used to have nomograms and little slide rules that would calculate this kind of stuff.  I think the difference between 80 and 100 dBc will be well below the other errors and not a real concern.

What comes to mind is situations where you're trying to measure items that are far away from 50 ohms, such as the impedance of an end-fed halfwave antenna where the resistive part is thousands of ohms. Then the VNA is trying to measure the slight difference from a perfect reflection.  Then your calibration method and your noise levels all have to be very good.

True- VNA/Reflection measurements have limits imposed by the coupler and the sensitivity and noise.  They aren't great for SWR's much over 10.  There are other techniques like RF I/V and others.  HP has a kind of extended app note about measuring impedance- it might be called a handbook.  Its basically a sales tool that goes through all the options pretty well.

Keysight has an app note called "Ultra-Low Impedance Measurements Using 2-Port Measurements" that discusses accurate VNA measurements of very low impedances, which relates to the point you're making. The limiting factor here though is receiver dynamic range (measurement down to 0.1ohm requires about 100dB dynamic range), not source noise.


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