Network analyzer vs Spectrum analyzer

[questionner]

I'm currently studying the network analyzer(NA) and spectrum analyzer(SA), and one thing makes me wonder;

Would it be possible to 'mimick' a NA with a SA and a signal generator(SG)?

Ofcourse some phase calculations may also be required, but from my naive understanding, if a NA sends various frequencies of signal and sees the amplitude of the incoming signal, similar situation can be simulated using a SA and a SG; we may vary the input signal with SG and record the amplitudes of the received signal with SA.

The latter is ofcourse an inefficient way but.. I have understood the mechanism here, right..? Say if one doesn't have a NA but instead have SA and SG, then he can naively mimick the role of NA with SA and SG?

 

[berkeman]

One version of what you are describing is available on some spectrum analyzers that have a "tracking generator" output. Can you find some examples of those?

 

[the_emi_guy]

Adding to berkeman, spectrum analyzer + tracking generator provides a "scalar" network analyzer. Only amplitude response is measured.
You mentioned "Of course some phase calculations may also be required", in fact, no phase information can be obtained, so "vector" (phase and amplitude) measurements are not possible.

Additionally, network analyzers often include directional couplers that allow measurement when stimulus and response are on the same cable (return loss measurements). SA can't do that without external directional coupler.

 

[tech99]

Yes, I have a miniature antenna analyser, Youkits FG7, which does exactly this and displays VSWR and modulus of impedance on a small screen.
I find that in antenna engineering the VSWR/frequency plot is frequently the best measurement to make. It must have some form of directional coupler circuit in it, possibly a resistance bridge.

 

[sophiecentaur]

Rhodes und Schwarz made a ‘sweeper’ which worked for UHF, a few decades ago (SDR?). Very handy and quick; all you needed was a length of good cable and some calibrating mis-terminations. Nothing fancy no sampling - just an analogue display of the varying volts on the output port over the frequency sweep. Built to last too.

 

[berkeman]

Yes, I have a miniature antenna analyser, Youkits FG7, which does exactly this and displays VSWR and modulus of impedance on a small screen.
I find that in antenna engineering the VSWR/frequency plot is frequently the best measurement to make. It must have some form of directional coupler circuit in it, possibly a resistance bridge.

Yeah, VSWR meters are golden.

[thread hijack]
I was on a remote HAM assignment one time supporting a marathon run through the boonies, and set up a remote HAM radio station with a portable antenna and my vehicle. When I tried to contact Net Control, they kept replying that they could barely hear me, and there must be some problem with my radio. It was my standard portable setup, so I had no idea what the issue was until I connected my VSWR meter between my radio and my antenna. Ack, terrible VSWR! It was then that I noticed that I hadn't attached my counterpoise elements to the main antenna structure -- Duh. Screwing in the three counterpoise elements cleared everything up, and we were able to support the race like we were supposed to.
[/thread hijack]

https://4.bp.blogspot.com/-gohsJymy9CY/UEY1uPes7ZI/AAAAAAAAAIw/ZNpINIqrEKE/s1600/v20004.jpg

 

[Baluncore]

Would it be possible to 'mimick' a NA with a SA and a signal generator(SG)?

First, welcome to PF.

Unfortunately it would not be possible. They are similar but quite different swept frequency instruments.

The NA has a swept reference input signal that is used to resolve the phase shift of the output as separate sine and cosine terms. The SA measures power, but has no ability to resolve phase.
The SA has a variable resolution bandwidth, usually from tens of Hz to tens of MHz. On the other hand, the NA has zero bandwidth by virtue of it's synchronous detector.
Each instrument has it's specific applications.

As analog to digital converters have become faster, with better quadrature mixers, and FFT algorithms, more can be done by the same hardware. The advent of SDR makes it possible to emulate a SA or a NA in the same hardware. They remain as separate swept frequency instruments, the SA resolving in-band power, without phase, while the NA resolves the phase and amplitude of a transfer function.