Help characterizing an Advantest R3361B Spectrum Analyzer Input with miniVNA.

[egirland]

Characterizing an Advantest R3361B Spectrum Analyzer Input with  miniVNA in the 1-180MHz range.

As per my knowledge of VNAs this should be a very easy REFLECTION measurement.
According to the results I get, ...it is not.

I know: the setup itself is questionable due to the differences between Golia (the R3361B) and David (the miniVNA). But this is what I have in hand and it is an experiment done for the purpose of learning something, not to make actual measurements.

Equipment:
    • ADVANTEST R3361B Spectrum Analyser;
    • vanilla miniVNA (no Pro, no lite, no v2, ...nothing);
    • VNA-J software versione 3.4.8;
    • BNC adaptors from China (!);
All calibrations are made for an OPEN over a 1-180MHz range with 6k points.

To make a prove of work I first calibrated the miniVNA for REFLECTION with a M2M BNC adapter followed by a F2F adapter. The calibration plan is expected to be located at the free side of that last adapter (far left end in the picture):


Connecting a 50 Ohms load ….


I’ve got exactly what I expected:


What is worth noting here is the constant behavior over the entire range (forgetting the spikes at the far ends of the range).

I then replaced the F2F adapter with a BNC2F_M adapter and re-calibrated the VNA:


then I connected the whole thing to the SA:


and here are the plots with different ATTENUATOR values:

SA input ATT = 0 dB


SA input ATT = 10dB


SA input ATT = 20dB


COMMENTS:
    • The ATT= 0dB attenuator plot is ugly.
    • The ATT=10db is a little better.
    • The ATT= 20db is acceptable.

Why the first two plots are so ugly?

    • wrong setup?
    • wrong vna?
    • wrong SA frontend (input attenuator)?
    • wrong data interpretation (what I see is correct);
    • the experimenter doesn’t know what is doing and needs help and/or pointers to training resources...

Thanks.
Emanuele.

[smaultre]

Beware of high output level from your VNA!!

[egirland]

Thank you very much for the replay.

The miniVNA output level is never higher than -5dBm.
It is well beyond the maximum level for this SA (+25dBm MAX).

Any other idea?

Thanks.
Emanuele.

[EggertEnjoyer123]

The Advantest probably has a preselector. When the preselector is at a different frequency than the miniVNA, it reflects all the energy back (filters reflect all the energy outside the passband back, while allowing all the energy in the passband through). This is why the S11 sucks when you're at 0dB. When you're at 10dB attenuation the worst S11 you can ever see is -20dB (since the signal gets attenuated twice when it goes forward and bounces back.

Edit: looks like your spectrum analyzer doesn't have that. The problem then probably is that you have a mixer with RF, LO, and IF inside the spectrum analyzer, and the miniVNA is looking at the RF port. If the IF has a bandpass filter after it, then you will see a bad match if the mixed frequency is outside the passband. The signal that gets mixed down will get reflected back at the mixer from the IF filter, and it will get converted back up to the same RF frequency. So you end up getting power reflected back at the VNA, which shows up as bad return loss. The jumps are because the LO and RF sweep at different rates so it will appear to jump around.

Try setting your spectrum analyzer to 100 MHz or something, zero span mode.

[egirland]

EggertEnjoyer123, you are an RF wizard!

Here is the SA front end block diagram:

It resembles exactly what you supposed.
Setting the SA to 90MHz and 0-SPAN solved the problem.
Here are the new, much more comfortable, plots:
ATT=0dB

ATT=10dB

ATT=20dB


Thank you very much indeed. I now have something to think about for an entire week (at least)!
Ciao from TORINO (Italy).