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SA video bandwidth affects signal level?
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pdenisowski:

--- Quote from: switchabl on March 22, 2023, 06:03:34 pm ---Some SAs definitely display "UNCAL" when you set an insufficient sweep time but I think on the FPC it just means that TG response is not normalized, no? In any case, I don't know why this should ever happen on Auto.

--- End quote ---

Agree.  This is something I need to check on.
Hexley:
Some experimental results from an FPC1500 with TG:

* The OP's results could be duplicated when the SA was first powered up and the TG was enabled. Changing the VBW affected the amplitude.
* pdenisowski's results could also be duplicated when the SA was changed from TG to CW mode, and a single 550 kHz tone was measured. In this case, changing the VBW did not affect the amplitude.
* When the SA was then changed from CW back to TG, the amplitude no longer changed as VBW was varied.
This sequence seems to be reproducible. After powering down the SA, I could repeat the sequence above. Sort of smells like a data initialization bug somewhere.
jmw:
Wow, that's interesting Hexley! I didn't think to try that sequence - I'm in a different building at the moment, but will see if I can confirm a little later today. Edit: able to reproduce the sequence & behavior described by Hexley. Once source is set to CW and back to TG, I'm actually seeing the opposite effect: the TG signal appears to get stronger at very low VBW. At 10 Hz VBW, the -20 dBm TG output reads as -19.81 dBm.  :-//

Yeah, it says "uncal" only because I didn't run a S21 normalization aka response calibration before taking the measurement. The sweep time effect on filter charge-up is a valid point, I wonder if I manually increase sweep time whether it will change the offset level. I'll give that a try too. Edit: TG forces Auto sweep time, can't force it higher or lower. With CW output, forcing a low sweep time distorts the signal but it also puts a red warning indicator on the screen to tell you that. No such indication when the TG is active with auto sweep time.

The reason I am cranking down the VBW is to clean up traces of measurements close to the noise floor. I'm doing shunt-thru measurement of milliohm resistors. S21 expected be about -70 to -90 dB below the TG output.
Detlev:
Hello,

I just recreated it, I hope all the parameters are the same.

First VBW to 1kHz, normalized and marker set and then VBW to 30Hz, the attenuation is 0.13dB on my FPL:



A good document on SAs is from Rohde & Schwarz

https://www.rohde-schwarz.com/de/produkte/messtechnik/analyzers/signal-und-spektrumanalysatoren/educational-note-fundamentals-of-spectrum-analysis-register_252824.html

(Thank you R&S for that  :) )



pdenisowski:
First, many thanks to jmw, Hexley, and Detlev for looking into this.

I've been working with the R&S North America support team this afternoon and this issue does seem strange enough that we'd like you to open a case.   

Could you send an email to Support@rohde-schwarz.com and describe the issue?  Please let them know that you've been working with me (Paul Denisowski) and include your description of the issue (as well as what Hexley found).  I'm assuming you're on the latest FW since they definitely will ask that.

Once they get the email they'll open a case and most likely they'll also contact me (internally) directly.  I've PM'ed you my email address already, so please reach out to me directly anytime as well.

Thanks again for the detailed description and all the investigation: by the time I had arranged for remote access to an FPC you had all beat me to it :)
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