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SA video bandwidth affects signal level?
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jmw:
I have a R&S FPC-1500 and looped the tracking generator to the input. Evidently the signal level drops when I lower the VBW setting. Goes from about -1 dB (1 kHz VBW) to -1.5 dB (100 Hz) to -6 dB (30 Hz). I thought VBW just smooths the displayed detector output. Is this a bug in the SA?

jonpaul:
suggest that you read and learn about Spectrum Analyzers.

The sweep speed, res BW and freq span are related. As video filters,are,applied the observed level is affected.

our SA are,all vintage TEKTRONIX and HP without FFT,

Ask the manufacturer of your SA.

Jon
Kean:
My understanding is that lowering the VBW causes additional filtering/smoothing that affects the displayed trace.  You can see an example of this in the FPC getting started video, but not explained in any detail.  This is similar to the effect of reducing RBW, but it affects only display instead of the measurement.



I don't know that Paul has done any videos that go deeper into affects of VBW or details of the FPC tracking generator specs, but I do seem to remember a post of his about this topic.  I cant find it right now, so maybe I am misremembering who wrote it.

I am admittedly an RF novice, so I may be using the wrong terminology - best to watch some of the related videos.

Ping @pdenisowski
jonpaul:
Rebonjour Cher Monsieur

The relationship between RBW, sweep speed, accuracy and sweep band is,well known.

tHe best application notes,are,the old HP, and Tektronix Circuits Concept books on Spectrum Analyzers circuits and SA measurement Concepts.

http://www.davmar.org/concepts.html

Jon
switchabl:

--- Quote from: jonpaul on March 22, 2023, 02:34:10 pm ---The relationship between RBW, sweep speed, accuracy and sweep band is,well known.

--- End quote ---

According to the screenshot, the sweep time is set to "Auto", so the SA should adjust it according to span, RBW and VBW to maintain a calibrated display. Also, if this were an analog swept-tuned SA, I am not sure how an insufficient sweep time could result in what appears to be an offset trace, rather than a distorted or smoothed-out one.

I suspect that this might be a bug. However, AFAIK the FPC has a purely digital IF with both swept and FFT modes and the tracking generator is actually a programmable synthesizer, so it is hard to be sure what exactly is going on and I may be missing something.
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