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IF output on a Spectrum Analyser - what is its main purpose?
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SHJH-13:
Hi,

I know it has been a long time but this topic has recently piqued my interest. I did try to look for the video on The Signal Path channel but didn't manage to find it. Can you help me with a little information? The video's name or topic? I searched for EXA and EXA spectrum analyzer on the channel but didn't find the video.

Thank you in advance.
pdenisowski:

As others have mentioned, one use of the IF output of our spectrum analyzers is to do wideband signal analysis by connecting this output to an oscilloscope.  For example, we have numerous customers who connect the 2 GHz (wide) IF output of our FSW spectrum analyzer to one of our RTO series oscilloscopes.  In fact, we have an entire app note on this setup:

https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/application_notes/1ef92/1EF92_1e_Wideband_Signal_Analysis.pdf

There are other applications, but in my experience, that's the most common one.
pdenisowski:

--- Quote from: Berni on January 25, 2019, 09:10:52 pm ---Today such functionality is not that useful anymore since modern spectrum analyzers have a lot more signal and modulation analysis features built right in, or they are themselves FFT based now, essentially making them SDR radios where the data can be streamed into a more powerful PC and analyzed there purely in software.
...
EDIT: Oh and the reason why the above SA+VSA setup uses the IF port rather than the video port is that the video port is essentially already a AM demodulated version of the signal of interest. By leaving the signal downmixed to a lower frequency LO lets it retain phase information that is needed by the VSA in some modulation schemes. Looking at the video port just gives you scalar information about the size of the signal.

--- End quote ---

Yes.  Generally speaking if you need to do vector signal analysis (i.e. something that requires phase information), this is nowadays usually done entirely within the analyzer. 

The exception is when the VSA's internal "analysis bandwidth" is not wide enough, in which case an external digitizer (in the form of a oscilloscope) is needed.  Usually the internal analysis bandwidth of a VSA is considerably less than is available with off-the-shelf oscilloscopes:  for example, there are plenty of scopes with bandwidths in the single-digit GHz range, but until recently there weren't many VSAs that could process a signal that wide internally.

Since wider bandwidths are becoming increasingly common (5G, 6G, radar, newer flavors of 802.11), higher-end VSAs have now more or less caught up with current bandwidth requirements, but the IF out of our signal and spectrum analyzers is still widely used by many of our customers.




TimFox:
On older spectrum analyzers, the IF output may be at a reasonable frequency (such as 100 kHz).
I found that useful to connect a true-RMS AC meter to use the analyzer at a fixed frequency to measure noise in the bandwidth selected by the analyzer.
Berni:

--- Quote from: TimFox on September 22, 2022, 06:24:41 pm ---On older spectrum analyzers, the IF output may be at a reasonable frequency (such as 100 kHz).
I found that useful to connect a true-RMS AC meter to use the analyzer at a fixed frequency to measure noise in the bandwidth selected by the analyzer.

--- End quote ---

Ah nice trick there.

Tho it seams to be IF frequencies that low are pretty rare to see, but even if it is in a few MHz you could use a digital scope as a RMS voltmeter on signals well into the MHz range.
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