IF output on a Spectrum Analyser - what is its main purpose?

[jpb]

Apologies if this has been covered elsewhere - I tried searching on a string including IF and of course it came back with every entry with "if" in it as well as lots with words that have "if" in!

I've been looking at the GWInstek 9300B Spectrum analyser and like a lot of older HP units it has an IF output at around 770MHz. (There is a connector on the back giving the IF out at -20dBm I think.)

I'm a bit puzzled as to what you can do with this. I guess you could feed it into your own mixer and LO and ADC but that is only doing what the Spectrum Analyser is doing anyway.

You could (if you have a 1GHz oscilloscope) look at it.

It doesn't help with synchronising an external tracking generator because it is not swept.

Perhaps it allows a better quality power measurement via an external power meter - the GWInstek 9300B has a dc power connector on the front for some unspecified option.

Am I missing something really obvious?

[glarsson]

External tracking generator.

[jpb]

External tracking generator.

Thanks for responding.

How does that work?

My understanding is that the IF is at a fixed central frequency that is the tracking happens at the LO of the first mixer so you have Freq_IF (fixed_difference_frequency) = Freq_LO (varying - swept) - Freq_Input (swept) so if you are looking at the IF you see a band of frequencies around a fixed central value but which varies with magnitude depending on the input signal at the currently swept frequency value. I don't see how you can know from this what the frequency value should be to set on the external tracking generator.

[DaJMasta]

At least on the SAs I've worked with, the IF output isn't an intermediate conversion stage LO or anything, it's the fully downconverted input signal (sometimes what the ADC sees, depending on architecture), and it's available as an output for external digitization/analysis/whatever.  That means in a zero span mode, your SA effectively just acts as a mixer and LO, downconverting the input and outputing on the IF output, but it also means you can send it into a scope or a signal analyzer to get some additional capability the SA doesn't include.  Notably, if your SA can't decode a wide enough bandwidth to demodulate a given signal, you could use the IF output to digitize it on a much higher bandwidth scope and run demodulation software in that, or even on another device just using the scope as a digitizer.

[Berni]

External tracking generator.

That's what you do with the LO output port, not the IF port.


One example of use i have is combining my HP 8566B (20GHz Spectrum analyzer) and the HP 89410A (10MHz Vector signal analyzer). The IF port of the SA on it can be fed (via a adapter box) into the input of the VSA. This lets the SA be a radio front end that amplifies and downmixes the high frequency signal of interest down to where the VSA can look at it. Because the VSA is a digital FFT based analyzer it lets it do more advanced stuff such as QAM demodulation and does have some options for decoding signals such as GSM.

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.

Tho you can still have some fun by connecting a speaker to the video output port on a old spectrum analyzer. This is not actually a video signal, but rather the output of the detector circuit that drives the trace up and down on the screen. If the spectrum analyzer is set for zero span and its center is placed on to a radio station the detector will start demodulating the radio station down into audio and you can listen to it. Works best with AM stations but also works fine on FM when the center frequency is placed onto the shoulder of the carrier. Tho its probably no good for any SSB modulation.

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.

[tmbinc]

That all makes sense, but if the IF is ~770MHz (The GW Instek for example has it at 886 MHz), wouldn't you need another downconverter first? Or is that typically integrated in the "baseband" VSAs?

[jpb]

That all makes sense, but if the IF is ~770MHz (The GW Instek for example has it at 886 MHz), wouldn't you need another downconverter first? Or is that typically integrated in the "baseband" VSAs?

That was exactly what I was about to ask.

My guess is that GW Instek put out what they have - if they put out the last IF stage it is probably too low a frequency (you don't gain anything over what the final stage of the Spectrum Analyser can do) so they put out the IF before that which is up at 886MHz. I suppose the user can add another mixer/LO stage but given that the Spectrum Analyser is only up to 3GHz you're not gaining a huge amount by dropping only down to a bit under 1GHz.

What I'm trying to say is I suspect it was put out as a "feature" to help sales but it is not really that useful in practice. This is perhaps a little harsh because at least you only need a fixed LO frequency as the spectrum analyser is taking care of the frequency sweep.

For comparison the Aeroflex 3250 outputs the 3rd IF at only 21.4MHz (16MHz bandwidth) which makes more sense.

[Berni]

Well there is still the benefit of the IF staying at that frequency and all the signals of interest sitting right next to it. Even if the SA is sweeping from 1MHz to 3000MHz. So while it might be high in frequency its very narrow in bandwidth.

But yeah i highly doubt anyone will ever use it for something useful. They likely just ended up at that frequency due to the internal architecture and put it out just because other spectrum analyzers often have it.

[DaJMasta]

Well there is still the benefit of the IF staying at that frequency and all the signals of interest sitting right next to it. Even if the SA is sweeping from 1MHz to 3000MHz. So while it might be high in frequency its very narrow in bandwidth.

But yeah i highly doubt anyone will ever use it for something useful. They likely just ended up at that frequency due to the internal architecture and put it out just because other spectrum analyzers often have it.

That's my guess.  There are Keysight analyzers with a 900MHz bandwidth IF output, but those are intended for niche high bandwidth applications, or something like simultaneous realtime analysis of the portion the SA itself can analyze plus other parts of the signal a second (or third) analyzer could be set to.  The likelihood this will come up outside of pretty high end R&D work is probably pretty slim.... somehow I don't expect GW Instek to be targeting that marketshare.

I believe TheSignalPath has a video on youtube of using an EXA spectrum analyzer as a frontend and the wideband IF output being decoded by a 1GHz scope.

[Wolfgang]

... I use it to decode broadband modulated signals on a scope.

[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]

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.

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]

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.

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.