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Zero span spectrum analyzer .FFT, sweep, filters and RBW.

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G0HZU:
A lot depends on what you want (or expect) from your spectrum analyser. You might not need a high performance analyser for your task as I'm not convinced you 'need' a 1Hz digital RBW filter. The Siglent might be fine for what you want to do.

However, if it turns out that you really do 'need' lots of spurious free dynamic range (SFDR) whilst using a narrow digital RBW then I think that you will find that the performance of the Siglent is going to be fairly poor compared to a proper lab spectrum analyser.

The Siglent is going to lose out in both the performance of the downconverter (phase noise, IIP3 and DANL) and I'd also expect it to lose out even further if it can't provide a 1Hz (or maybe even a 10Hz) digital RBW filter when in swept or zero span mode.

It looks like it is limited to a minimum of a 30Hz digital RBW. This is still quite reasonable, but it does limit the SFDR in swept mode if you want to look for tiny signals near very large ones for example.

FFT mode can offer great speed advantages, but it comes at a price in terms of how it can limit the spurious free dynamic range. Swept mode is slower but it usually offers the best SFDR.

Having said all that, most users won't need the extra performance and they may be perfectly happy with the Siglent analyser. They may not be aware of its limitations compared to a proper lab analyser but this probably doesn't matter.

G0HZU:
The other thing to be wary of with the cheaper analysers (like the Siglent) is that they are unlikely to offer much (if anything) in the way of analogue preselection ahead of the ADC when narrow spans are selected.

A decent lab analyser will have a suite of adjustable preselectors ahead of the ADC and the bandwidth of the preselector is usually adjusted to be a bit wider than the bandwidth of the digital RBW filter (down to maybe 1kHz).

This means the ADC is only exposed to a limited spectrum of signals when set to narrow spans or narrow RBW settings.

I'm not sure what the Siglent does here, but I expect it might only have one (or maybe two) IF preselection bandwidths. This limits the spurious free dynamic range and also makes the analyser very prone to ADC overload if there is a very large signal maybe 100kHz or maybe even several MHz away.

Many users will never realise this limitation because they probably don't ever explore the limits of the analyser during typical use. However, having a wide bandwidth digital IF can sometimes be a disadvantage if there is no means to reduce the bandwidth that the ADC is exposed to.
 




G0HZU:
If this all sounds a bit confusing, then consider the case of FFT vs swept for a span of 500kHz.

In the swept case, the digital RBW could be 1kHz and there could also be a 3kHz wide preselector ahead of the ADC. So the ADC is only ever exposed to a spectrum 3kHz wide even though it is sweeping through a span of 500kHz. This protects the ADC from overload from large, nearby signals.

In FFT mode, the analyser may well present the whole digital IF bandwidth to the ADC and this could be several MHz wide. So there is much more chance of ADC overload either from a single large signal or from the voltage summing of all the signals in the IF bandwidth.

If the analyser does support an adjustable IF preselector, then in FFT mode, the preselector could be set to about 2MHz bandwidth or the analyser could select a narrower preselector and do several smaller (narrower) FFTs whilst stepping or hopping the LO each time. So the analyser could use a narrower preselector bandwidth and provide more protection of the ADC. This would sacrifice some of the speed advantage of the FFT mode though.

I don't know if the Siglent can do this multiple FFT trick, but it would ideally need to have a suite of IF preselectors fitted in order to make it truly worthwhile.

G0HZU:
Note that for a 500kHz span in FFT mode, you can't choose a preselector that is also about 500kHz wide as there will be droop in the preselector band edges and this will spoil the flatness of the analyser across a 500kHz span. Usually, the preselector is selected to be several times wider than the span in FFT mode in order to preserve flatness.

If multiple FFTs are available, the the preselector bandwidth could be reduced significantly and maybe ten or more smaller FFTs could be done whilst retuning the LO for each FFT. Building up the spectrum in chunks like this obviously takes longer than doing a single FFT but it means the analyser could auto adjust the gain to prevent overload for some of the smaller FFT chunks if they contain huge signals in their passband.

ftg:
Yeah I have hit the dynamic range and phase noise limits of SSA3032X on occasion.
And indeed it only offers 1Hz RBW in swept mode and 30Hz in Zero Span mode, which makes sense I guess, as 1Hz wide Zero Span would require rather good frequency stability on higher frequencies.

What measurement requires 1Hz RBW on Zero Span?
Especially on UHF and up?

At least on my SSA3032X the VBW does go down to 1Hz, so for long term monitoring of some signals output amplitude the RBW can be wider and then the VBW setting can be used to filter out all quick variations.

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