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Zero span spectrum analyzer .FFT, sweep, filters and RBW.
xugmu:
It is logical that you do not understand my answer since my answer was wrong. I confused digital filter with fft.
The only way I have right now to get close to 1 Hz resolution is with an SDR. In fact, I opened the thread with that intention, to know what I see on the screen using 1 Hz RBW using fft technology or scanning technology. The same could be applied to the topic of zero span, the doubt was between owon and siglent, owon only offers fft technology at those resolutions and siglent thus offers two options.
Bringing the zero span to 1 Hz resolution has also been the result of my lack of awareness. In fact, seeing the images that have been posted in this thread, I have doubts about the use of a variable RBW when using the zero span and how that influences what is seen on the screen.
The signals that interest me the most are DVB-T signals (COFDM), specifically the pilot carriers, which are simpler. They interest me in two ways: interference can be appreciated relatively well since they have quite a bit of "free space" and on the other hand, observing the pilots in the time domain (a simple sinusoidal signal) may perhaps provide information (an almost abandoned objective would be to obtain the impulse response (my television analyzers only reach 400us))
Although I have demodulated signals complete with gnuradio I have been trying to do the same for some time with an individual data carrier with absolutely no success . The field of quadrature signals seems very interesting to me.
G0HZU:
I don't know if this helps, but if we confine things to just the ancient analogue RBW filters and digital RBW filters (and we ignore FFT for now) then it's possible to predict the sweep time of the analyser for a given span setting and RBW.
The sweep time goes down as the RBW is increased and there should typically be a RBW^2 relationship. So if you increased the RBW by a factor of ten from 100Hz to 1kHz then the sweep time would be 10*10 = 100 times faster.
For an analogue RBW filter the sweep time in seconds is typically = Span/(0.33*RBW*RBW) where Span and RBW are in Hz.
For a digital RBW filter the sweep time in seconds is typically = Span/(0.8*RBW*RBW) where Span and RBW are in Hz.
This shows about a 2.5 times improvement in sweep time for a digital filter.
However, if you want to explore very narrow RBW filters then the sweep time goes up very rapidly because of the RBW^2 term in the equation.
A 1kHz Span and 10Hz RBW would have a sweep time of 12.5 seconds for a digital RBW filter. If you reduced the RBW to 1Hz the sweep time would go up to 1250 seconds. This is obviously impractical. If the span was reduced to just 100Hz then the sweep time would be 125 seconds and this is still really slow, but it is tolerable. But you only get to see a 100Hz span and not a 1kHz span.
The modern solution for this slow sweep time is to select FFT mode.
i.e. to improve on the 1250 second sweep time, a modern analyser can digitise the whole IF spectrum using a suitable sample rate and do a large FFT on the sampled data.
The display update rate would hugely improve and if you wanted a 1kHz span and a 1Hz RBW and you selected a typical window function (eg Blackmann-Harris window), the screen would update about once every two seconds with the FFT mode selected.
This is a massive improvement. However, the FFT mode comes with it's own compromises in terms of spurious free dynamic range.
Most analysers will be set up to automatically use digital RBW filters above a certain span setting and then swap across to FFT on narrower spans. This give the best spurious free dynamic range in wider spans (using digital RBW) and it gives the best sweep time on narrower spans (using FFT mode) where the sweep time of a digital RBW filter would be too slow.
G0HZU:
Note that the really ancient HP 8568B and HP 8566B spectrum analysers could also support FFT analysis but only on the post detection data. I think they supported several window functions including flat top and hanning. It allowed modulation analysis at frequencies 'very' close to the carrier.
However, hardly anybody will have ever used this FFT feature in these ancient analysers.
G0HZU:
Note that I don't recommend that you buy an HP 8568B or HP 8566B for your task. These analysers are too big and heavy and they produce lots of fan noise. They have old school analogue RBW filters and they don't support digital RBW.
The FFT mode they support is impressive for the age of the instrument, but the FFT performance will be fairly grim by modern standards.
You are probably best off using an SDR although the Siglent analyser might be OK for some of the things you want to do.
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