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Show us your square wave

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TunerSandwich:
The RIS sample rate was 200GS, and the non RIS is 10GS (at that timebase), not 5GS......no boxcar on that.....just sin x/x and i believe the settings were power spectrum and flattop.....I can duplicate the result and show the setup screens.....

as you can see in the "broadband" plot the spurs extended out to around 100GHz and then began to roll off up to 200GHz....where there was a dramatic rolloff....pretty close to the way your scope behaved. 

It would be interesting to do a true quantitative comparison.....I will document the EXACT settings, and if you are inclined to spend the time, we could compare again. 

Maybe even load the same waveform into arb?  My signal source and cable were very much inferior to yours. 

The uniformity of the spurs is highly suspicious....in the "1THz" plot you can see the roll-off above 100GHz.....just to confirm you have a 5GHz front end on your WaveMaster?

joeqsmith:
Yes, I would like to see if I could replicate your results it just because they seemed so strange.   

The data I showed was also sin x/x.  My DSO must also be in sin x/x to get 1THz with the FFT.   I also show the FFT in power spectrum and flat top.   

I don't think the 5G spurs has anything to do with the incoming signal but if you don't mind, try it and let me know.   I can always put it into the ARB if you find its a factor without any problem.   I just need a CSV or something file.   I can always translate it.   Just raw voltage is good enough.   

Yes, my 8500A has a 5GHz front end.   It's pretty old compared to yours but for my home use it is over kill.     

TunerSandwich:

--- Quote from: joeqsmith on January 29, 2015, 02:28:23 am ---Yes, I would like to see if I could replicate your results it just because they seemed so strange.   

The data I showed was also sin x/x.  My DSO must also be in sin x/x to get 1THz with the FFT.   I also show the FFT in power spectrum and flat top.   

I don't think the 5G spurs has anything to do with the incoming signal but if you don't mind, try it and let me know.   I can always put it into the ARB if you find its a factor without any problem.   I just need a CSV or something file.   I can always translate it.   Just raw voltage is good enough.   

Yes, my 8500A has a 5GHz front end.   It's pretty old compared to yours but for my home use it is over kill.     

--- End quote ---

Regardless of age, that is a superior scope....it obviously doesn't have the bells and whistles etc, but 5GHz front end is 5GHz front end....and I believe you can also do 10GS/s interleaved?   I have extended mem on mine plus all options, but I think you essentially have the same, or better "horsepower" in your wavemaster.  I will stay away from "wavestream" view (which I normally do anyhow, because it negates any sinx/x or ERES).  i didn't use ERES for the broadband plots....but did use it on the pulses focusing on the zoomed impulse responses....

I will see if I can load the proper signal into arb (PITA on the ds2000a-s).  if not I will try to get much more specific.  Also there is that cable issue (although I don't see how that should have anything to do with the spurs on my 600MHz front end). 

I will also try the same on CH3....and another option is to use the AP020 probe......do you have one?  That way we can have a more apples to apples comparison (again I doubt the cabling has anything to do with such a dramatic result). 

When I did the plot I didn't pay much attention to where the spurs were.....I just zoomed in to the region (roughly) where they had equivalent amplitude and assumed it was harmonic centers.....but now that it has been brought up, and I look closer, that doesn't seem right at all.....that is far too uniform across a bandwidth my front end doesn't even cover, to be signal....*scratches head* :-//

Jay_Diddy_B:
Hi group,

Here is a little simulation to show what happens with sampling and FFTs. This is to help explain the scope waveform.

I have constructed a model with a square wave with a 66ns period (15.1515 MHz) with a 1ns rise and fall times.

I have also modelled a 100MHz Bessel filter, this will soften the edges without introducing ringing.

And then I constructed a sample and hold to simulate sampling the waveform at 1 Gsps.

Here is the model:



This what the waveforms look like in the time domain:




And this is what the FFT results look like:




The top waveform shows the 1ns rise and fall time square waves. There are spectral lines at 15MHz, 45 MHz, 75 MHz .... There are only even harmonics because the square wave has a 50% duty cycle. There a dips at 1GHz 2GHz etc this comes from the 1ns rise and fall times.

In the second waveform the harmonics are attenuated by the response of the 100 MHz Bessel filter, so no real surprises here.

In the third waveform, we can see the effects of the sampling frequency. We have the baseband spectral lines, then we have components at sampling frequency +/- the baseband components, we also components at 2x sampling frequency +/- the baseband components etc.
This is a form of aliasing because in calculating the FFT we are looking at frequencies greater than half the sampling frequency (Nyquist Theorem) The FFT is only valid up to 500 MHz.



Regards,

Jay_Diddy_B

joeqsmith:

--- Quote from: TunerSandwich on January 29, 2015, 03:01:43 am ---Regardless of age, that is a superior scope....it obviously doesn't have the bells and whistles etc, but 5GHz front end is 5GHz front end....and I believe you can also do 10GS/s interleaved?   I have extended mem on mine plus all options, but I think you essentially have the same, or better "horsepower" in your wavemaster.  I will stay away from "wavestream" view (which I normally do anyhow, because it negates any sinx/x or ERES).  i didn't use ERES for the broadband plots....but did use it on the pulses focusing on the zoomed impulse responses....

When I did the plot I didn't pay much attention to where the spurs were.....I just zoomed in to the region (roughly) where they had equivalent amplitude and assumed it was harmonic centers.....but now that it has been brought up, and I look closer, that doesn't seem right at all.....that is far too uniform across a bandwidth my front end doesn't even cover, to be signal....*scratches head* :-//

--- End quote ---

For home, even my old 7200s have way more features than I use. 

My 8500A will do 20Gs/s interleaved, 200Gs/s RIS.    I would expect them to behave the same (for those 5G spurs).   Obviously, I was expecting to see 10G spurs.   I would not have thought to even look at it until you posted.    See, there was a point to this thread after all!   

I don't believe this is caused by the cable or input signal but give it a try.   Those spurs are so nice and flat on yours where mine has nothing. 


--- Quote ---AP020 probe......do you have one? 
--- End quote ---
I only have resistive probes.    I plan to make some low freq buffers at some point to allow me it to work with 10X passive probes as well.   
Best to stick with coax. 


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