Products > Test Equipment
Rigol DS1000z/2000/4000/6000 Waveform Capture Utility
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AlessandroAU:

--- Quote from: pa3bca on December 21, 2014, 01:00:33 pm ---
--- Quote from: nixfu on December 21, 2014, 07:09:32 am ---I am just curious if you guys thank that this could replace the need to purchase an actual SA?

I have a DS1074z, and I need to maybe check some filter responses, and maybe look at RFI or such stuff, or even look for harmonics on a transmitter.

I have almost every piece of gear I ever have told myself I needed, except for an SA because they are soooo expensive compared to most other gear.

So, for a hobbyist can this replace the need for a SA for mostly simple stuff when combined with a good signal generator, and a DS10xxZ scope?

--- End quote ---
Up to a point: yes. If you look at my comparison between the DSA815 and the FFT analysis you can see that up to say 3 MHz and -50dBm they are almost exactly the same. But further down in amplitude they begin to differ somewhat.
So i think that is useable as a SA substitute to possibly a few tens of MHz (remember the scope's BW is only 100 MHz and de sampling rate "only"  1GSa/s). And signals below say -50 dBm are also under suspicion. The scope's AD converters are only 8 bits, and by massively oversampling (say 12 to 25 MPts) you can up the dynamic range to possibly a practical 12 bits (I'm guessing here but the -70 dBm noisefloor of the FFT's seem to suggest this).
A real SA goes to at least 1.5 GHz (as my DSA815) and has a much larger dynamic range. the DANL is -130 dBm for the 815....

--- End quote ---


You must consider fft processing gain which is equal to 10log (m/2) where m is the number of samples. So the noise floor of the fft can actually reach that low (-130dbm)!

Edit: See Image

pa3bca:

--- Quote from: AlessandroAU on December 21, 2014, 02:13:26 pm ---You must consider fft processing gain which is equal to 10log (m/2) where m is the number of samples. So the noise floor of the fft can actually reach that low (-130dbm)!

--- End quote ---
Yes theoretically. But -130 dBm starting from a cheap scope with an 8 bits ADC?.
The scope itself is 8 bits, so almost 50 dB dynamic range. To get to 130 dB you would have to get 200 Mpts (80 dB = 10 log (200x10^6  /2). Perfectly doable.
But... Using these formulas, with 24 MPts (see screenshots below) the FFT would add 10 log ((24 x 10^6)/2) = - 71 dB!
If all were perfect I would expect the noisefloor to be at -50-71 = -120 dB. I only see -80 (also at 200 MHz and there I would not expect to see any "real" signal out of the scope). So there seems to be 40 dB missing somewhere. I am no expert on FFT (calculations) so I do not have the answer to this puzzle.

Also if you look at the screenshots below there is a significant discrepancy after say 5 MHz. The FFT (with 24 MPts) shows peaks that are not there (according to the DSA815).
So the FFT is certainly usable as a substitute for an SA, but up to a point (obviously in frequency, but also in amplitude. Although I do not know how to discern where the useability ends...)
Rigol DSA815:

24 Mpts FFT from DS1074Z:


JDubU:
Is that a new version of the capture utility that has been updated for DS2000 and DS4000?
Is there a download link for it?

AlessandroAU:
Fair enough the dynamic range is not that high, but its definitely possible to have a noise floor at -130ish with 24mpoints (by zooming in). However I concede that the best dynamic range appears to be around 105dBm, which is still very good.

Also, just to show that is possible to take measurements exceeding 8 bits limit; Look at image 3, the scope is set to 5v/div or 40v max range. A 10mv pk-p signal is applied.

This is 20*log10(0.01/40) = -72db below full scale, a full 22db or so below the 8 bit resolution limit. Even though the waveform looks like barely like anything, thanks to the mathematics of the Fourier transform this signal is still measurable. Very neat!

Edit:
The tektronix MDO series also only uses an 8bit adc internally but has a DANL of -130dbm from 50khz to 5mhz (http://www.tek.com/datasheet/mdo4000/mdo4000b-series-datasheet-0) so it is definitely possible approach this kinda of performance with 8 bits.

also see: https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=22&ved=0CC4QFjABOBQ&url=http%3A%2F%2Fwww.tek.com%2Fdl%2F48W-28882-2%252520%252520%252520MDO%252520Performance%252520WP_.pdf&ei=p_2YVOHwJcLZ8gWIsoKABg&usg=AFQjCNHaLpRkqk9Fxl4CPInDNz58hWFfLQ&sig2=vefpuXdZKo0KQWZVw9PI1g&bvm=bv.82001339,d.dGc&cad=rja

Just wanted to make people aware of this fact! I would trust the scope for use as an SA up to its bandwidth of 100MHz personally.
AlessandroAU:

--- Quote from: JDubU on December 22, 2014, 05:51:08 pm ---Is that a new version of the capture utility that has been updated for DS2000 and DS4000?
Is there a download link for it?

--- End quote ---

I am still working on support for other scopes, currently it is quite hard as I only have access to a 1074z. So far my program only detects which series scope is selected. It does not yet load configuration changes for the higher model scopes. If you have a ds2000 I could send you a WIP to try if you would like.
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