Marek Peca wrote:
My point was, that DSO is basically an ADC. Therefore, there is some amount of noise, nonlinearity and drift, limiting the jitter measurement. Do you think any method can dig more information from given data than sinc() interpolation and zero-crossing computation?The cross-spectrum averaging does indeed do just that, relying on two ADCs to produce uncorrelated noise, which can be averaged out.Or am I misunderstanding your point?Nothing against that. It depends on what noise level after averaging you require. I only posted my experience with a very low-quality DSO, which has 100psRMS single-shot. Using sinc() interpolation, but my point was, that I suppose there is no way to obtain better single-shot performance than this. To average out 100psRMS to, say, 1psRMS, it would require 10^4 edges (under the assumption, that the 100psRMS is well behaved noise).What performance it could yield with a better scope? I hope I'll try LC584AL some day, I guess it might give sth like 10psRMS single-shot...
John Miles' Timepod uses 16-bit ADCs which by definition can't have better than roughly 100dB noise floor, yet it is able to measure down to around -170 dBc phase noise, isn't it?
A scope like the RTO which I mentioned has 8-bit ADCs with 10 GS/s, which could for example be downsampled by a ratio of 128 to yield an effective sampling rate similar to what is used in the Timepod, with a corresponding increase in resolution. It would still not be equivalent to a 16-bit ADC, but as long as there are no prominent spurs, it should not be radically worse. And since it is not a low-quality scope, I would expect reasonable jitter performance from the oscillator in the scope. The oscillator used in the Timepod isn't going over the top, either, since the measurement method does not rely on that clock being pristine.
Still, I may very well have overlooked something important, so tell me if my reasoning is faulty.
Cheers Stefan
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