[time-nuts] Phase noise measurement with a scope

[Azelio Boriani]

The problem with sampling 'scopes is that you cannot get a continuos
samples stream. I think that the TimePod correlates continuously in
time.

On Thu, Jun 13, 2013 at 11:56 AM, Stefan Heinzmann
<stefan_heinzmann at gmx.de> wrote:
> 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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