[time-nuts] Phase noise measurement with a scope

[Stefan Heinzmann]

Marek Peca wrote:
> Hello,
>
>> given that digital scopes have a multichannel ADC for acquisition, 
>> which is similar to what a cross-correlating phase noise measurement 
>> instrument has, it occurred to me that phase noise measurement might 
>> also be possible with a standard digital scope and some 
>> post-processing software. The scope usually will have only 8 bits of 
>> resolution, but it will have a rather high sampling rate. With 
>> oversampling math, one may be able to trade one for the other, at 
>> least if the scope's analog frontend is not too bad.
>>
>> Has anyone investigated or tried this? Is it a silly idea to start with?
>
> yes, did it last week. I think it may have a sense with >1Gsps scope 
> with good quality guts (should check with LC584AL at work).
>
> I have tried it with a very cheap one, Rigol 2-channel, originally 
> 50MHz, reflashed to 100MHz. 2 signals, ref&measured, into Ch1, Ch2. 
> Waveforms (2x500Msps) acquired, sinc() interpolated. Results: 
> short-term single-shot jitter around 100ps RMS. Long-term was of no 
> interest for my purpose now, so no observations here.
>
> Therefore, it is almost of no use at all for higher precision needs.

I was thinking about using a 4-channel scope with cross-spectrum 
averaging. Look at the Timepod by John Miles for an example of the 
method. I'm trying to guesstimate if the R&S RTO scope, perhaps with the 
aid of the I/Q option, is capable of doing such measurements, and with 
what kind of performance.

Cheers
Stefan

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