[time-nuts] Phase noise measurement with a scope

[Stefan Heinzmann]

Magnus Danielson wrote:
> On 06/13/2013 04:26 PM, Stefan Heinzmann wrote:
>> Azelio Boriani wrote:
>>> The problem with sampling 'scopes is that you cannot get a continuos
>>> samples stream. I think that the TimePod correlates continuously in
>>> time.
>> Does that matter for phase noise measurements? Doesn't that just make
>> the measurement take correspondingly more time?
>
> It matters a lot, since the length of memory will limit the how close 
> in you can do it. You can naturally make multiple runs, and that's 
> what the TimePod do, but with decimations done in realtime by the 
> firmware.
>
> You can do it with high speed ADCs, but it won't bee cost efficient 
> and it will cost you in speed, as you need to do much in software 
> processing to get there.
>
> The TimePod is in that context a fairly well balanced design as in 
> bang for the buck. Another aspect I like is that it can do pretty neat 
> long-term measurements.

That's of course true when you want to build or buy an instrument just 
for this job. I don't question the Timepod, on the contrary I think it 
is a very good instrument. My aim was rather to find another use for a 
scope that's already there (it isn't in my case yet, but will be).

The R&S RTO seems to have a few unusual capabilities for a scope, which 
might help here. It does seem to do decimation and a number of other 
math functions in hardware and in real-time. Looking at their 
description of the I/Q option reminded me of the Timepod manual, 
specifically the block diagram in there, and brought me to the question 
I'm asking here. Have a look if you're interested:
http://www.rohde-schwarz.de/file/1TD01_0e_RTO_IQ_Software_Interface.pdf

While the scope may not be able to continuously acquire and 
cross-correlate, with no dead time, I would think it capable of taking 
fairly long shots by storing only the decimated data. There's a chance 
of it being suitable for phase noise down to 10 Hz from the carrier, I 
think. Perhaps closer than that. That would already be quite useful, I'd 
say.

Cheers
Stefan