On 06/13/2013 11:02 PM, Stefan Heinzmann wrote:
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.
Now, that would indeed be very handy.
It's not that you can use todays realtime sampling scopes and use two
channels for cross-correlation, it will work, it will just take many
runs to get anywhere. Cross-correlation is pretty cheap if done via FFT,
and wrapping FFTW to do it only takes about 20-30 lines of code.
Having a scope able to do decimation up-front is quite similar to what
the TimePod do, and when quickly browsing the manual it looks useful.
Cheers,
Magnus
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