On 08/03/2012 04:20 AM, John Miles wrote:
Cross correlation is one of life's few free lunches, but you do have to wait
for it to work. Magnus is correct in that you can get a 3 dB advantage in
noise reduction per sweep, but that only applies to the first sweep.
Agreed. Did not mean to say anything else.
The actual improvement in the instrument noise is related to the square root of
the number of averages taken. This means that a cross-correlating phase
noise analyzer is very effective for broadband noise, where minimal
decimation has to be done prior to the FFT. The input buffers fill up very
rapidly in that case, and you can get hundreds of thousands of averages in
just a few minutes. But measurement of low levels of 1/f^n noise close to
the carrier can take a lot longer.
I have mostly done 1 hour measure runs, so the gain is limited.
One can view the gain for the various sections, which helps to
illustrate the above statement.
As an example, one customer just sent me a plot of the residual noise of a
high-quality distribution amplifier. The indicated PN was below -160 dBc/Hz
at 10 Hz, but the instrument floor at 10 Hz was still -165 dBc/Hz after a
50-hour run! Over that time, the 10 kHz-100 kHz FFT segment had undergone
almost 30 million averages, improving the instrument floor in that region by
over 35 dB. But the segment containing 10 Hz had been averaged only 17000
times for a 21 dB improvement.
Interesting. When more data has been received, the more interesting it
would be to spend processing time on the closed in noise rather than
getting more data on far out noise, as they come quickly. Pondering if
some overlapping strategy could be be used with succsess, similar to
what we do to ADEV and friends in the overlapping, total and theo
strategies to improve long tau measurement quality.
As far as concrete examples go, in addition to Magnus's suggestion of the
NIST documents, I'd recommend this one:
http://www.congrex.nl/EFTF_Proceedings/Papers/Session_14_Oscillators_and_Noi
se/14_04_Bale.pdf
Here, the authors use a pair of 1980s-vintage HP 3048A phase noise
measurement systems with a newer signal analyzer that performs
cross-correlation. They are measuring additive noise from two-port devices,
rather than oscillator noise, but the same principles apply to both.
http://cp.literature.agilent.com/litweb/pdf/5989-1617EN.pdf
The modern E5052A/B signal analyzers do the same thing in one box,
basically.
http://arxiv.org/pdf/1003.0113v1.pdf
A good introductory article by Enrico Rubiola, with more specific math than
the other two links.
Enrico has been using cross-correlation and averaging for quite a while,
in combination with interferometric setups. Do read the article!
For the buffer-amplifier, they would make good use of an interferometric
front-end to the TimePod in order to reach deeper into the noise. I
haven't built one yet. Something to do in the fall.
Cheers,
Magnus
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
