[time-nuts] Plot phase noise spectrum from DMTD
demianm_1 at yahoo.com
Sat Mar 12 18:47:13 UTC 2011
I would like to see more in one place about implementing these ideas. The
bits are scattered in different places and I'm not sure where to find them
One resource for getting the signals sampled at SOTA performance for a
reasonable price is this demo board from TI: PCM4222EVM
http://focus.ti.com/lit/ug/sbau124/sbau124.pdf $149 from TI
It supports differential input at 192 KHz sampling with spdif/aes outputs.
It has on board crystal oscillators from Pletronics. You can also run it
from an external oscillator. I'm not sure how far from the standard
frequencies you can go and still have the system get good data. You would
need a sound card that has an SPDIF input that runs at 192 KHz. Some
motherboards may be able to do this. The board is a very good implementation
of their best audio frequency ADC. I don't think you could build it anywhere
near the price of the demo board.
You would need a sound card that has a native ASIO driver to keep Windows
from mucking around with the signals ideally. I would not recommend a USB
audio interface at this time. They are linked to the PC through some ugly
mechanisms if accuracy is important (adaptive sample??). Better solutions
are happening for USB but they are few and expensive for now. You can get
PCI cards from ESI, M-audio, EMU that would do the job pretty well with
drivers that work. For Linux I would suggest the ESI Juli@ since I know it
works well in Linux.
It may be possible to create a plug in for Audacity that will do the math
and conversion to get the phase noise plots etc from the system. Audacity
will do the job of collecting the data. http://audacity.sourceforge.net/
Date: Fri, 11 Mar 2011 14:21:55 -0800
From: "John Miles" <jmiles at pop.net>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Subject: Re: [time-nuts] Plot phase noise spectrum from DMTD
Message-ID: <HMEBKHFEACMNLMHIAFDNOEAEAEAH.jmiles at pop.net>
Content-Type: text/plain; charset="us-ascii"
> Ok some cool advice - this thread is an interesting thought exercise. I'm
> going to think about it a some more, but it seems, in comparison at least,
> the loose phase-lock technique remains the simplest. Provided you have a
> low-frequency spectrum analyser handy.
It would be a better idea from the standpoint of VCO modulation bandwidth as
well. If you measure PN by looking at the tuning voltage in the tight-PLL
configuration, you'll probably be limited to offsets of a few kHz before the
response rolls off.
> The sound card idea is clever as well - however, I'd assume one needs to
> measure the ADCs clocking oscillator offset, since that will be apparent
> when plotting the beat frequency phase (what I mean is that sampling will
> then look like another mixing process). What I usually due is to
> clock the
> sampling system off a clock that's correlated to the clock under
> test. This
> resolves that issue.
Sound cards will usually end up running within 1 Hz of the desired sampling
rate, but it's important to pick a sampling rate that's native to the
hardware, or the driver will resample the data. On Windows, many drivers
for popular sound cards rely on some imprecise resampling code that
apparently was distributed by Microsoft in the DDK. Stick with 44100 or
48000 Hz, or you'll be lucky to land within a few dozen Hz in some cases.
Warren's been getting some really nice ADEV plots from a tight PLL sampled
with a USB sound card, running a quick and dirty command-line utility I put
together to acquire the data and downsample it. I'll post the next build on
my web page if anyone else is interested in playing with it.
> However, I'd like to experiment with the cross-correlation idea,
> since I've
> got a setup that will lend itself perfect to that. Maybe I could
> save myself
> some time, with clever post-processing.
> Can anyone recommend a fundamental text on the cross-correlation
The easy way out is to look for a dual-channel FFT analyzer or one of its
successors. The HP 3562A and 3563A models are pretty affordable these days,
and they've been used in a number of papers on cross-correlation
measurements of various types of noise. SRS also sells some nice
If you're looking to write your own processing code, you should search for
information on 'cross spectrum' as well as 'cross correlation' techniques,
because the former is the correct term for the scenario where the sampled
data from both channels is already time-aligned. For its part, the cross
spectrum is just a $5 mathematical buzzword for the vector length between
the corresponding output bins of two FFTs, obtained by multiplying one array
by the complex conjugate of the other. When averaged over time, the real
component of the cross spectrum will converge to the common signal at the
ADC inputs. Unlike the DUT signal, the ADCs' noise contribution is randomly
distributed in phase space. It will converge to zero when averaged and fall
out of the measurement... at least to the extent that the channels are truly
Unwanted channel correlation is one of several reasons to use a lab-grade
FFT analyzer instead of a sound card for multichannel measurements. At a
minimum it would be better to use two USB sound cards and run them from a
Enrico Rubiola's "The cross-spectrum experimental method" is a good survey
of the basic principles (http://arxiv.org/abs/1003.0113); also see
http://tycho.usno.navy.mil/ptti/ptti2001/paper42.pdf and Walls's original
"Cross-correlation phase noise measurements" paper, as well as the various
white papers at aglient.com and symmetricom.com.
Rubiola's material is the most helpful I've found, really. I've been doing
a lot of R&D in this area lately, and I've found that most DSP textbooks are
too far removed from real applications (no pun intended) to be of much use.
The math is not rocket surgery, but you couldn't tell that from a survey of
the academic literature.
-- john, KE5FX
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