I think that no one can say anything on that setup. Anyway, try this: run
the GPSDO 10MHz into the radio and take the measurements. Then run your
signal and let us know the difference. Usually running the reference
against itself (if possible) gives the noise floor of the measurement
setup... if you can't see any difference then maybe the noise is too high.

On Wed, Jul 4, 2012 at 10:23 PM, Bill Dailey docdailey@gmail.com wrote:

Hi Bill,

On 07/04/2012 10:23 PM, Bill Dailey wrote:

At this point you should have 1 kHz sampling rate, and the FFT window
will be 32,768 s long. So far my only question is what your radio is
dialled into, as it will give the beat frequency on the audio side and
also the scaling factor for the phase-deviations.

How do you do that? If you have a 32,7 s long FFT window, you only get
results at that rate. Are they interleaved? 32 interleaved will give you
measures every 1,024 s.

If you taking the readings out of the top bin on your spectrum plot,
then the FFT window time will scale the "instruments" hardware
bandwidth, as to be expected from the Nyquist theorem. This has proven
not to be all the truth, as there will be a droop (bias error) for the
short-tau ADEV measures, which wears off and is essentially gone after a
decade or so. Hence, using long FFT windows will reduce the rate of
samples and cause short tau measure of interest to be biased and untrusted.

The recommended practice is to use a higher sampling rate, and in your
case shorter FFT window, maybe skip the decimation, such that you can
afford to remove low tau0 multiples such that what you see is what you
can trust.

Please have a look at the Allan Deviation wikipedia page, I think you
will find some useful stuff there. I'd happy to adjust it if needed.

It's a fair question to ask. It's also a good question to ask.

It would be good if you could calibrate your setup by using known
sources with known phase-noises. While I like the tool I have, I will
sure that I do my calibrations one way or another.

Cheers,
Magnus

Yes, I'm also interested in how-to. At the moment I think it is a hack:
there is no sound card AFAIK that accepts a reference input. I have
recently bought an Acqiris/Agilent DP105/U1067A 150MHz 500Ms/s digitizer
PCI card that accepts an external 10MHz as a reference for the sampling
process.

On Fri, Jul 6, 2012 at 4:43 PM, Stewart Bryant stewart@g3ysx.org.uk wrote:

I think a number of higher-end sound cards accept a "word clock" or
"world clock" (I've seen it both ways) that's intended to allow syncing
to an external source.  The challenge I've seen is that the frequency
(either in the 12 or 24 MHz range) is one that's not simple to
synthesize precisely (i.e., zero offset from nominal) and with low
jitter/phase noise from a 5 or 10 MHz reference.

I think it would be a great project to come up with a synthesizer block
that could do that.  It's been on my list for a long time, but hasn't
yet risen near the top.

John

On 7/6/2012 10:51 AM, Azelio Boriani wrote:

On 7/6/12 7:51 AM, Azelio Boriani wrote:

while there are no "sound cards" (in the sense that they physically plug
into the PC bus) with external clock inputs, there are quite a few sound
interfaces that take a sample clock input (e.g. at 44.1, 48,96, 192) of
some sort.

A lot of them use 1394/Firewire (for historical reasons)

what you're looking for is "word clock in"  or similar things

On 7/6/12 7:51 AM, Azelio Boriani wrote:

oops I was wrong.. there ARE people making PCI multichannel audio
interfaces with external clock.. RME in Germany makes more than one.

OK, found it: the RME HDSPe RayDAT PCIe audio card has this reference but
with the optional extension card (of course). It is $950 for the card + the
expansion card for the world clock...

On Fri, Jul 6, 2012 at 5:09 PM, Jim Lux jimlux@earthlink.net wrote:

Word clock generators appear to exist that will accept standard external reference frequencies.  One of the vendors also sells a stand alone 10 MHz rubidium reference for driving their word clock generator.

Sent from my iPod

On 2012-07-06, at 8:09 AM, Jim Lux jimlux@earthlink.net wrote: