Lux, James P skrev:
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Rex Moncur
Sent: Monday, June 01, 2009 3:00 PM
To: 'Discussion of precise time and frequency measurement'
Subject: [time-nuts] Sound Cards for locking to GPSDO 10 MHz
references
Hi all
Does anyone have any experience of locking a USB external
soundcard to a GPSDO 10 MHz reference.
I am interested in advice on any good quality soundcards that
can be readily locked to either 10 MHz or if necessary to
some other frequency that we can derive from a GPSDO source.
I have done some tests with the SignalLink soundcard that
uses a Texas Instruments PCM2904 chip and requires a 12 MHz
lock frequency. This requires some cutting of tracks to
remove the internal oscillator feedback and insert the
locking frequency. 12 MHz is readily derived from 10 MHz but
I have not been able to get it to lock. The Texas
instruments data sheet suggests that it is possible to use an
external refernce but also says this is not recommended.
With this expereicne I would rather find a sound card that is
designed for external locking that does not require the
cutting of tracks.
For info the purpose of this request is that we are looking
at using very narrow bandwidth modes at less than 1 mHz for
light wave communcation. To date using LEDs and cloud
reflection we have worked over 200 km with WSJT but we should
be able to do 20 dB better if we can get down to milli-Hz
bandwidths (at the expense of spending all night to complete
a QSO). Our expereince to date is that standard sound cards
are just not stable to better than 5 milli-Hz at 1000 Hz
which should be readily solved by GPS locking let us get down
to sub milli-Hz levels.
Rex VK7MO
Some of the "pro" sound interfaces have a "word clock" input.
There are a variety of things that take a external input and generate a S/PDIF that's properly timed, as well. Lots of boxes will take a S/PDIF sync input (e.g. the Edirol FA-66 which was used by lots of Flex-Radio folk), so maybe that's something you could easily generate from your 10MHz.
A chart at Cakewalk shows that MOTU has a USB interface (828MkII) which has a
word clock sync. It's going to be a pricey beast though, with 8in/8out ($800?)
Even if you have a word clock input, you're going to have to
synthesize that from the 10 MHz. Maybe it's easier to just
make a S/PDIF which is a MUCH more common sync signal. ( I
think S/PDIF is something like 3 MHz)
S/P-DIF [iec60958-3] has a baudrate which is 128 x sample rate and a bit
rate which is 64 x sample rate, which is inherited properties from
AES/EBU [aes3] [tech3250] [iec60958-4].
Locking up a S/P-DIF (128 x sample rate) is about the same job as
locking up a superclock (256 x sample rate) or wordclock (1 x sample rate).
As long as the signal is samples with low jitter and A/D converted in a
good fashion, delivery over S/P-DIF should not be too hard. An ADC is
slammed onto a AES/EBU/S/P-DIF chip which is fairly trivial extra work.
Cheers,
Magnus
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