[time-nuts] Sound Cards for locking to GPSDO 10 MHz references

Rex Moncur rmoncur at bigpond.net.au
Mon Jun 1 17:59:36 EDT 2009

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.


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