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

Lux, James P james.p.lux at jpl.nasa.gov
Mon Jun 1 19:06:42 EDT 2009


> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] 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)

The HPSDR folks also might have something...



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