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

Magnus Danielson magnus at rubidium.dyndns.org
Tue Jun 2 13:07:45 EDT 2009


Lux, James P skrev:
>> -----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)

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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