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

[John Miles]

The concern I'd have with modifying a USB sound card, or any of them for
that matter, is that the glue logic between the ADC and the USB chip may be
designed for a certain relationship between the ADC and USB clocks.  Running
the ADC asynchronously may or may not be robust depending on the assumptions
baked into the gate array.  It might be OK if your app can tolerate
occasional misclocking or dropouts but I'd be reluctant to use a hacked
sound card for anything timing-critical.

I just (last week) got an AD7760 ADC eval board working with the Digilent
Nexys2 FPGA platform, with the EVAL-AD7760 board running from its own 40 MHz
clock.  It will accept an external 40 MHz clock source that, in turn,
wouldn't be hard to derive from 10 MHz.  Way overkill for ultra
low-bandwidth work, but if anyone is looking for a clean digitizer for audio
rates in general, you could do a lot worse than this approach.  Cost isn't
too bad either, at $130 for the Nexys2 and $150 for the ADC7760 eval board.
Of course the big drawback is the lack of any sort of standardized audio
driver on the host side.

If/when I spin a PCB for this project I'll definitely include a 10 MHz
input.

-- john, KE5FX


> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of Jeffrey Pawlan
> Sent: Monday, June 01, 2009 3:08 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Sound Cards for locking to GPSDO 10 MHz
> references
>
>
> Soundcards for USB are poor at best.
>
> I have a set of PCI cards that were previously made by EMU and
> they accept
> external reference input.  They no longer make the model I have
> but perhaps
> they have another PCI card with an external ref input.
>
> I am interested in your modulation technique which allows you to use WSJT.
> Please let me know exactly what you are doing. I also do not know
> how you are
> using 5 milliHertz with WSJT since the group of discrete tones
> occupy more
> bandwidth.
>