At 04:17 AM 6/12/2005, John Ackermann N8UR wrote:
[EMAIL PROTECTED] wrote:
>The current cal scheme is unlikely to ever work well for most users. The
>issue
>is the error in the oscillator in everyone's sound card. Without taking this
>into
>account, most of us can achieve very good accuracy at one WWV frequency
>but will find it is way off on another. My sound card was several hundred Hz
>off (at 24 MHz). Unlike the DDS error correction, it does not scale with
>tuned
>frequency. I think this is on Bob's list of things to look at.
>
>Mike W3IP
>
>
I'm not sure I follow why the sound card error would result in accurate
results at one RF frequency but not another. Error in the sound card
clock is like an error in the last local oscillator -- it generates a
constant offset that doesn't change with frequency.
It's more like a scaling error. If you sample at, e.g., 40 kHz, and your
software thinks you were sampling at 44 kHz, the frequencies will all have
the same percentage error. An 8 kHz input will "look" to the software like
it's at 8*44/40 or 8.8 kHz. A 10 kHz input will look like 11 kHz, and so
forth.
Despite those results, my goal is still to have the whole frequency
chain stabilized.
Stabilized, or calibrated? Overall, if you've got the "real time margin"
to do calibration (that is, you can wait the few milliseconds for the
calibration calculations on your realtime data), you'll get better
performance with a high quality source that's calibrated, rather than a
flexible source that's adjusted to stabilize.
One thing I've thought about is hardware hacking the
Delta to use an external clock. I haven't dug into that possibility
yet.
Or how about the AC97 codec on most PCs. Is the performance limiting of
conventional sound cards the crummy clock, or the codec performance, or the
noise from the environment.
Several of the other M-Audio allow an external "word clock" which
the pro audio world uses to lock the sampling rates of multiple cards.
It's a shame that feature isn't on the Delta 44 as it would allow an
easy way to use an external reference.
I think the external pilot injection and tracking, in the long run, has
more potential, because it works with ANY sound card. It's an easy matter
to divide down your system reference clock and make a quiet cal tone.
James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875
