At 02:44 PM 11/21/2005, Robert McGwier wrote:
I completely agree with the external oscillator software fix for offset
and I even use it for experiments where I am tuned to 10 Mhz. I can't
figure out how to keep the reference tuned and the signal of interest
tuned with the single IF! Please fix my ignorance.
Bob
Don't measure the external reference with the radio. Measure it with an
external counter.
Same hardware as the usual disciplining system. here's a disciplining system:
Disciplined oscillator feeds a counter which is gated by a "precision"
reference (usually 1pps tick from GPS). Counts are processed through some
sort of filtering algorithm to generate a steering signal to try and drive
the oscillator to a particular frequency.
The disciplined oscillator is then used to lock yet another oscillator at
whatever frequency you need to use. (e.g. you multiply a 10MHz disciplined
oscillator up to 200 MHz)
The filter is needed because the usual GPS receiver has a 50 ns jitter in
the 1pps, so you want to average over many seconds.
===
here's the software solution
Measure the frequency of the 200 MHz oscillator in the SDR1000 (couple off
a low power tap somehow.. should be a simple mod..) using your precision
tick. For instance, drive a counter and latch it once a second with your
tick.
use the measured frequency in your "dial frequency to DDS frequency" algorithm.
Interestingly, you could probably just latch the LSBs of the counter (which
DOES have to run at 200 MHz, though), since the MSBs can be inferred (you
know the approximate frequency already).
You could also probably get away with using a prescaler. Almost certainly,
you could get away with running two or three low speed counters with
different prescalers in front of them. All you have to do is be able to
figure out what the "real count" is by looking at lowest common multiples.
The other approach is to beat the 200 MHz from the SDR1000 against an
external reference using a mixer, and then look at the mixer output. If
you have a 10MHz reference, for instance, you could beat the 200 MHz
against the 20th harmonic of the 10MHz, and then count that using the 1pps.
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
