But on the SR-40 the oscillator is fixed! if you want to listen to 7.058 you have 2KHz audio coming out, the only thing you can do is change the crystal (good luck) or use a DDS instead of the built in fixed crystal.
If you want to listen to a signal on 7.0563, good luck!
Now, if you are changing the crystal, try to get the frequency of interest away from 0 Hz.
At 08:44 AM 9/30/2005, you wrote:
--- In softrock40@..., Ramakrishnan Muthukrishnan
<vu3rdd@g...> wrote:
I couldn't understand how the mixing down to the magic number 11250
is
obtained.. Would you mind explaining it a bit more?
Sorry, typo. it's 11025. I got it right in a subsequent paragraph :-)
What I thought was that the signal at 7.056 gets downconverted to
zero
IF and because of the soundcard sampling rate of 48 khz, we are able
to tune +-24khz..
11025 * 2 = 22050.
22050 * 2 = 44100.
11025Hz is the center of the presumed "good" passband on typical
soundcards capable of handling audio CD rates.
You don't want to downconvert to 0 because most soundcards have
bandpass filtering that roll off towards 0 and the Nyquist frequency.
If you downconverted to 0, you'd be filtering off most of the signal
before it got to your signal processing.
Therefore you downconvert such that the signal of interest is in the
"best" part of the soundcard passband.
The subsequent mixing stage (complex oscillator at -11025 or whatever)
then moves the signal down to 0 once it's in digital form. Then the
remainder of the processing takes place.
73
Frank
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