At 06:03 PM 7/4/2005, John Ackermann N8UR wrote:
Jim Lux wrote:
>
>
> Consider two RF carriers, at 10.001 and 10.002 MHz
>
>
> changes) and tuning high or low with SSB (which does not).
And this is where I'm confused. I'm not (for this experiment) looking
at the linearity of the passband, but rather the absolute accuracy of
the frequency transformation.
The change in pitch is what I'm measuring -- if everything is perfect, I
know that an input precisely on the frequency the radio is tuned to will
yield an output (audio tone) that's precisely 600Hz. If the sampling
rate is off, the 600Hz tone will be off, which translates into a
frequency error -- if the tone is 599Hz, that's the same as the radio
being tuned 1Hz low in frequency. That's the frequency error I'm trying
to measure.
But how are you measuring it? By looking at a .WAV file (or equivalent)
with the digitized data, or by looking at an analog signal generated by the
sound card? If the former (or, the equivalent, looking for the peak in the
spectrum display), then you are measuring sound card sampling rate
error. If you're looking at the analog output of the sound card, which is
essentially the "looped back" analog input to the card, then the sampling
rate may not make any difference.
John
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
