-----Original Message-----
From: Brett gazdzinski <[EMAIL PROTECTED]>
Just what drives the choice of 15 KHz?
Here's my semi-educated guess. Correct me if I'm wrong, folks!:
The reason for the conversion to a low last-IF is to feed the DSP
filter-decoder system.
The lower you go, the better, because you get more samples per Hz of
signal. (If you are
sampling a 15 kHz signal 150,000 times per second, that's 10,000
samples per Hz, but if you
were to sample a 150 kHz signal the same number of times per second you
only get 1000
samples per Hz.) All else being equal, more samples per Hz is better,
as is more bits per sample.
But increasing either means more processor power is needed.
You can't go much lower than 15 kHz without getting down into audio.
Plus you also
have roofing-filter issues at low IFs (if you tried to convert from the
first IF to, say, 5 kHz, the
oscillator is only 5 kHz from the filter passband, and the secondary
image is only 10 kHz away.)
Every design is a series of tradeoffs. 15 kHz is the optimum tradeoff
for all these issues given the
available parts and other design issues.
---
You are probably familiar with receivers of the 1950-60s era which used
a last IF in the 50 to 250
kHz range. From the Hallicrafters SX-88/S-76 to the Drake R4B and many
in between, this was
done because it allowed a reasonable number of practical LC circuits to
provide the selectivity.
Again, a tradeoff - lower IF was better for selectivity but made the
secondary-image problem
worse, while a higher IF meant more tuned circuits were needed.
The introduction of practical high frequency xtal filters ended that
design.
73 de Jim, N2EY
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