Q-multiplier

[talen]

Daniel, good deal on the Advanced exam....

A Q-multiplier (should be in any ARRL Handbook) is basically a
very sharp (high Q) tuned circuit with feedback added after the
last IF stage in a receiver (it's tuned to the IF frequency....that's
why you can't just use any Q-multiplier on any receiver). The
sharply tuned circuit can be used in "Peak" mode (enhance the
desired signal) or "Null" mode (reduce the unwanted signal). As
an example of how well they work.....a given receiver IF with a
1000Hz passband at -3db might have a 100Hz passband at -3db
with a Q-multiplier. At -60db it may go from a 4000Hz passband
to 500Hz with a good Q-multiplier. They are very usefull in
"Peak" mode for CW/RTTY, etc (narrow bandwidth signals) and
good in "Null" mode for SSB (I've found that they are a little too
sharp for SSB in "Peak" mode...and they will require hands-on
tuning, but are still very useable).

Most receivers today use filters which have VERY sharp sides
to accomplish the same selectivity, but they are potted, you
can't see what's inside, you couldn't duplicate it in your shop if
you could see inside, and cost lots. Crystal filters, sharp IFs,
high Q frontends, and Q-multipliers are fun, cheap, work, and
you learn something.

Go for it. First thing is get an ARRL Handbook or Bill Orr's Radio
Handbook (very good).

73s  Kees K5BCQ

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