On Sun, 2008-01-20 at 18:11 -0500, John wrote:
> Hello all ! I have a question . I was just listening to the TT net on
> 14.329 and some one made reference to listening to his Orion II with the
> RFgain way down and then putting on the radio's preamp . Does this procedure
> cancel each other out or what am I missing here ? I thought the preamp
> increases the noise along with the signal ? Thanks for setting me straight .
> John kb2huk
Partly. Could be the preamp has lower internal noise than the standard
RF stage. And less gain, so for the noise and nearby QRM conditions the
combination heard weak signals better and wasn't as often overloaded by
close by (frequency or location) strong signals. Mixers are nearly
always the weak link and minimizing the RF gain before the mixer can
often be a great benefit in rejecting strong unwanted signals (which
could include power line noise), but having no gain in front of a mixer
(which is inherently noisier than an amplifier stage) can set a signal
detection threshold higher than the strength of the desired signal.
Could also be that there is an RF gain setting that works without the
preamp just as well as the preamp on with lowered RF gain, but sometimes
"RF" gain also controls IF stage gains too and lowered RF gain sets up a
threshold condition where the noise is reduced while the not quite
really weak signal is copied just fine making for a better S/N ratio at
the speaker. There are many ways to run a receiver, and that's why it
has knobs and buttons to allow the user to optimize it for his own needs
and the conditions at the time.
73, Jerry, K0CQ
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