[TenTec] jpreamp and rf gain

[John]

Thanks for your reply Jerry .  John kb2huk
----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at storm.weather.net>
To: <tentec at contesting.com>
Sent: Monday, January 21, 2008 12:12 PM
Subject: Re: [TenTec] jpreamp and rf gain


> 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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