[TenTec] Attn vs RF Gain

[Dr. Gerald N. Johnson]

On Wed, 2006-08-02 at 15:06 -0400, jcharley at bellsouth.net wrote:
> Gentlemen and Ladies.
> Maybe a silly question but ...
> Is there an actual difference in results if I reduce RF gain or use the attenuator on my Corsair II.
> In researching the topic on the reflector I see both actions listed and in different orders,
> Attenuator first then RF gain, and vice versa.  Any science here or just personal preference?
> Thanks.
> 73/Crit/K4BXN
> 
> 
The Attenuator switch in the Corsair II is a misnomer. Its not an
attenuator. It literally switches the RF stage out of the circuit and
connects the input bandpass filters directly to the mixer. By reducing
the receiver RF gain in a chunk (and raising the noise figure) it
effectively is an attenuator but technically it is not. And on the lower
HF bands unless you are listening with a very inefficient antenna (like
a Beverage) that RF stage does nothing but make the S-meter rise higher
and make the receiver a bit more susceptible to adjacent channel
interference because of the stronger unwanted signals at the mixer
input.

Now that I've studied the manual and its schematics I find the "RF Gain"
is not truly and RF gain but is an IF gain. It controls the gain of the
first two IF stages in the fixed IF after the bandpass tuning module and
filters. So it reduces the receiver gain without affecting the input
noise floor or the maximum unwanted the receiver will handle. Though
with the IF gain low enough the front end noise no longer overrides the
IF noise and it does effectively reduce the receiver sensitivity along
with the gain.

So if you just want to enhance the output S/N and aren't noticing any
adjacent channel signal splatter (providing its not in the offending
transmitter), either or both will help. If you are getting signals
strong enough to overload the receiver front end, the "Attenuator" will
help and the RF gain will have no effect on improving the rejection of
those unwanted signals. Which means that what should be used first
depends on the situation on the band at that moment.
-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer