[Elecraft] K2 RX audio output - again

[Geoffrey Mackenzie-Kennedy]

Ron AC7AC wrote:

Going waaaaaay back to when I was first building receivers, that RF gain
actually controlled the gain of the RF amplifier (preamplifier in modern
jargon) at the input to the receiver as well as the gain of the intermediate
frequency (i.f.) amplifiers ahead of the detector. Nowadays in
state-of-the-art receivers like the K2 we control the gain at the input by
switching the rf amplifier (preamp)into our out of the signal path or we can
add an attenuator (ATT) at the input to reduce the signal gain reaching the
receiver. The RF gain now controls only the gain of the i.f. amplifier, but
the term "RF Gain" is still used to describe that control (after all, the
intermediate frequency is still an RF signal).

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Digressing from the subject of audio output for a moment, having the "RF
Gain" control the gain of the IF amplifier only can lead to some
undesireable effects unless the IF amplifier design is done very carefully
using suitable devices. The problem is that the linearity of some of the
more popular low cost amplifier chips actually gets worse as their gain
starts to be reduced, although some types of these chips will recover as the
gain is further reduced. Assuming a situation where the Preamp is "Off" and
the Attenuator is "In" leaving two or more signals are in the passband, some
strong and the one that you want is weak, any negative effect on linearity
caused by changing gain is of course unwelcome. The overall effect is
determined by the receiver's Gain Distribution and the characteristics of
the stages both before and after the controlled IF amplifier.

Many modern (not ham) HF receivers designed to handle *very* strong signals
while allowing copy of an adjacent signal close to the noise still do use
"RF Gain" to control some type of front end variable attenuator, and a 
strong
RF amp with fixed but very low gain embedded in reasonably high loaded Q
variable frequency tuned circuits. But these receivers consume a lot of
power.

73,
Geoff
GM4ESD