In a message dated 98-05-24 14:14:35 EDT, N4KG writes:
<< Actually, it is not necessary to turn the AGC off. Reducing the RF
gain raises the AGC threshold as indicated by the S-meter position
with no signals (or high level noise) present. This leaves the entire
receiver in the LINEAR region where any increase (or decrease) in
signal level results in a corresponding INSTANTANEOUS increase (or
decrease) in audio level.
>>
The "RF Gain" control in today's receivers is actually RF + IF Gain control.
The ideal situation with RX gain control is to have separate gain control in
the first RF stage(s) and IF stages. The best performing RX in the tube era I
had was modified Drake R4B, where I replaced preselector tube with 6EJ7 (or
was it 6EH7?) and had sepaate RF gain control in cathode. This gave abt 15 dB
extra gain needed especially on 160 and 10 m and ability to keep the band
noise level around the S1 mark. Ability to adjust RF and IF gain separately
allows to tailor the RX response as needed for weak in the noise signals.
Makes difference in hearing/not hearing the really weak ones.
With new solid state receivers this can be roughly emulated by using RF
attenuator, it is not as slick as continuous gain adjustment but better than
nothing. Another way of doing it is to use RF preamps, especially on 160 with
receiving antenas and having set the gain similar to TX antenna, and then use
RF gain or attenuator to bring them all down to band noise treshold. Helps to
save ears and controls when doing quick switching between different RX
antennas.
Looks like there is still room for muti stage - tuned circuit preselectors to
be put in the front of bell and redundand whistles radios of today.
Yuri VE3BMV, K3BU/7 at Signal Hill, Teton Park, breathtaking WY
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