RF - Gain / AGC

 
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überarbeitet am 21.10.2010

In communication receivers, the audio volume (AF-Gain, volume) and the high frequency amplification (RF-Gain) can be regulated independently.
Excessive high frequency gain can cause excessive noise and distortions of the received signal on one hand, as well as intermodulations, that will cause "ghost stations" to appear on strange places on the dial.

The high frequency amplification gain should be turned up only as much, that a sufficient signal strength will reach the demodulation stage.
Usually, the regulation is fully automatically controlled by means of the so-called AGC (Automatic Gain Control) circuitry, the automatic RF-gain control. Particularly regular fluctuations the signal strength due to the wave propagation, the so called "fading" typical for short wave propagation, requires rapid regulation of the high frequency amplification gain. Only with this, continuously changing audio volume can be avoided.

In fading situation with rapidly changing signal strengths, it can happen, that the AGC circuitry will up-regulate the RF-gain in the moment when signal strength is rising and down-regulate in the moment when signal strength fades. To prevent this so-called "pumping", in some high end receivers, You can switch different time constants of the AGC circuit.

Under normal conditions, the AGC should be activated. Normally a FAST AGC-time constant with a short attack and slightly longer decay time is suitable for bandcruising. In some cases, when the time constant is similar to the frequency of the fading, the ACG should be switched to SLOW, the decay time will be prolonged. Also for SSB and CW (morse code) reception, the AGC should be set to SLOW, to avoid more noise by up-regulation of the RF-gain in moments of silence between spoken words or sentences or between the morse code pips.

Usually the automatic regulation performs quite well. In some special cases, the AGC can be switched off and You can adjust the RF-gain completely by hand with the RF gain knob.
When RF-Gain is turned too low, You won't hear anything. When the gain is turned up too much, the audio will suffer from severe distortiondue to overloading of the demodulation stage.

When using long or very high gain antennas, the input voltage can be too high and will cause overloading of the receiver front end.
Especially smaller travel radios won't support high RF signal voltages of an outdoor aerial and will react with distortion due to overloading. In many receivers You find an attenuator or a DX / LOCAL switch (gives attenuation of the signal when switched to local) as a useful remedy against overloading problems.

Also in commercial grade communication receivers, sometimes an attenuator is provided in case of possible overloading from very long antennas.

Take care of the RF gain control knob or an inadvertedly activated attenuator: it may cause Your receiver appearing completely deaf...