Receiver circuit basics.

Basic block diagram.

RF Amp.
This amplifies all signals within the band that the receiver operates on. The gain of this must be just right, as not enough gain will mean that it does not pick up very far but too much gain will overload the following circuits.

VFO
The VFO creates the signal for the first mixer to combine with the incoming signal. This oscillator shifts frequency to control the frequency the receiver is tuned to

1st Mixer
This mixes the incoming signal with the output of the VFO, producing sum (Fin + VFO) and difference (Fin - VFO or VFO - Fin) frequencies.

Osc
This fixed frequency oscillator creates a signal for the second mixer to combine with the output of the first mixer.

2nd Mixer
This mixes the first IF signal with the output of the Osc, producing sum and difference signals.

Filtering
Although not shown on the diagram, filters will usually be included before the RF amp and after each mixer stage.

IF Amp.
At this point only the wanted signal remains, although it is now at the 2nd IF (usually 455KHz). This must now be amplified to a suitable level for the detector stage.

Level detect
This stage produces a DC signal proportional to the incoming signal level, which may be used to control gain, operate the squelch and give the user a meter reading.

AGC
This controls the gain of the RF amplifier stage, reducing the gain when strong signals are received.

Audio Detect
This stage removes the carrier, passing just the audio into the following stages.
For AM, a couple of diodes is sufficient to detect the audio. Most modern FM CBs simply use an IC, which has all the required circuits built in. Some older UK CBs used separate components to detect FM sound.

Squelch
When there is no signal, the squelch circuit cuts off the sound. This is particularly useful with FM equipment, where the constant background noise could drive you crazy.

Audio Amp
This amplifies the output from the detector to a level high enough to drive a speaker.

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