Superhet radios.

What is Superhet?
Superhet, which is short for Super-heterodyne, is a type of radio where the incoming signal is converted to another frequency before the sound is recovered.
A dual Superhet or dual conversion Superhet receiver changes the frequency twice.

How is the conversion done?
The radio generates a second signal, which is mixed with the wanted signal to create the new frequency. The new frequency is called the Intermediate Frequency (IF), and carries all the sound information that was on the incoming signal.

Why use Superhet?
While it is possible to simply take a radio signal and directly detect the sound, the superhet system is much more practical.

Are there any bad points?
If the radio is not properly designed and filtered, it will pick up unwanted signals at the Intermediate Frequency or at an "image" frequency.
Also, Superhet receivers may radiate signals (this is what TV detectors may pick up) which, if at strong enough levels, can interfere with other equipment.
The parts required to convert the frequencies will cost more, but semiconductors are extremely cheap to large manufacturers.

What and where is this image?
The image frequency is a second frequency which a poorly designed receiver will treat as if it is tuned to it.
For CB with a single 455KHz IF, this would be 910KHz below your channel. Anyone using that frequency will come through as if on your channel... but they will not hear you.

Why use dual conversion?
To get the best out of a Superhet, the Intermediate Frequency should be low. The problem with this is that it places the image frequency close to the one you are tuned to - making it impossible to filter out.
The dual conversion uses a high IF before converting that to the lower IF. The unwanted "image" signal of the first conversion is easily stopped at the receiver input. The image from the second conversion, which would have been too close to stop, can now be stopped by a filter after the 1st conversion.

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.