# Receivers

The four fundamental qualities of any receiver are selectivity, sensitivity, frequency stability and bandwidth. A very simple receiver is the tuned radio frequency (t.r.f.) receiver, which consists of an RF amplifier to select and amplify the required signal, followed by a detector to change the RF signal to an AF signal. This signal is then fed to an audio amplifier which drives a loudspeaker.

The superheterodyne receiver has a mixer whereby the RF signal is mixed with a signal from a local oscillator to give a fixed intermediate frequency. This, however, can create a problem of second channel interference. A high value of IF will reduce the possibility of this interference but will give decreased selectivity. A double superheterodyne receiver operates with two different IF’s, thus giving the advantages of low probability of second channel interference, and high selectivity.

The purpose of a detector is to demodulate. The diode (or envelope) detector is probably the simplest detector. A product detector is used for the reception of SSB signals and is essentially a mixer circuit. The carrier (suppressed in an SSB transmission) must be re-inserted in an SSB receiver and is done by a carrier insertion oscillator.

A beat frequency oscillator (BFO) is used for the reception of c.w. signals and has an output frequency of the IF, but can be varied each side of this by about 3 kHz.

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