# Receivers

## Direct-conversion receivers

The basis of the superhet is the conversion of all signals to a fixed intermediate frequency, followed by demodulation and AF amplification to drive a loudspeaker. The incoming signals can, however, be converted directly to AF, and thus a much simplified type of superhet results. This is known as the 'synchrodyne'. It is not a new principle but has become popular in amateur radio as the direct-conversion receiver.

The local oscillator operates very close to the signal frequency so that the output of the mixer (which is equivalent to a product detector) is in the AF range. This is normally followed by a low-pass filter to restrict the audio bandwidth to about 3kHz and a high-gain audio amplifier to drive a loudspeaker.

The mixer is usually preceded by a simple untuned RF stage. Due to the difficulties of making a sufficiently stable oscillator, the direct-conversion receiver is often restricted to the lower-frequency amateur bands where it is capable of surprisingly good performance, although obviously not in the same class as a good-quality communication receiver.

The oscillator followed by a buffer amplifier can also be used to drive a power amplifier, thus creating a low-power transceiver, and a number of these are now available commercially.

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