Receivers

A complete receiver

A block diagram of a double-superhet receiver incorporating the points discussed so far in this chapter is shown in Fig 5.12. For obvious reasons this is known as a 'communication receiver' - it is intended for communication purposes and not entertainment.

Fig 5.12. Block diagram of double super heterodyne receiver

Consideration of Fig 5.12 raises the query as to which local oscillator is tuned. In fact, either can be tuned as follows:

(a) LO1 tuned and LO2 fixed frequency;
(b) LO1 fixed frequency and LO2 tuned.

For stability reasons, the fixed-frequency oscillator should be crystal controlled.

The arrangement (a) allows, by appropriate design of the RF/mixer/local oscillator tuned circuits, each amateur band to be spread over the whole of the tuning scale which is obviously a very convenient arrangement.

In arrangement (b) the RF/mixer/local oscillator tuned circuits are designed to cover a small range, usually 500kHz. Each amateur band apart from 28MHz can be covered in one such range: for full coverage of the 28MHz band four 500kHz segments are required.

The second local oscillator frequency (crystal controlled) is chosen to convert the above RF ranges to the IF range required, which may be for example 5000-5500kHz. The rest of the receiver is therefore a single-superhet having a single tuning range of 5000-5500kHz.

The intermediate frequency would be typically 455kHz and the IF amplifier would ideally contain crystal filters having bandwidths of about 0.4kHz, 3kHz and 6kHz which would be switched in to suit the mode in use.

This is a very common arrangement - it provides a constant tuning rate on each band which is slow enough for satisfactory tuning of an SSB signal, ie a tuning rate of no more than 25kHz per revolution of the tuning knob.

Photos 5.1 & 5.2. Two HF communications receivers. The left picture is of an analogue tuned receiver from the mid 1970s (FRG7), on the right its present day counterpart (FRG100).