Radio and Electronics Cookbook

9

A beat-frequency oscillator

Introduction

Many readers will know that, although they have a short-wave radio which covers at least one of the amateur bands (e.g. 7 MHz or 14 MHz), they are unable to listen to SSB or Morse signals. This is because the receiver lacks a Beat-Frequency Oscillator (BFO). We need the ‘carrier’ frequency of a BFO to replace the carrier that has been removed from the signal at the transmitter. When listening to Morse signals, the BFO signal ‘beats’ with the incoming signal to produce a note in the loudspeaker. If you are a musician, you will be familiar with the method of using ‘beats’ to tune one musical instrument from another; in the BFO, the beat frequency produced is the tone signal you hear.

In the more complex ...