Beat Frequency Oscillator

Beat-Frequency Oscillator

 

The beat-frequency oscillator (bfo) is necessary when you want to receive cw signals. Cw signals arenot modulated with an audio component, you remember, so we must provide one. The action of the rfamplifier, mixer, local oscillator, and IF amplifier is the same for both cw and AM; but the cw signalreaches the detector as a single frequency signal with no sideband components. To produce an af output,you must heterodyne (beat) any cw signal with an rf signal of the proper frequency. This separate signal isobtained from an oscillator known as a beat-frequency oscillator.

 

Figure 2-20 is a block diagram of a superheterodyne receiver capable of receiving and demodulatinga cw signal. The bfo heterodynes at the detector and produces an af output. The detector (second detector)is used primarily because the mixer (first detector) is normally used as the source of agc.

Figure 2-20.—Placement of the beat frequency oscillator.

If the intermediate frequency is 455 kilohertz and the bfo is tuned to 456 kilohertz or 454 kilohertz,the difference frequency of 1 kilohertz is heard in the output. Generally, you will tune the bfo from thefront panel of a receiver. When you vary the bfo control, you are varying the output frequency of the bfoand will hear changes in the tone of the output audio signal.

 

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