Beat Frequency Oscillator

Posted on Feb 6, 2014

The beat frequency oscillator (bfo) is necessary when you want to receive cw signals. Cw signals are not modulated with an audio component, you remember, so we must provide one. The action of the rf amplifier, mixer, local oscillator, and IF amplifier is the same for both cw and AM; but the cw signal reaches the detector as a single frequency sign

Beat Frequency Oscillator
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al 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 is obtained from an oscillator known as a beat frequency oscillator. The picture below is a block diagram of a superheterodyne receiver capable of receiving and demodulating a 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. If the intermediate frequency is 455 kilohertz and the beat frequency oscillator 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 the front panel of a receiver. When you vary the bfo control, you are varying the output frequency of the bfo and will hear changes in the tone of the output audio signal. The sensitivity of a receiver is maximum when no signal is being received. This condition occurs, for example, when a receiver is being tuned between stations. At this time background noise is picked up by the antenna, and you will hear noise greatly amplified. This noise is highly annoying and occurs because receiver gain is maximum without a signal. You can often overcome this problem by using a circuit called a SQUELCH, NOISE SILENCER, NOISE SUPPRESSOR, or NOISE LIMITER. All of these noise type circuits...

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