2-23be received, the setting should be high (no agc until the signal level is high). However, you should set itas low as possible to prevent overloading of the last IF amplifier by stronger signals.Finally, you must have two diodes to obtain delayed agc. If only one diode were used, the agc wouldbe developed from the detector diode, and there would be no delayed action. Or, if a signal diode werebiased to provide the delaying action desired, no signal would pass to the audio amplifier until the biaswas exceeded by the input signal.Beat-Frequency OscillatorThe 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.SquelchThe sensitivity of a receiver is maximum when no signal is being received. This condition occurs, forexample, when a receiver is being tuned between stations. At this time background noise is picked up bythe antenna, and you will hear noise greatly amplified. This noise is highly annoying and occurs becausereceiver gain is maximum without a signal. You can often overcome this problem by using a circuit calleda SQUELCH, NOISE SILENCER, NOISE SUPPRESSOR, or NOISE LIMITER. All of these noise typecircuits just clip the peaks of the noise spikes. Squelch will actually eliminate noise. Figure 2-21 is a
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business