2-23
be received, the setting should be high (no agc until the signal level is high). However, you should set it
as 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 would
be developed from the detector diode, and there would be no delayed action. Or, if a signal diode were
biased to provide the delaying action desired, no signal would pass to the audio amplifier until the bias
was exceeded by the input signal.
Beat-Frequency Oscillator
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 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 is
obtained from an oscillator known as a beat-frequency oscillator.
Figure 2-20 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.
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 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.
Squelch
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 just clip the peaks of the noise spikes. Squelch will actually eliminate noise. Figure 2-21 is a
