Elenco Electronics AM/FM-108CK, Сборка и инструкция по эксплуатации, Страница: 16 / 56

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SECTION 2
AM DETECTOR AND AGC STAGE
Figure 12
Switch
J2 - Jumper Wire
(use a discarded lead)
The purpose of the automatic gain control (AGC) circuit
is to maintain a constant level at the detector,
regardless of the strength of the incoming signal.
Without AGC, the volume control would have to be
adjusted for each station and even moderately strong
stations would clip in the final IF amplifier causing audio
distortion. AGC is accomplished by adjusting the DC
bias of the first IF amplifier to lower its gain as the signal
strength increases. Figure 12 shows that the audio at
the top of the volume control is actually “riding” on a
negative DC voltage when strong signals are encountered. This negative DC component
corresponds to the strength of the incoming signal. The
larger the signal, the more negative the component. At
test point five (TP5), the audio is removed by a low pass
filter, R36 and C32, leaving only the DC component.
Resistor R35 is used to shift the voltage at TP5 high
enough to bias the base of transistor Q8 to the full gain
position when no signal is present. Resistors R35 and
R36 also forward bias diode D4 just enough to minimize
“On Condition” threshold voltage.
The purpose of the detector is to change the amplitude
modulated IF signal back to an audio signal. This is
accomplished by a process called detection or
demodulation. First, the amplitude modulated IF signal
is applied to a diode in such a way as to leave only the
negative portion of that signal (see Figure 12). The
diode acts like an electronic check valve that only lets
current pass in the same direction as the arrow (in the
diode symbol) points. When the diode is in conduction
(On Condition), it will force the capacitors C33 and C38 to charge to approximately the same voltage as the
negative peak of the IF signal. After conduction stops
in the diode (Off Condition), the capacitors will
discharge through resistors R36 and R42. The
discharge time constant must be small enough to follow
the audio signal or high frequency audio distortion will
occur. The discharge time constant must be large
enough, however, to remove the intermediate
frequency (455kHz) and leave only the audio as shown
in Figure 12.
ASSEMBLY INSTRUCTIONS
1/8”