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 11). 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 capacitors C9 and C10 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 R11, R12 and the volume control. The
discharge time constant for this circuit 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 at the volume control as shown in Figure 11.
The purpose of the automatic gain control (AGC) circuit
is to maintain a constant audio 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 11 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 three (TP3), the audio is removed by a low
pass filter, R11 and C4, leaving only the DC
component. Resistor R5 is used to shift the voltage at
TP3 high enough to bias the base of transistor Q2 to
the full gain position when no signal is present.
Resistors R5 and R11 also forward bias diode D1 just
enough to minimize “On Condition” threshold voltage.
-16-
SECTION 2
Figure 11
AM DETECTOR AND AGC STAGES
THEORY OF OPERATION
Summary of Contents for SUPERHET AM-550TK
Page 31: ... 30 SCHEMATIC DIAGRAM ...
