Elenco AM Radio Kit - Manual (Page 22)

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 19). 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 19.
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 19 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.
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Figure 19
AM DETECTOR AND AGC STAGES
THEORY OF OPERATION
SECTION 2