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AM DETECTOR BANDWIDTH TEST
Connect your test equipment as shown in Figure 15.
Set the generator at 455kHz with 80% modulation at a
modulation frequency of 1kHz. Set the oscilloscope to
read .1 volts per division. Turn the power ON and set
the volume at the minimum. Increase the amplitude of
the generator until the signal on the oscilloscope is 4
divisions peak to peak. Check the signal to make sure
that it is free of distortion. Leave the frequency of the
SECOND AM IF AMPLIFIER
The purpose of the second IF amplifier is to increase
the amplitude of the intermediate frequency (IF) and at
the same time provide SELECTIVITY. Selectivity is the
ability to "pick out" one radio station while rejecting all
others. The second IF transformer (T8) acts as a
bandpass filter with a 3dB bandwidth of approximately
6kHz. The amplitude versus frequency response of the
second IF amplifier is shown in Figure 16.
Both IF amplifiers are tuned to a frequency of 455kHz
and only need to be aligned once when the radio is
assembled. These amplifiers provide the majority of the
gain and selectivity needed to separate the radio
generator at 455kHz, but increase the modulation
frequency until the output drops to .28Vpp. Record the
modulation frequency on the generator here:
This frequency should be greater than 5kHz. Turn the
power OFF.
SECTION 3
stations.
The gain at 455kHz in the second IF amplifier is fixed
by the AC impedance of the primary side of transformer
T8, and the DC current in Q9. The current in Q9 is set
by resistors R39, R40 and R41. Both C36 and C37
bypass the 455kHz signal to ground, making Q9 a
common emitter amplifier. The signal is coupled from
the first IF amplifier to the second IF amplifier through
transformer T7. The IF transformers not only supply
coupling and selectivity, they also provide an
impedance match between the collector of one stage
and the base of the next stage. This match allows
maximum power to transfer from one stage to the next.
kHz
kHz
kHz
Figure 16
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