Elenco SEE AMFM108CK - Manual (Page 18)

SECTION 3
-17-
AM DETECTOR BANDWIDTH TEST
Figure 16
kHz kHz
kHz
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 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.
The purpose of the second IF amplifier 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 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.
SECOND AM IF AMPLIFIER