Elenco Electronics AM/FM-108TK Assembly And Instruction Manual page 40

Generator
Hz
AC GAIN
Connect the RF generator and oscilloscope to the
circuit as shown in Figure 36. The scope probe must
have an input capacitance of 12pF or less otherwise the
probe will detune T3 resulting in a false reading of the
AC gain. Set the generator at 10.7MHz no modulation
and minimum voltage output. Set the scope to read
50mV per division and turn the power ON. Slowly
increase the generator until 150mVpp or 3 divisions are
seen on the scope.
screwdriver adjust T3 for a peak. Reduce the generator
until 150mVpp or 3 divisions are seen on the scope.
With an alignment tool or screwdriver adjust T3 for a
peak. Reduce the generator input to maintain 3
divisions on the scope. Move the probe to the base of
Q5 and record the input voltage here:
Vb = __________ mVpp.
Turn the power OFF. The AC gain can be calculated as
follows:
AC Gain = 150mV / Vb
Your calculated answer should be about 20.
Record your calculation:
AC Gain = __________
.001µF
TP15
With an alignment tool or
Probe
Figure 36
BANDWIDTH
With the power turned OFF, connect your test
equipment as shown in Figure 36. Set your generator
at 10.7MHz no modulation and minimum voltage
output. Set the scope to read 50mV per division. Turn
the power ON and slowly adjust the generator
amplitude until 150mVpp is seen on the scope. Realign
T3, if necessary, for maximum output while adjusting
the generator to maintain 150mVpp. Slowly decrease
the frequency of the generator until the voltage drops
.707 of its original value, 2.1 divisions or 106mVpp.
Record the frequency of the lower 3dB drop-off point
here:
Increase the frequency until the voltage drops to .707 of
its original value, 2.1 divisions or 106mVpp. Record the
frequency of the high frequency 3dB drop-off point
here:
The bandwidth of the 2nd IF can be calculated as
follows:
Bandwidth = Fh - Fl
Your results should be between 300 - 500kHz.
Record your calculation:
Bandwidth = __________
-39-
Oscilloscope
Fl = _________MHz.
Fh = ___________MHz.
TP15