Elenco AM/FM Radio Kit - Manual (Page 48)

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AC GAIN
Connect the RF generator and oscilloscope to the
circuit as shown in Figure 44. 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. With an alignment
tool or 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 = __________
BANDWIDTH
With the power turned OFF, connect your test
equipment as shown in Figure 44. 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 0.707 of its original value, 2.1 divisions
or 106mVpp. Record the frequency of the lower 3dB
drop-off point here:
Fl = _________MHz.
Increase the frequency until the voltage drops to
0.707 of its original value, 2.1 divisions or 106mVpp.
Record the frequency of the high frequency 3dB
drop-off point here:
Fh = ___________MHz.
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 = __________
Figure 44
GND
TP15
Oscilloscope
GND
TP15
Generator
.001 μ F
DYNAMIC MEASUREMENTS