Elenco AM/FM Radio Kit - Manual (Page 45)

DYNAMIC MEASUREMENTS
AC GAIN
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Figure 40
If you don’t have an RF generator and oscilloscope, skip to Section 7.
The AC gain of the ratio detector is set by the AC
impedance of the primary side of T4 and the current
through Q6. The current is set by R20, R21 and R22.
Capacitors C22 and C19 bypass the AC signal to
ground. Connect your RF generator and oscilloscope
to the circuit as shown in Figure 39.
Your scope probe must have an input capacitance of
12pF or less, otherwise the probe will detune T4
causing an incorrect measurement of the AC gain.
Set the generator for 10.7 MHz no modulation and
minimum voltage output. Set the scope to read
50mV/division. Turn the power ON and slowly
increase the amplitude of the generator until 3
divisions or 150mVpp are seen on the scope. With an
alignment tool or screwdriver, adjust T4 for a peak. Reduce the generator input to maintain 150mVpp on
the scope. Move the scope probe to the base of Q6
and record the 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 approximately 20.
Record your calculation:
AC Gain = __________
With an alignment tool or a screwdriver, turn both
coils T4 and T5 fully counter-clockwise until they stop. DO NOT FORCE THE COILS ANY FURTHER.
Now turn both coils in about 1¼ to 1½ turns.
RATIO DETECTOR ALIGNMENT
METHOD #1
ALIGNMENT WITH NO TEST EQUIPMENT
Figure 39 GND
TP15
Oscilloscope
GND
TP15
Generator
.001 μ F
GND
TP15
Oscilloscope
GND
TP15
Generator
.001 μ F