Ratio Detector Alignment - Elenco Electronics AM/FM-108TK Assembly And Instruction Manual

DYNAMIC MEASUREMENTS
AC GAIN
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 31.
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 10.7MHz 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
Generator
Hz

RATIO DETECTOR ALIGNMENT

METHOD #1
ALIGNMENT WITH NO TEST EQUIPMENT
With an alignment tool or a screwdriver, turn both coils
T4 and T5 fully counter-clockwise until they stop. DO
Generator
.001µF
Hz
TP15
.001µF
TP15
on the scope. With an alignment tool or screwdriver,
adjust T4 for a peak. Reduce the generator input to
maintain 150mVpp on the scope.
probe to the base of Q6 and record the voltage here:
Turn the power OFF. The AC gain can be calculated as
follows:
Your calculated answer should be approximately 20.
Figure 31
NOT FORCE THE COILS ANY FURTHER. Now turn
both coils in about 1 1/4 to 1 1/2 turns.
Figure 32
-36-
Move the scope
Vb = __________ mVpp.
AC GAIN = 150mV / Vb
Oscilloscope
TP15
Oscilloscope
TP15