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FM RF AMPLIFIER, MIXER AND OSCILLATOR
In a superheterodyne receiver, the radio waves are
emitted and then mixed with the local oscillator to
produce the intermediate frequency (IF). The first stage
is the RF amplifier which selects a radio station and
amplifies it. The second stage is the local oscillator
which oscillates at a frequency 10.7MHz above the
desired radio station frequency. The third stage is the
mixer stage where the amplified radio waves are
heterodyned with the local oscillator. During the mixing
process, a difference frequency of 10.7MHz is
produced. This difference frequency is used as the IF
in FM radios. The collector of transistor Q3 contains an
IF transformer (T1) which is tuned only to the difference
frequency.
This transformer rejects all frequencies
MIXER ASSEMBLY INSTRUCTIONS
R10 - 10kΩ Resistor
(brown-black-orange-gold)
C12 - .005µF Discap (502)
R8 - 22kΩ Resistor
(red-red-orange-gold)
R7 - 6.8kΩ Resistor
(blue-gray-red-gold)
TP13 - Test Point Pin
(see Figure A)
SECTION 9
except those near 10.7MHz.
10.7MHz signal to the first FM IF amplifier. The RF
amplifier and the oscillator are the only two resonant
circuits that change when the radio is tuned for different
stations.
above the oscillator frequency, it is important that the
RF stage rejects this station and selects only the station
10.7MHz below the oscillator frequency. The frequency
of the undesired station 10.7MHz above the oscillator is
called the image frequency. Since this FM receiver has
an RF amplifier, the image frequency is reduced
significantly.
decouple the voltage of the tuner from the voltage of the
IF stages.
-42-
T1 also couples the
Since a radio station may exist 10.7MHz
The resistor R9 and capacitor C12
R9 - 100Ω Resistor
(brown-black-brown-gold)
TP12 - Test Point Pin
(see Figure A)
CAUTION:
not touch can of T1 FM Mixer
Coil.
Q3 - 2N3904 Transistor
(see Figure C)
R11 - 1.8kΩ Resistor
(brown-gray-red-gold)
Test Point must
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