Elenco SEE AMFM108CK - Manual (Page 42)

-41-
SECTION 9
FM RF AMPLIFIER, MIXER, OSCILLATOR, AND AFC STAGE
R10 - 10k
Ω
Resistor
(brown-black-orange-gold)
C12 - .005
μ
F Discap (502)
R8 - 22k
Ω
Resistor
(red-red-orange-gold)
R7 - 6.8k
Ω
Resistor
(blue-gray-red-gold)
TP13 - Test Point Pin
(see Figure A)
R9 - 100
Ω
Resistor
(brown-black-brown-gold)
TP12 - Test Point Pin
(see Figure A)
CAUTION: Test Point must
not touch can of T1 FM Mixer
Coil.
Q3 - 2N3904 Transistor
(see Figure C)
R11 - 1.8k Ω Resistor
(brown-gray-red-gold)
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 except those near 10.7MHz. T1 also couples the 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.
Since a radio station may exist 10.7MHz 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. The
resistor R9 and capacitor C12 decouple the voltage
of the tuner from the voltage of the IF stages.
MIXER ASSEMBLY INSTRUCTIONS