Elenco AM Radio Kit - Manual (Page 29)

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In a superheterodyne type receiver the radio wave at the
antenna is amplified and then mixed with the local
oscillator to produce the intermediate frequency (IF).
Transistor Q1 not only amplifies the RF signal but also
simultaneously oscillates at a frequency 455kHz above
the desired radio station frequency. Positive feedback
from the collector to the emitter of Q1 is provided by coil
L2 and capacitor C3. During the heterodyne process, the
following four frequencies are present at the collector of
Q1.
1. The local oscillator frequency, LO.
2. The RF carrier or radio station frequency.
3. The sum of these two frequencies, LO + RF.
4. The difference of these two frequencies, LO - RF.
The “difference frequency” is used as the intermediate
frequency in AM radios. The collector of Q1 also
contains an IF transformer (T1) tuned only to the
difference frequency. This transformer rejects all
frequencies except those near 455kHz. T1 also couples
the 455kHz signal to the base of Q2 to be processed by
the IF amplifiers.
The antenna and the oscillator coils are the only two
resonant circuits that change when the radio is tuned for
different stations. Since a radio station may exist 455kHz
above the oscillator frequency, it is important that the
antenna rejects this station and selects only the station
455kHz below the oscillator frequency. The frequency of
the undesired station 455kHz above the oscillator is
called the image frequency. If the selectivity of the
antenna (Q factor) is high, the image will be reduced
sufficiently.
The oscillator circuit must also change when the radio
is tuned in order to remain 455kHz above the tuning of
the desired radio station. The degree of accuracy in
keeping the oscillator frequency exactly 455kHz above
the tuning of the antenna is called tracking accuracy.
MIXER AND OSCILLATOR
THEORY OF OPERATION
SECTION 5
ASSEMBLY INSTRUCTIONS - ANTENNA, MIXER AND OSCILLATOR
L1 - Antenna with Holders
(see Figures K & L)
C2 - .02
μF or .022μF Discap
(marked 203 or 223)
TP1 - Test Point Pin
(see Figure F)
R2 - 12k Ω Resistor
(brown-red-orange-gold)
R3 - 3.3k
Ω Resistor
(orange-orange-red-gold)
White
Black
Red
White
Black
Red
Green
} }
}
R=9 - 11 Ω
R=1 - 1.5 Ω
R=9 - 11 Ω
R=1 - 1.5
Ω
4 Wire 3 Wire Figure K
Determine if you have a three wire or four wire coil. Resistance
measurements will be used to check the configuration of the coil. Slide one
holder off the ferrite core of the antenna assembly. Then slide the coil off
the the ferrite core. Measure the resistance of the coil. Your readings should
match the approximate values as shown.
Note: If the end of a wire from the antenna should break off,
strip the insulation off the end with a hot soldering iron.
Lay the wire down on a hard surface and stroke the
wire with your iron. The insulation should come
off very easily. CAUTION: The soldering
iron will burn the hard surface that you are
working on.
}
R1 - 56k
Ω Resistor
(green-blue-orange-gold)
L2 - Oscillator Coil (red)
Q1 - 2N3904 Transistor NPN
(see Figure A)
C3 - .01
μF Capacitor
(marked 103)
C1 - Tuning Gang Capacitor
2 Screws M2.5 x 3.8mm
Knob (dial)
Screw M2.5 x 8mm
Label (dial knob)
(see Figure M)