Icom IC-21 Instructions Manual page 13

SECTION МІ — THEORY OF OPERATION
1.
Receiver RF Amplifier and First Mixer (Module U—1) (В-006А)
The input signal is amplified by the field effect cascade amplifier, О1 and 02 (2SK19s),
and passed into the RF filter. This filter is a high О helicalized cavity resonator, with
excellent bandpass characteristics and shockproof construction. From the output of the
helical resonators the signal is fed to the gate of the first mixer, Q3 (2SK19), where it is
mixed with the output from the receiver crystal oscillator unit, which is fed into the soürce
of Q3 to obtain the 10.7 MHz. IF signal. The output of the mixer is fed through the first
ceramic filter (FL1 SFC 10.7MA) to the IF amplifier. All transistors in the receiver RF unit
are field effect transistors, which have the highest possible sensitivity and signal to noise
ratio and the lowest possible cross-modulation.
Receiver and Transmitter Oscillator and Modulator (Module U—9) (В-012А)
The receiver crystal oscillator is a fundamental oscillator circuit at 14 MHz. using Q1
(28C372). The trimmers in series with the crystals make it possible to alter the crystal
frequency several KHz. The output is taken from the emitter and tripled by Q2 (2SC372),
and again tripled by Q3 (2SC784). The output signal is then fed into the First Mixer in the
receiver RF unit, Q3 (U—1) (2SK19), giving it the first IF of 10.7 MHz. In the Transmitter
Crystal Oscillator transistor Q4 (2SC373) operates as a fundamental oscillator at a frequency
of 18 MHz. The output of this oscillator, taken from its emitter, is fed to the base of
transistor Q5 (25С373) which operates as a buffer amplifier between the oscillator and
phase modulator circuit. Trimmer capacitors are provided in series with each crystal which
allow the frequency of the oscillator to be varied by several KHz. Thus the final output
of the transmitter can be set precisely on the desired frequency.
The Phase modulation circuit consists of Varactor diode D2 (MV3047), R24, 26, 27, 28,
29, C26 and C27. Resistors R24 and R29 provide a bias voltage for the reverse biasing of
the Varactor diode D2. An audio voltage is coupled into this same bias point through the
1 mh inductor. This audio voltage is derived from the speech amplifier circuit (U—4).
The modulated signal is amplified by Об (2SC372), then this signal is fed into the
exciter.
Receiver Ist IF Amplifier, 2nd Mixer, 2nd IF р Limiter апа Discriminator
(Module U—2—1) (В--009)
The 10.7 MHz signal from the first ceramic filter in the RF unit is amplified by Q1
(2SC371) and fed to the gate of the 2nd Mixer Q2 (2SK19) through the 2nd ceramic
filter FL1 (SFC 10.7MA). The variable resistor in the emitter circuit of the First IF
transistor Q1 adjusts the IF gain and S meter sensitivity. The 2nd Oscillator (U—12) is
crystal-controlled and operates оп 10.245 MHz + 5 KHz, its signal is fed into the gate of the
2nd Mixer Q2 (2SK19) to obtain the 455 KHz IF signal, which is passed through the 3rd
ceramic filter (FL2-CFP 455B) and amplified by Q3, 4, 5, 6, 7, and 8 (2SC371s). The
output of Q8 is then demodulated in the discriminator circuit, consisting of L4, L5, and
D7 and D8 (13605),
2nd Oscillator and Meter DC Amplifier (Module U—12) (В-013)
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