Icom IC-22S PLL Synthesized 2-Meter Transceiver
Instruction Manual and Service Notes
9
SECTION VI
Theory of Operation
TRANSMITTER
Microphone, pre-amplifier circuit
The pre-amplifier circuit is composed of Q30, Q29, in an NPN, PNP direct-coupled 2-stage amplifier
configuration. The low noise transistors used and application of a large amount of feedback in the
1st stage gives a high signal-to-noise ratio and high stability.
Since DC voltage is supplied through Rl39 to the microphone connector, the ICSM2 (electrolytic
condenser microphone) can be used also. C 166, R140, C65 constituting a which filter suppress
high frequency regeneration and C 163 provided between base and emitter of the 1st stage
transistor prevents oscillation due to regeneration.
Pre-amplifier output is through R132 to the microphone circuit.
IDC circuit (Instantaneous Deviation Control)
Passing of signals through narrow band filter stage can result in distortion if the signal is over-
modulated and consequent degrading of following channels. To give improved limiting
characteristics Q28, Q27, Q26, are connected in a 3-stage direct-coupled configuration that results
in less distortion and protects succeeding stages from the effects of excessive input.
Since feedback is supplied to the 3-stage DC circuit and the input impedance is low, the frequency
characteristic of the differentiation by R130 and C159 is improved. R124 through which the
feedback circuit connects to ground serves for adjusting the operating point of Q26 and insuring
symmetry of clipped waveforms. Dl 8 and D19 are temperature compensating elements for the 3-
stage DC. circuit.
The limiter output is close to square waves in form, and since it includes harmonics an active filter
is provided to eliminate anything over 3 kHz. To prevent the frequency deviation from becoming
too large as temperature increases, compensation is made by thermistor R13, after which
adjustment is made by R12 to narrow the frequency deviation range.
Frequency modulation, 10.7 MHz Oscillator
Because of the quartz crystal characteristics in the frequency modulator Q24, a non-Controlled VXO
configuration, the circuit is tolerant of temperature variations, and there is less drift. Output signals
from the IDC circuit are supplied to the anode of varicap diode D17. To improve the temperature
characteristics, temperature compensation is effected by thermistor Rl06 connected to the cathode
of D17. The signal is set to 10.7 MHz by L4 connected to the cathode of D17.
Transmission Mixer
1C3 includes a constant current circuit and differential amplifier. 1C9 provides balanced output
from the 10.7 MHz oscillator and drives the two inputs of the differential amplifier. Local oscillator
output enters via the constant current input and differential amplifier output is filtered by L36,
opposite phase components of the 10.7 MHz output and local oscillator cancel, and so signals
obtained at the secondary side of L36 are LO+/ 10.7 MHz. The required LO+ 10.7 MHz signals are
obtained from the Band Pass Filter following L36.
