Icom IC-22S PLL Synthesized 2-Meter Transceiver
Instruction Manual and Service Notes
11
RECEIVER
R.F. Amplifier
Antenna input or self contained antenna signals pass through switching diode D-40 located in the
PA section to the RF amplifier Q2 where it is amplified and passed to the R.F. filter section. Out-of-
band signals are attenuated by the band pass filters.
Mixer Filter
The amplifier signal is injected into Gate-i of the mixer Q-3. The LO frequency is also applied to Q3
where a resultant 10.7 MHz IF signal is derived. This signal is passed through a crystal BP filter
that greatly attenuates other in-band signals. The 10.7 MHz signal is again mixed with a second
local oscillator, Q8 operating at 10.245 MHz at mixer 2, Q4. The resulting mixer output is 455 kHz.
I. F. Amplifiers
Two ceramic filters provide the low frequency selectivity and the adjacent channel rejection needed
in today’s crowded repeater world. I.F. amplifiers Q5 and Q7 drive IC1 limiter. The signal is
detected by the ceramic discriminator.
Audio
Lower frequency audio components (300 Hz to 3 kHz) are amplified by Q1O and passed by Q11
active filter. These (desired) audio signals are adjusted to level by the volume control and amplified
to I watt power by IC-2.
Squelch
At point J-5, higher frequency discriminator noise is taken at a selected level by the R1 squelch
Control back via J4 and amplified by Q14 and Q13 rectified by D7 and applied to Q12’s base. Under
no signal conditions, when noise is high, this rectified voltage is high, and Q12 turns off Q11. The
reverse is true when a signal is of sufficient strength to reduce noise and the squelch opens
permitting the audio signal path to operate normally.
During transmit, positive voltage is fed to the Q9 base, silencing the audio system. After switching
back to receive, a delay in Q9 base voltage change provided by C-56 allows a silent transition.
PHASE LOCKED LOOP
Voltage controlled oscillator
Use of a clap oscillator in the form of a junction FET in Q8 gives an improved signal-to-noise, and
by use of other elements having excellent temperature characteristics, frequency stability of the
order of +/50 ppm/C is achieved.
Varicap diode D3 serves to broaden the range of frequency permitted and by contributing to the
linearity improves stability of the circuit as a whole.
Buffer amplifier
In Q5, a MOSEFT using very little feedback minimizes the effects of load variation, and the
necessary LO output of 400 mV of the main unit is obtained.
