Icom IC-R5 Service Manual Download Page 8

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(3) 30–117.995 MHz, 175–329.995 MHz
The 30 MHz–117.95 MHz, 175 MHz–329.995 MHz signals
pass through the band switching circuit and low-pass filter
(C40–C43, C665, C666, L9, L10, L92), and are then applied
to the RF amplifier (Q36). The amplified signals are amplified
at the RF amplifier (IC11, pin 1) via band switching diodes
(D34, D71). The amplified signals are applied to the 1st mixer
circuit (IC1, pin 1).

(4) 833 MHz–1309.995 MHz
The 833 MHz–1309.995 MHz signals pass through the band
switching diode (D13) and bandpass filter (C5, C45–51,
L11–L13, L43), and are then applied to the RF amplifier
(Q26). The amplified signals are amplified at the RF amplifi-
er (IC11, pin 1) via band switching diodes (D36). The ampli-
fied signals are applied to 1st mixer circuit (IC1, pin 1).

4-1-3 1ST MIXER CIRCUIT (RF UNIT)

The 1st mixer circuit converts the received RF signals to a
fixed frequency of the 1st IF signal with a PLL output fre-
quency. By changing the PLL frequency, only the desired fre-
quency will pass through the bandpass filters at the next
stage of the 1st mixer.

The filtered RF signals are mixed with 1st LO signals at the
1st mixer circuit (IC1) to produce a 266.7 MHz 1st IF signal.
The 1st IF signal is output from pin 6, and passed through the
bandpass filter (FI1) to suppress unwanted harmonic com-
ponents. The filtered 1st IF signal is applied to the 2nd mixer
circuit.

The 1st LO signals are generated at the 1st VCO (Q28, Q30,
D54) and are applied to the 1st mixer (IC1, pin 3) directly or
passing through the doubler circuit (Q31) after being ampli-
fied at the buffer amplifiers (IC4, Q40).

4-1-4 1ST IF AND 2ND MIXER CIRCUITS (RF UNIT)

The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal.

The filtered 266.7 MHz 1st IF signal from the bandpass filter
is mixed with the 2nd LO signal at the 2nd mixer circuit (IC10,
pin 1) to produce a 19.65 MHz 2nd IF signal. The 2nd IF sig-
nal pass through (except WFM mode) or bypass (WFM
mode) the bandpass filter (FI3), and is then amplified at the
2nd IF amplifier (Q5). The amplified signal is applied to the
demodulator circuit.

4-1-5 DEMODULATOR CIRCUITS (RF UNIT)

The demodulator circuit converts the 2nd IF signal into AF
signals.

The 19.65 MHz 2nd IF signal from the 2nd IF amplifier (Q5)
is applied to the 3rd mixer section of the FM IF IC (IC2, pin
16) and is then mixed with the 3rd LO signal for conversion
into a 450 kHz 3rd IF signal.

IC2 contains the 3rd mixer, limiter amplifier, quadrature
detector and S-meter detector, etc. A frequency from the PLL
reference oscillator is used for the 3rd LO signal (19.20
MHz).

(1) FM mode
The 3rd IF signal is output from the FM IF IC (IC2, pin 3) and
passes through the ceramic bandpass filter (FI2). The filtered
signal is fed back and amplified at the limiter amplifier section
(pin 5), then demodulated AF signals at the quadrature
detector section (pins 10, 11) and detector coil (L21). The
demodulated AF signals are output from pin 9 and are
applied to the AF circuit (LOGIC unit) via the “FMDET” signal.

Limiter
amp.

3rd IF filter
450 kHz

PLL IC

IC3

19.2 MHz

RSSI

IC2 TA31136F

2nd IF (19.65 MHz)
from Q5

"RSSI" signal to the CPU
(LOGIC unit; IC1, pin 4)

Active
filter

FI2

Noise