Icom IC-R2 Service Manual Download

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4 - 2

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 FM IF IC (IC2, pin 3) and
passes through the ceramic bandpass filter (FI2). The fil-
tered signal is fed back and amplified at the limiter amplifier
section (pin 5), then demodulated AF signals at the quadra-
ture 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).

(2) WFM mode
The 3rd IF signal from the 3rd mixer bypasses the ceramic
filter (FI2) and fed back to the limiter amplifier section (pin 5).
The amplified signal is demodulated at the quadrature
detector section (pins 10, 11) and detector coil (L21). The AF
signals are output from pin 9 and are applied to the AF cir-
cuit (LOGIC unit).

By connecting R55 to R54 in parallel, the output character-
istics of pin 12, “RSSI”, change gradually. Therefore, the FM
IF IC can detect WFM components.

(3) AM mode
The filtered 3rd IF signal from the bandpass filter (FI2) is
amplified at the 3rd IF amplifier (Q1). The amplified IF signal
is applied to the AM detector circuit (Q4) to converted into
AF signals, and the signals are applied to the AF circuit
(LOGIC unit).

4-1-6 AF AMPLIFIER CIRCUIT (LOGIC UNIT)

The AF amplifier circuit amplifies the demodulated AF sig-
nals to drive a speaker.

While in FM mode, AF signals from the demodulator circuit
(RF unit) are passed through the de-emphasis circuit (R118,
C66, C68) with frequency characteristics of –6 dB/octave,
and are then applied to the pre-amplifier (Q31) via the band-
pass filter (Q30).

While in AM mode, AF signals are pass through the band-
pass filter and are then applied to the pre-amplifier (Q31).

While in WFM mode, AF signals are applied to the pre-
amplifier (Q31) directly.

The pre-amplified AF signals pass through the AF mute cir-
cuit (Q37) and are then applied to the electronic volume con-
trol circuit (IC14, pin 6). The level controlled AF signals are
output from pin 7 and applied to the AF power amplifier
(IC15, pin 1) via the buffer amplifier (Q36). The power ampli-
fied AF signals are applied to the internal speaker via the
[EXT SP] jack.

The electronic volume control circuit controls AF gain, there-
fore, the AF output level is according to the [VOL] setting
and also the squelch conditions.

4-1-7 SQUELCH CIRCUIT (LOGIC AND RF UNITS)

• NOISE SQUELCH
The noise squelch circuit cuts out AF signals when no RF
signals are received. By detecting noise components in the
AF signals, the squelch circuit switches the AF mute switch.

A portion of the AF signals from the FM IF IC (RF unit; IC2,
pin 9) are applied to the active filter section (IC2, pin 8). The
active filter section amplifies and filters noise components.

The filtered signals are applied to the noise detector section
and output from IC2 (pin 13) as the “SDET” signal.

The “SDET” signal from IC2 (pin 13) passes through the
noise detector (LOGIC unit; IC1), and is then applied to the
CPU (LOGIC unit; IC11, pin 12) via the “SQL” line. The CPU
analyzes the noise condition and outputs the “AMUTE” sig-
nal to the AF mute switch (Q37).

Even when the squelch id closed, the AF mute switch (Q37)
opens at the moment of emitting beep tones.

• 2nd IF AND DEMODULATOR CIRCUITS

Mixer

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
pin 7 (Logic unit)