Circuit Description; Receiver Circuits - Icom IC-M88 Service Manual

SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT
(MAIN UNIT)
The antenna switching circuit functions as a low-pass filter
while receiving and as resonator circuit while transmitting.
The circuit does not allow transmit signals to enter receiver
circuits.
Received signals from the antenna connector pass through
the low-pass fileter (L131, L132, C131–C136) and antenna
switching circuit (D151, D152). The filtered signals are then
applied to the RF amplifier circuit (Q165).
4-1-2 RF AND 1ST MIXER CIRCUITS (MAIN UNIT)
The 1st mixer circuit converts the received signals to a fixed
frequency of the 1st IF signal with a PLL output frequency.
By changing the PLL frequency, only the desired frequency
will be passed through a pair of crystal filters at the next
stage of the 1st mixer.
The signals from the antenna switching circuit are passed
through the 2-stage bandpass filters (D154, D155, L154,
L155) and amplified at the RF amplifier (Q165). The ampli-
fied signals are passed through another 2-stage bandpass
filters (D181, D182, L181, L182), and then applied to the 1st
mixer circuit (Q199).
The filtered signals are mixed at the 1st mixer (Q199) with a
1st LO signal coming from the PLL circuit to produce a 31.05
MHz 1st IF signal. The 1st IF signal is passed through a pair
of crystal filter (FI211) and is then amplified at the IF ampli-
fier (Q211).
4-1-3 2ND IF AND DEMODULATOR CIRCUITS
(MAIN UNIT)
The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal. A double convertion superheterodyne system (which
converts receive signal twice) improves the image rejection
and obtain stable receiver gain.
• 2ND IF AND DEMODULATOR CIRCUITS
R240
R241
"SQCON" signal to the D/A
convertor IC (IC251, pin 2)
"SQLOUT" signal to the D/A
convertor IC (IC251, pin 1)
C242
AF signal "DET"
C238 C239
R239
8 7
Noise
detector
Active
filter
IF amp.
FM
detector
9 10
11
C232
R242
R231
C244
C243
X231
450 kHz
The 1st IF signal is applied to a 2nd mixer section of the FM
IF IC (IC231, pin 16). The signal is then mixed with a 2nd LO
signal for conversion into a 450 kHz 2nd IF signal.
IC231 contains the 2nd mixer, limiter amplifier, quadrature
detector and active filter circuits. A 30.6 MHz 2nd LO signal
is produced at the PLL circuit using the reference frequency.
The 2nd IF signal from the 2nd mixer (IC231, pin 3) passes
through ceramic filters (FI231, FI232) to remove unwanted
heterodyned frequencies. It is then amplified at the limiter
amplifier section (IC231, pin 5) and applied to the quadra-
ture detector section (IC231, pins 10 and 11) to demodulate
the 2nd IF signal into AF signals.
4-1-4 AF CIRCUIT (MAIN AND LOGIC UNITS)
AF signals from the FM IF IC (IC231, pin 9) are fed to the
analog switch (IC282).
The AF signals (detected signals) passes throught the AF
mute swtich (IC281A, pins 2 and 1) via "DET" singal, and are
then applied to the analog switch (IC282, pin 1). The signals
are then applied to the low-pass filter (IC261B, C266, C267,
R270–R272).
The filtered AF signals are applied to and adjusted audio
level at the [VOL] control (VR unit; R801) via the "VOLIN"
signal. The level controlled signals are passed through the
AF mute switch (LOGIC unit; Q411) which is controlled by
"AFMS" signal from the CPU (IC661, pin 84). The passed
signals are applied to the AF power amplifier (IC421, pin 4),
and then output to the internal speaker or [EXT SP] jack
after being passed through the de-emphasis circuit (R411,
C413) to obtain the –6 dB/octave frequency characteristics
2nd IF filter
450 kHz
FI232
5 3
2nd
Mixer
Noise
comp.
14
12
R232
R5V
4 - 1
30.6 MHz
2
17 16
Q221
PLL IC
2
IC1
X1
15.3 MHz
IC170 TA31136FN(D)
16
"IF" (1st IF signal: 31.05 MHz)
from RF unit, Q211
"NOISV" signal to the CPU (pin 32)
"RSSIV" signal to the CPU (pin 33)