Circuit Description; Receiver Circuits - Icom IC-M87 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 fi lter
while receiving and a resonator circuit while transmitting. This
circuit does not allow transmit signals to enter the receiver
circuits.
Received signals enter the antenna connector (CHASSIS;
J1) and pass through the low-pass fi lter (L131, L132, C132
–C136). The fi ltered signals are passed through the
antenna switching circuit (D151, D152) and then applied to
the RF circuit.
While receiving, no voltage is applied to the D151, D152.
Thus, the receive line and the ground are disconnected and
L151, L152, C152, C153 function as low-pass filter which
leads received signals to the RF circuit.
4-1-2 RF CIRCUIT (MAIN UNIT)
The RF circuit amplifi es signals within the range of frequency
coverage and fi lters out-of-band signals.
The signals from the antenna switching circuit pass through
the two-stage tunable bandpass fi lters (D154, D155, L154,
L155). The fi ltered signals are amplifi ed at the RF amplifi er
(Q165) and then passed through another two-stage tunable
bandpass fi lters (D181, D182, L181) to suppress unwanted
signals.
The fi ltered signals are applied to the 1st mixer circuit.
The tunable bandpass filters (D154, D155, D181, D182)
employ varactor diodes to tune the center frequency of the
RF passband for wide bandwidth receiving and good image
response rejection. These diodes are controlled by the CPU
(LOGIC unit; IC661) via the D/A converter (IC251, pins 14,
15, 22, 23).
• 2ND IF AND DEMODULATOR CIRCUITS
2
IC251
1
AF signals
1
8 7
Active
filter
Limiter
amp.
Quadrature
detector
9 10 11
X231
4-1-3 1ST MIXER AND 1ST IF CIRCUITS (MAIN UNIT)
The 1st mixer circuit converts the received signal into the
fi xed frequency of the 1st IF signal with the PLL output fre-
quencies. By changing the PLL frequency, only the desired
frequency passes through a monolithic fi lter at the next stage
of the 1st mixer.
The filtered signals from the bandpass filter are applied to
the 1st mixer circuit (Q199). The applied signals are mixed
⁄ type
4
with the 1st LO signals coming from the RX VCO circuit (Q41)
to produce a 31.05 MHz 1st IF signal. The 1st IF signal is
passed through the monolithic fi lter (FI211) to suppress out-
of-band signals.
The fi ltered signal is amplifi ed at the 1st IF amplifi er (Q211)
and is then applied to the 2nd IF circuit.
4-1-4 2ND IF AND DEMODULATOR CIRCUITS
(MAIN UNIT)
The 2nd mixer circuit converts the 1st IF signal into the 2nd
IF signal. The double-conversion superheterodyne system
(which convert receive signals twice) improves the image re-
jection ratio and obtains stable receiver gain.
The amplifi ed 1st IF signal from the 1st IF amplifi er (Q211) is
applied to the 2nd mixer section of the FM IF IC (IC231, pin
16), and is mixed with the 2nd LO signal to be converted into
a 450 kHz 2nd IF signal.
The FM IF IC (IC231) contains the 2nd mixer, limiter ampli-
fi er, quadrature detector, active fi lter and noise amplifi er, etc.
The 2nd LO signal (30.6 MHz) is produced at the PLL circuit
by doubling it's reference frequency (X1: 15.3 MHz) at dou-
bler circuit (Q221).
2nd IF filter 450 kHz
FI232
FI231
5
Noise
amp.
Noise
detector
14
12
5V
4 - 1
30.6 MHz
2
2
PLL IC
Q221
3 2
IC1
X1
15.3 MHz
Mixer
IC231
TA31136FN
16
1st IF from IF amplifier (Q211)
"NOISV" signal to CPU (LOGIC unit; IC661)
"RSSIV" signal to CPU (LOGIC unit; IC661)
1