Icom ID-RP2 Service Manual page 62

SECTION 4
4-1 RECEIVER CIRCUITS
4-1-1 ANTENNA SWITCHING CIRCUIT
(MAIN-D 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 from the antenna connector (CASE; W4)
are applied to the MAIN-D unit via J1 (CHASSIS-D). The
applied signals are passed through the low-pass fi lter which
contains strip-line and C1198, and are then applied to the
type antenna switching circuit (D1161–D1162, L1162).
While receiving, no voltage is applied to D1161–D1162.
Thus, the receive line and ground are disconnected and re-
ceived signals are applied to the RF circuit.
ID-RP2D's transmitting and receiving are different timing.
4-1-2 RF CIRCUIT (MAIN-D UNIT)
The RF circuit amplifi es signals within the range of frequen-
cy coverage and fi lters out-of-band signals.
The signals from the antenna switching circuit pass through
the high-pass fi lter (L1–L4, C2–C5) and then applied to the
RF amplifi er (Q1). The amplifi ed signals are passed through
the bandpass fi lters (FI1, FI2) to suppress unwanted signals.
The fi ltered signals are applied to the 1st mixer circuit.
• RF AND IF CIRCUITS
1240–1300 MHz
HPF
LOGIC-D unit
CIRCUIT DESCRIPTION
Q1
FI1
AMP
BPF
IC271
Q271
MSK
AMP
receiver
4-1-3 1ST MIXER AND 1ST IF CIRCUITS
(MAIN-D UNIT)
The 1st mixer circuit converts the received signals into
fi xed frequency of the 1st IF signal with the 1st LO signal.
By changing the 1st LO signal, only the desired frequency
passes through the bandpass fi lter at the next stage of the
1st mixer circuit.
The RF signals from the bandpass filter (FI2) are mixed
with the 1st LO signal, where come from the 1st VCO circuit
(Q471, Q472, D471), at the 1st mixer circuit (IC71) to pro-
duce a 243.95 MHz 1st IF signal. The 1st IF signal is passed
λ
⁄
4
through the bandpass fi lter (FI71) to suppress unwanted sig-
nals and pass only the desired signals.
The fi ltered signal is applied to the 2nd IF circuit.
4-1-4 2ND MIXER AND 2ND IF CIRCUITS
(MAIN-D UNIT)
The 2nd mixer circuit converts the 1st IF signal into the 2nd
IF signal with the 2nd LO signal.
The fi ltered 1st IF signal from the bandpass fi lter (FI71) is
mixed with the 2nd LO signal (233.25 MHz), where come
from the 2nd VCO circuit (Q631, D630), at the 2nd mixer
circuit (Q131) to produce a 2nd IF signal (10.7 MHz). The
2nd IF signal is passed through the bandpass fi lter (FI272)
to remove unwanted heterodyned frequencies via the mode
switch (D273). The fi ltered signal is amplifi ed at the IF ampli-
fi er (Q270) and then passed through another bandpass fi lter
(FI273). The fi ltered signal is applied to another IF amplifi er
(Q271) and then applied to the MSK receiver (IC271).
1st LO:
996.05–1056.05 MHz
FI2
BPF
1st mixer
1st IF:
IC71
243.95 MHz
Q270
FI273
AMP
BPF
D - 4 - 1
2nd LO:
233.25 MHz
Fl71
BPF
2nd mixer
Q131
FI272
D273
BPF
2nd IF: 10.7 MHz