Circuit Description; Rf Unit; Att, Bpf And Pre-Amplifier Circuits - Icom IC-7200 Service Manual

SECTION 3

3-1 RF UNIT

The RF UNIT is composed by BPFs for both TX and RX,
attenuator and pre-AMP for RX line, YGR AMP, SWR detect
circuit and ALC circuit for TX line.
• TX/RX SW
RL1 connects the RX/TX circuit to the antenna connector
according to the operation (receiving/transmitting).
While receiving, RL1 connects the antenna connector and
the attenuator through the LPF. While transmitting, RL1
connects the antenna connector and J10 which guides the
TX signal from the FILTER UNIT.
• ATTENUATOR (ATT)
The attenuator is composed of D164, D165, R163 and
R164.
While the ATT function is inactivated, D164 turns ON and
D165 turns OFF to apply the RX signals to next RX stage
via D164 without attenuation.
While the ATT function is activated, D164 turns OFF and
D165 turns ON and the RX signals are divided by R163 and
R164 for 20 dB of attenuation.
• BPF
The RX signals from the ATT are passed through one of
9 BPFs. The BPF for MF band contains an attenuator to
prevent the saturation caused by strong RX signals. The
BPF for 50 MHz band contains a pre-AMP (Q380) to make
up the loss for high sensitivity.
These BPFs are commonly used in TX and RX, but
exclusive BPF for 50 MHz TX is also equipped to secure the
transmit signal path for 50 MHz.

- ATT, BPF AND PRE-AMPLIFIER CIRCUITS -

<1.6 MHz
LPF ATT
1.6-2 MHz
BPF
2-4 MHz
LPF
4-8 MHz
TX
BPF
8-11 MHz
BPF
11-15 MHz
BPF
20 dB
ATT
15-22 MHz
D164,165
R163,164
BPF
22-33 MHz
BPF
33-60 MHz(RX)
RF
HPF
AMP
33-60 MHz(TX)
BPF
To YGR AMP

CIRCUIT DESCRIPTION

TX&PRE OFF
Q 441
PRE
AMP
Q442,443
PRON
PREAMP
SW
PROF
HPF
RX
Q155
ATT
SW
ATTS
ANTENNA
LPF
SWR
DET
S1
RX
TX SW11
TRXS
From FILTER UNIT
• PRE-AMP
While the pre-amp. function is activated, the RX signals
from the BPF are applied to the noiseless feedback AMP
(Q441). This AMP provides low-noise, high RF gain and
good IP characteristic. The amplified RX signals are entered
to the MAIN UNIT via J7.
• YGR AMP
While TX, the transmit signal (YGR signal) is entered to the
MAIN UNIT via J7. The transmit signal is by-passed the pre-
AMP and passed through one of BPFs, and amplified to the
level required in PA UNIT by IC181, then output through J8.
• SWR DETECT CIRCUIT
The TX signal which is amplified to the output power level
in the PA UNIT is entered to the RF UNIT via J10. The TX
signal is passed through the TX/RX SW (RL1) and the SWR
detect circuit (L1), then fed to the antenna connector. The
detected forward and reflected signals are rectified by D1
and D2 to be converted into DC voltage.
• ALC CIRCUIT
The detected forward signal is applied to IC462 and
compared with TX power setting voltage "POCV" which is
supplied from the CPU (voltage is differ according to the
TX output power), and the resulting voltage controls the TX
power according to the setting.
The ALC voltage is also used for two APC cicuits. One is
the SWR APC circuit which is controlled by reflected voltage
"REF", another is ID APC circuit which is controlled by
driving current of the power amplifier.
To/From MAIN UNIT
3 - 1