Circuit Description - Icom IC-725 Service Manual

SECTION 4 CIRCUIT DESCRIPTION
4·1 RECEIVER CIRCUITS
4-1·1 RF SWITCHING CIRCUIT
(PA AND MAIN UNITS)
RF signals from the antenna connector pass through the
transmit/receive switching relay (RL13) and a low-pass
filter, and are applied to the MAIN UNIT via P2 (MAIN
UNIT: J12).
The signals from the PA UNIT either bypass or are
attenuated at 20 dB attenuator (R102, R103, RL1). There
are no non-linear components between the antenna
connector and attenuator to prevent distortion caused
by strong signals. The signals are then applied to RF
filters.
4·1·2 RF BANDPASS FILTER CIRCUIT
(MAIN UNIT)
The RF UNIT has 7 RF. bandpass filters (BPF) for signals
above 1.6 MHz and 1 low-pass filter (LPF) for signals below
1.6 MHz. The signals pass through one of the bandpass
or low-pass filters depending on the receive frequency
range.
(1) 0.5-1.6 MHz
A diode is not used at the low-pass filter entrance
removing diode distortion from very strong signals.
Signals bypass a preamplifier by the bypass switch (012).
(2) 1.6 MHz AND ABOVE
Signals are applied to a high-pass filter consisting of L42,
L43, C143-C146. This filter suppresses strong signals
below 1.6 MHz such as a broadcasting station.
The filtered signals are applied to one of 7 bandpass filters
depending on the frequency of the signals and then to the
preamplifier circuit (08, 09).
(3) FILTER SWITCHING CIRCUIT
An RF bandpass filter is selected with BPF switching
voltage (BO-B7) from the CPU via IC16 current amplifier.
The switching voltage of the BPF entrance is higher than
the BPF exit to improve multi-signal and strong signal
characteristics.
4·1·3 PREAMPLIFIER CIRCUIT (MAIN UNIT)
The preamplifier circuit uses low-noise junction FETs
(2SK125 x 2) to provide 1 O dB gain over a wideband
frequency range.
When the [PRE] switch is turned ON, the signals from
the RF filter are amplified by the preamplifier circuit (08,
09). When the [PRE] switch is turned OFF, the signals
bypass the preamplifier through 030 and 032. When the
operating frequency is below 1.6 MHz, 012 turns ON and
the signals bypass the preamplifier regardless of the
[PRE] switch.
4-1
Amplified or bypassed signals are applied to the 1 st mixer
circuit via the low-pass filter. The low-pass filter cuts
off at 35 MHz to suppress image frequency at the 1st
mixer circuit (013, 014).
PREAMP CIRCUIT
RF signal
from BPF
030
Q10
Fig.1
4·1·4 1 ST MIXER CIRCUITS (MAIN UNIT)
The signals from the low-pass filter enter the 1 st mixer
circuit (013, 014) to be converted to a 70.45 MHz tst IF
signal.
EXACTNESS 1ST IF FREQUENCY
MODE
FREQUENCY (MHz)
SSB
70.4515
cw
70.4506
AM, FM 70.4500
The 1 st mixer circuit employs a balanced mixer using low-
noise junction FETs (2SK125 x 2) to expand the dynamic
range.
The 1st LO signal (70.9515-100.4515 MHz) enters the
MAIN UNIT from the PLL UNIT via J5. The signal is
amplified at 04, filtered by a low-pass filter, and then
applied to the 1st mixer circuit (013, 014). The low-pass
filter employs a ring core inductor to prevent 1 st LO leakage
signals. The output level from 04 is approx. 25 dBm.
The 1st IF signal is applied to an MCF (Monolithic Crystal
Filter; Fl 1) to suppress out-of-band signals. The signal is
amplified at the 1st IF amplifier (015), and then applied
to the 2nd mixer (IC1).