Circuit Description; Receiving Circuits - Icom IC-730 Maintenance Manual

SECTION 3 CIRCUIT DESCRIPTION
3·1 RECEIVING CIRCUITS
3·' • 1 RF CIRCUITS
The receiving signal from the antenna is fed from P3 of the
LPF unit to J2·1, where frequencies less than 3.5MHz are
attenuated about 4Od8 by a High-Pass filter. This filter
reduces intermodulation by strong BC signals.
The signal is usually fed to 03 directly through the contacts
of RLl.
The set employs the OFM (Direct Feed Mixer) system at
the front end, to get wide dynamic range.
When the PREAMP switch on the front panel is in the ON
position, the signal is fed to the preamplifier. The pre-
amplifier, consisting of 01 and 02, is designed to provide
the gain of about 10dB and the intercept point of 26dBm
on the entire band.
To the preamplifier, power source (13.8V) is always applied
regardless if the preamplifier is turned on or off. When the
PREAMP switch is pushed in, the emitter of 03 is grounded
through the switch and RL 1 is actuated in the receive mode,
as R8V is applied to the base of 03 through R4. In the
transmit mode or when 08 is turned on as the TRV signal
is applied to its base, the bias voltage is not applied to 03
and RL 1 is not actuated even if the PREAMP switch is
pushed in.
05 is turned on in the transmit mode and shunts the re-
ceiver input line to ground to prevent RF feedback.
The signal passed 03 is fed to a band-pass filter which is
selected by the band switching signal sent from the BAND
switch through the PRE·MIX unit.
The band-pass filters are provided for each band, and one
is selected for the band of operation by turning ON the
diodes located at the input and output circuits of the
filter. These filters have about 2dB insertion loss res-
pectively.
The signal passed the band-pass filter is fed to the 1st mixer
consisting of 04 • 07 (Doubly Balanced Mixer). This OBM
has a +18dBm intercept point and 6dB insertion loss.
The 1st Local Oscillator functions at the operating fre-
quency plus 1st IF (39.7315MHz) frequency. It is fed from
the BPF unit through J6.
This 1st LO is changed with 1 kHz steps and its frequency
range for each band is as follows;
BAND
3.5MHz
7.0MHz
10.0MHz
14.0MHz
18.0MHz
1st LO FREOUENCY
43.1319MHz "" 43.8315MHz
46.6315MHz "" 47.3315MHz
49.6315MHz"" SO.3315MHz
53.6315MHz "" 54.3315MHz
57.6315MHz "" 58.3315MHz
3-1
21.0MHz
24.5MHz
28.0MHz
28.5MHz
29.0MHz
29.5MHz
60.6315MHz "" 61.3315MHz
64.1315MHz "" 64.8315MHz
67.6315MHz "" 68.3315MHz
68.1315MHz "" 68.8315MHz
68.6315MHz "" 69.3315MHz
69.1315MHz "" 69.8315MHz
The center frequency of the 1st IF is varied 39.7305MHz-
39.7315MHz due to the 10Hz step tuning.
The 1st IF signal converted at the 1st mixer is fed to the
monolithic crystal filter FI2 through 020 transmit/receive
switching diode, then fed to the 1st IF amplifier 06. FI2
has a pass band of 15kHz at -3dB points.
The 1 st IF ampl ifier 06 is aMOS F ET, and an AGC voltage
is applied to its 2nd gate. The attack time constant is
determined by R36 and C67. C81 prevents VH F parasitic
oscillation.
The amplified 1 st I F signal is fed to F 11 through 021
transmit/receive switching diode. Fll is the same as F12,
and totally the 2nd image rejection ratio is more than SOdB.
The 1st IF signal passed Fll is then fed to the 2nd IF unit.
3 -1 - 2 SECOND IF CIRCUITS
The 2nd IF signal fed from the RF unit is input to J2 and
fed to the Noise Amplifier and Noise Blanker gate circuits.
The signal (39.7315MHz) is amplified with 01 and 02, dual
gate MOS FETs, and IC1, high gain amplifier with AGC.
The amplified signal is detected by 014 and then fed to
IC2, voltage comparator and noise pulses are detected.
016 genarates the reference voltage, 1.2V, for IC2. 015
shunts over-voltage to prevent long delay-times.
A part of the detected signal from 014 is used for the AGC
of IC2. The detected signal is fed to the base of 07 through
R42. When the detected voltage exceeds 0.6V, 07 is turned
ON which turns 08 ON as well. 2.5V is usually applied to
pin 3 of IC2. This voltage is increased when 08 turns ON,
with time constant of R34 and C42 (attack-time), This
provides the AGC function. This time constant (attack·
time) can be changed by the NB WIDTH switch (S4 on the
MAl N unit). When it is set at the WI DE position, the time
constant is determined by R46 and C42. The release-time
of the AGC is determined by R34, R38, R47 and C42.
Average voltage at pin 2 of IC2 will be 0.6V due to the
AGC function. Only when the detected voltage exceeds
1.2V caused by a noise pulse, pin 3 output terminal of IC2
puts out lV pulse, and it turns ON the NB gate switch 03.
The NB gate circuit is composed of 020, 021 and 024,
and usually 020 and 021 are turned ON and 024 is OFF
by the reverse voltage (6.6V) applied to the cathode. When
OJ is turned ON by a noise pulse, 024 is turned ON and
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