Section 3 Circuit Description; Receiving Circuits - Icom IC-730 Maintenance Manual

SECTION 3 CIRCUIT DESCRIPTION

3 – 1 RECEIVING CIRCUITS
3 – 1 – 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 40dB by a High Pass filter. This filter
reduces intermodulation by strong BC signals.
The signal is usually fed to D3 directly through the contacts of
RL1.
The set employs the DFM (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 RL1 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
RL1 is not actuated even if the PREAMP switch is pushed in.
Q5 is turned on in the transmit mode and shunts the receiver
input line to ground to prevent RF feedback.
The signal passed D3 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 respectively.
The signal passed the band-pass filter is fed to the 1st mixer
consisting of D4 • D7 (Doubly Balanced Mixer). This DBM has
a +18dBm intercept point and 6dB insertion loss.
The 1st Local Oscillator functions at the operating frequency
plus 1st IF (39.7315MHz) frequency. It is fed from the BPF
unit through J6.
This 1st LO is changed with 1kHz steps and its frequency
range for each band is as follows;
Band
3.5 43.1319MHz ~ 43.8315MHz
7.0 46.6315MHz ~ 47.3315MHz
10.0 49.6315MHz ~ 50.3315MHz
14.0 53.6315MHz ~ 54.3315MHz
18.0 57.6315MHz ~ 58.3315MHz
1st LO FREOUENCY
21.0
24.0
28.0
28.5
29.0
29.5
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 F12 through D20 transmit/receive
switching diode, then fed to the 1st IF amplifier 06. Fl2 has a
pass band of 15kHz at -3dB points.
The 1st IF amplifier 06 is a MOS FET, and an AGC voltage is
applied to its 2nd gate. The attack time constant is
determined by R36 and C67. C81 prevents VHF parasitic
oscillation.
The amplified 1st IF signal is fed to F11 through D21
transmit/receive switching diode. Fl1 is the same as Fl2, and
totally the 2nd image rejection ratio is more than 80dB.
The 1st IF signal passed Fl1 is then fed to the 2nd IF unit.
3 – 1 – 2 SECOND RF 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 D14 and then fed to IC2,
voltage comparator and noise pulses are detected.
D16 generates the reference voltage, 1.2V, for IC2. D15
shunts over-voltage to prevent long delay-times.
A part of the detected signal from D14 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, 07is 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 MAIN 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
1V pulse, and it turns ON the NB gate switch 03. by the
reverse voltage (6.6V) applied to the cathode. When Q3 is
turned ON by a noise pulse, D24 is turned ON and
3 – 1
60.6315MHz ~ 61.3315MHz
64.1315MHz ~ 64.8315MHz
67.6315MHz ~ 68.3315MHz
68.1315MHz ~ 68.8315MHz
68.6315MHz ~ 69.3315MHz
69.1315MHz ~ 69.8315MHz