Circuit Description - Icom IC-730 Instruction Manual

SECTION 6 CIRCUIT DESCRIPTION
The IC-730 employs a pre-mixed local oscillator for both
transmit and receive. The local oscillator consists of a digital
phase-locked-loop controlled by a microcomputer system,
and crystal oscillator for each band. The frequency of the
local oscillator is approximately 39.7MHz above the
operating frequency, thereby spurious is kept to a minimum.
The basic circuit uses an up-conversion with a First IF of
39.7315MHz and a Second IF of 9.0115MHz. The receiver
front end employs the ICOM DFM (Direct Feed Mixer)
system, that incoming signals are directly fed to the mixer,
and provides wide dynamic range and high two signal
selectivity.
The circuits are divided several units. The units are:
6 - 1 LOW-PASS FILTER UNIT
The Low-Pass Filter Unit has a low-pass filter for each band
and a VSWR detecting circuit.
During receive, the signal from the antenna passes through
the Low-Pass Filter Unit which switches for the proper band
eliminating the undesired signals from out-of-band, and then
is sent to the R F Unit.
During transmit, the output signal from the PA Unit is fed to
the Low-Pass Filter of the proper band and eliminates all
harmonic components, then is fed to the SWR detecting
circuit, and detected for forward power and reflected power,
then are fed to the antenna terminal.
6 - 2 RF UNIT
This unit consists of a receiver preamplifier, band-pass filter
for each band, first mixer, 39MHz crystal filters, receiver first
IF amplifiers, and transmitter buffer amplifier.
During receive, the incoming signal passes through the low-
pass filter, is fed to the preamplifier circuit. If the PREAMP
switch on the front panel is in ON position, the signal passes
through the preamplifier and is amplified about 10 dB, then
fed to the band-pass filter. If the
PREAMP switch is in OFF position, the signal is directly fed
to the band-pass filter. The band-pass filter is selected its
respective filter depending on the band selected.
The signal is then passed through a high-level Double
Balanced Mixer which has an 18 dBm intercept point,
where it is mixed with the 1
BPF unit, becoming a 39MHz signal, passes through a third
over-tone monolithic filter and is amplified by O6, 1
amplifier, passes again through a monolithic filter, and is
nd
fed to the 2 IF unit.
During transmit, the 39MHz signal from the 2
passes through the monolithic filter, then the 1
reverse of receive, then to the band-pass filter, and is am-
plified by O4 and fed to the PA unit.
nd
6 - 3 2 IF UNIT
Composed of the 2
blanker circuit, and a crystal filter for AM and optional SSB
and CW filter circuits.
During receive, the 39MHz signal from the RF unit is fed to
nd
the 2 mixer which is a high level DBM, and mixed with
nd
30.72MHz 2 local oscillator signal to convert it in
nd
9.0115MHz 2 IF signal.
-- 19 --
st
Local Oscillator signal from the
nd nd
mixer, 2 local oscillator, noise
st
IF
nd
IF unit,
st
mixer, the