Icom IC-720A Instruction Manual page 27

_ SECTION 6 CIRCUIT DESCRIPTION
The IC-720A uses a phase-locked-loop (PLL) consisting of
three loops for the local oscillator for both transmit and
receive. The PLL is controlled by a Microcomputer system.
The basic circuit uses an up-conversion with a First IF of
39.7315MHz and a Second IF of 9.0115MHz. Included
within the IF Unit is the Pass Band Tuning circuit. Alto-
gether there are 11 units in the IC-720A, with overall con-
trol exercised by the Logic Unit. The units are:.
6-1 SWR UNIT
This unit consists of a VSWR detecting circuit, an RF am-
plifier circuit, and an attenuator circuit.
When a signal is received by the antenna it passes through
the SWR Unit and is fed into the Low-Pass Filter, then again
sent to the SWR Unit, then amplified by a wide-band
amplifier about 10dB. This amplifier uses push-pull of
junction ҒЕТ5, 25К1255, to achieve low noise and wide
dynamic range.
When the Attenuator is turned ON, the signal bypasses the
RF amplifier and goes directly to the Attenuator Circuit
and is attenuated 20dB.
During transmit, signals passed through the Low-Pass Filter
unit, are applied to this unit, and detected for forward power `
and reflected power, then are fed to the antenna terminal.
6-2 LOW-PASS FILTER UNIT
The Low-Pass Filter Unit has a low-pass filter for each band,
a rotary relay switching circuit, and a diode matrix which
takes the signal from the Logic Unit and processes it for the
relay control voltage. During receive, the signal from the
SWR Unit 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 again to the SWR
Unit. During transmit, the output signal from the PA Unit
is fed to the Low-Pass Filter of the proper band and eli-
minates all harmonic components, then is fed to the SWR
Unit.
RF UNIT BLOCK DIAGRAM (Arrow shows the signal path in Receive)
(39.7315)
B.P.F DBM ХЕ
ЕВОМ
LPF UNIT
г-----
151 [0
39.7315 - 69. 7315 МН?
6-3 RF UNIT
Composed of a Band-Pass Filter for each band, 1st Mixer,
2nd Mixer, ist IF, and Noise Blanker. During receive the
Logic Unit sends the appropriate signal to the Band-Pass
Filter which chooses the suitable pass band signal, then is
fed to the 15% Mixer, which is a double balanced mixer
(DBM), where it is mixed with the 1st Local Oscillator
signal from the PLL, becoming a 39MHz signal, passes
through the monolithic filter and is sent to the 2nd Mixer
after amplification. In the 2nd mixer, also a DBM, the
signal is mixed with the signal of the 2nd Local Oscillator
from the PLL Unit апа а 9MHz signal is obtained, again
amplified, and sent to the IF Unit. The circuit for the Noise
Blanker consists of a noise amplifier, a noise detector and a
monostable multivibrator which blocks the IF Amplifier
when noise is present.
In transmit the 9MHz signal from the IF Unit is mixed with
the signal of the 2nd Mixer, then the 1st Mixer, the reverse
of receive, then to the Band-Pass Filter, a Double Balanced
Mixer especially developed by ICOM which has an 18dBm
intercept point.
The figure below shows the Block Diagram of the RF Unit.
6-4 IF UNIT
Composed of a crystal filter for both SSB and AM, and an
optional Narrow CW Filter, IF Amplifier, Pass Band Tuning
circuit, RF Speech Processor, BFO, and the SSB, AM
Detectors. During receive, the signal from the RF Unit is
fed to the crystal filter, to the IF Amplifier, then to the
Pass Band Tuning circuit composed of a DBM (as shown in
the preceding figure), a crystal filter, and the VXO circuit.
When the P.B.T. is turned ON the VXO frequency can be
changed +1.5KHz. The circuit makes a choice of what
frequencies will pass electrically through the crystal filter.
The PBT will operate in all modes except AM. When the
optional AM filter (F L-34) is installed the PBT will function
in that mode. When the PBT is turned ON the signal is sent
AMP DBM AMP
TO IF UNIT
2nd Lo
30.72 MHz