Circuit Description; Logic Unit; Main Unit - Icom IC-R71A Instruction Manual

SECTION 6 CIRCUIT DESCRIPTION
The !C-R71 uses a phase-locked-loop (PLL) consisting of
two loops for the local oscillator. The PLL is controlled
by a Microcomputer system. The basic circuit uses an
up-conversion with a First IF of 70.4515MHz and a Second
IF of 9.0115MHz. Included within the MAIN Unit is the
Pass Band Tuning circuit.
6-1 RF UNIT
Composed of a band-pass filter for each band, preamplifier,
attenuator, 1st mixer and a 1st IF amplifier.
When a signal is received by the antenna, it passes through
an appropriate band-pass filter which is chosen by a band
signal from the LOGIC unit.
When the PRE/ATT switch is set at the PRE position, the
signal is amplified by a wide-band amplifier about 10dB.
This amplifier uses push-pull of junction FET's, 2SK125's
to achieve low noise and wide dynamic range.
When the PRE/ATT switch is set at the ATT position, the
signal bypasses the RF amplifier and goes to the attenuator
circuit and is attenuated 20dB.
When the PRE/ATT switch is set at the OFF position, the
signal bypasses both the RF amplifier and attenuator, and
goes directly to the 1st mixer.
The 1st mixer is a double balanced mixer (DBM), where it
is mixed with the 1st local oscillator signal from the PLL
unit, becoming a 7OMHz signal, passes through a pair of
monolithic filter and is sent to the 2nd mixer in the MAIN
unit through the 1st IF amplifier.
6-2 MAIN UNIT
Composed of the 2nd mixer, a crystal filter for each, SSB,
AM and CW (option), IF amplifiers, Pass Band Tuning
circuit, BFO, the SSB, AM detectors and IF amplifiers. In
the 2nd mixer, (which is also a DBM), the signal is mixed
with the 61.44MHz signal of the 2nd local oscillator signal
from the PLL unit, and a 9MHz signal is obtained, and sent
to the IF circuitry.
The circuit for the Noise Blanker consists of noise ampli-
fiers, a noise detector, a pulse amplifier and AGC circuit
which blocks the noise pulses at the IF when noise is present.
The signal from the 2nd mixer is fed to the crystal filter,
through an amplifier, to the IF amplifier, then to the
Pass Band Tuning circuit composed of two mixers, 455KHz
ceramic filters and the VXO circuit.
When the P.B.T. is turned ON the VXO frequency can be
changed +1.8KHz (+3.3KHz in AM). The circuit makes a
choice of what frequencies will pass electronically through
the ceramic filter.
The P.B.T. will operate in all modes except FM.
Then the signal is sent to the second mixer then to the pro-
duct detector, the squelch control and the AF attenuator.
In the case of AM, the signal goes to the AM detector, then :
to an AF amplifier, the squelch control and the attenuator.
The AF signal from the attenuator is applied to a high
performance and low distortion AF power amplifier 1C, and
amplified and fed to the speaker.
The BFO has two crystal units, one is for USB and the
other is for LSB, CW and RTTY. The oscillation frequency
is shifted to an appropriate value for each mode.
6-3 LOGIC UNIT
This unit is composed of proprietary CPU for treating
data such as up/down signal detection control, the dial lock
control circuit, the tuning rate select circuit, the band
switching: control circuit, code converter circuit, and the
{/O control circuit.
The dial clock signals are generated by the rotary encoder
directly connected to the tuning knob. The up/down cont-
rol circuit detects the rotating direction of the tuning knob,
and puts out the up/down control signal for the CPU.
The CPU is an 8-bit microcomputer which has a program to
control the set. The CPU's functions are determined by the
input signals, such as the dial clock signal, up/down signal
and the signals from the function switches, and the CPU
puts out signals to control the operating band, frequency,
mode, tuning steps, display, and so on.
6-4 PLL UNIT
This unit is composed of two Phase-Locked-Loops for the
first local oscillator and a crystal oscillator for reference
frequency and the second local oscillator.
The crystal oscillator oscillates 30.72MHz. A part of this
output is multiplied by 2 and a 61.44MHz signal is fed to
the second mixer as the local oscillator signal. The other
parts are divided for 10KHz and 5KHz as the reference
frequencies of the main loop and sub loop.
The sub loop oscillates at 115 ~ 119.99MHz with 5KHz
steps. This signal is divided by 500 to get 230 ~ 239.99
KHz with 10Hz steps, and mixed with 30.72MHz signal of
the crystal oscillator (the mixer puts out 30.950 ~ 30.9599
MHz signal), then fed to the mixer of the main loop as the
local oscillator signal.
The main loop puts out 70.55MHz ~ 100.45MHz, as the
first local oscillator of the set, with 10Hz steps, for entire
band. This loop has four VCO's which divide the oscilla-
tion frequency range into segments of; 70.55 ~ 78.45MHz,
78.45 ~ 85.45MHz, 85.45 ~ 92.45MHz and 92.45 ~
100.45MHz to improve the C/N (Carrier/Noise) ratio.
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