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Reception and transmission are switched by "RX"
and "TX" lines from the microprocessor unit (MPU).
The receiver uses double-conversion super-hetero-
dyne circuitry, with a 43.95MHz 1st IF and 450 kHz
2nd IF. The 1st LO, produced by a PLL synthesizer,
yields the 43.95MHz 1st IF.
The 2nd LO uses a 43.5 MHz (43.95 MHz-450 kHz)
signal generated by a crystal oscillator. The 2nd mix-
er and other circuits use a custom IC to convert and
amplify the 2nd IF, and detect FM to obtain demod-
ulated signals. During transmit, the PLL synthesiz-
er oscillates at the desired frequency directly, for
amplification to obtain RF power output. During
transmit, voice modulation and CTCSS (or DCS)
modulation are applied to this synthesizer. Trans-
ceiver functions, such as Tx/Rx control, PLL synthe-
sizer settings, and channel programming, are con-
trolled using the MPU.
Receiver
Incoming RF signals from the antenna connector
are delivered to the MAIN Unit, and pass through a
low-pass filter (LPF) antenna switching network
consisting of coils L1008, L1004, L1022 and L1003,
capacitors C1338, C1337, C1336, C1335, and C1037,
and antenna switching diodes D1047, D1048 and
D1049 for delivery to the receiver front end.
Signals within the frequency range of the trans-
ceiver are then passed through a varactor-tuned
band-pass filter consisting of L1007, L1020 before RF
amplification by Q1018 (2SC3357).
The amplified RF is then band-pass filtered again
by varactor-tuned resonators L1009, L1011 to ensure
pure in-band input to 1st mixer Q1188, Q1189, Q1190,
Q1191 (2SK520 x 4).
Buffered output from the VCO Unit is amplified
by Q1032 (2SC5107), Q1192 (2SC3357) and low-
pass filtered by L1041 / L1045/ L1060and C1153 /
C1154 / C1174/C1089, to provide a pure 1st local sig-
nal between 177.95 and 217.95 MHz to the 1st mix-
er.
The 43.95MHz 1st mixer product then passes
through dual monolithic crystal filters XF1001 and
XF1002 (12 kHz BW)/ XF1003 and XF1004 (7kHz BW)
and is amplified by Q1041 (2SC4215Y) and deliv-
ered to the input of the FM IF subsystem IC Q1034
(TA31136FN).
This IC contains the 2nd mixer, 2nd local oscilla-
tor, limiter amplifier, FM detector, noise amplifier,
and squelch gates.
Circuit Description
The 2nd LO in the IF-IC is produced from crystal
OSC X1001 (14.500MHz), and the 1st IF is converted
to 450kHz by the 2nd mixer and stripped of unwant-
ed components by ceramic filter CF1001 or CF1002.
After passing through a limiter amplifier, the signal
is demodulated by the FM detector.
Demodulated receive audio from the IF-IC is
amplified by Q2019 (CXA1846N). After volume ad-
justment by the AF power amplifier Q2018
(TDA7240AV), the audio signal is passed to the op-
tional headphone jack or 4 W loudspeaker.
PLL synthesizer
The 1st LO maintains stability from the PLL syn-
thesizer by using a 14.500 MHz reference signal from
crystal OSC X1001. PLL synthesizer IC Q1033
(SA7025DK) consists of a prescaler, reference
counter, swallow counter, programmable counter,
a serial data input port to set these counters based
on the external data, a phase comparator, and charge
pump. The PLL-IC divides the 14.500 MHz refer-
ence signal by 725 using the reference counter (20.0
kHz comparison frequency). The phase detector
comparison frequency to be eight times the channel
spacing (2.5kHz). The VCO output is divided by the
prescaler, swallow counter and programmable
counter. These two signals are compared by the
phase comparator and input to the charge pump. A
voltage proportional to their phase difference is de-
livered to the low-pass filter circuit, then fed back to
the VCO as a voltage with phase error, controlling
and stabilizing the oscillating frequency. This syn-
thesizer also operates as a modulator during trans-
mit.
The RX-VCO is comprised of Q1019 (2SK520) and
D1010, D1011, D1016, D1018 (1SV282 x 4), and os-
cillates between 177.950MHz and 217.950MHz ac-
cording to the programmed receiving frequency.
And the TX-VCO is comprised of Q1020 (2SC4226-
R24) and D1014, D1015 (1SV282 x 2), D1020
(1SV284) and oscillates between 134.000MHz and
174.000MHz according to the programmed transmit
frequency. The VCO output passes through buffer
amplifier Q1026 (2SC5107), and a portion is fed to
the buffer amplifier Q1029 (2SC5107) of the PLL IC,
and at the same time amplified by Q1032 (2SC5107)
to obtain stable output. The VCO DC supply is reg-
ulated by Q1006 (2SC4154E). Synthesizer output is
fed to the 1st mixer by diode switch D1021 (1SS321)
during receive, and to drive amplifier Q1031
(2SC3356) / Q1025 (2SC3357) for transmit.
4-1
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