Circuit Description - Vertex Standard VX-5500L Service Manual

Reception and transmission are switched by "RX"
and "TX" lines from the microprocessor unit (MPU).
Signal Path Overview
The receiver uses double-conversion superhetero-
dyne circuitry, with a 17.7 MHz 1st IF and 450 kHz
2nd IF. The 1st LO, produced by a PLL synthesizer,
yields the 17.7 MHz 1st IF.
The 2nd LO uses a 17.25 MHz (17.7 MHz-450
kHz) signal generated by a crystal oscillator. The 2nd
mixer and other circuits use a custom IC to convert
and amplify the 2nd IF, and detect FM to obtain de-
modulated signals. During transmit, the PLL syn-
thesizer 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 con-
sisting of coils L1011, L1012, L1014, and L1001, ca-
pacitors C1024, C1029, C1030, C1034, and C1002, and
antenna switching diodes D1032, D1033, and D1034
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 T1001 and T1002 be-
fore RF amplification by Q1006 (GN010100R).
The amplified RF is then band-pass filtered again
by varactor-tuned resonators T1003 and T1004 to
ensure pure in-band input to 1st mixer Q1040
(SPM5001).
Buffered output from the VCO Unit is amplified
by Q1027 (2SC5415E) and low-pass filtered by
L1020, L1023 and C1161, C1152, C1143, C1245, to pro-
vide a pure 1st local signal between 47.4 and 67.7
MHz to the 1st mixer.
The 17.7 MHz 1st mixer product then passes
through dual monolithic crystal filters XF1001 and
XF1002 (7.5 kHz BW), and is amplified by Q1035
(2SC4215Y) and delivered 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.
The 2nd LO in the IF-IC is produced from crystal
X1001 (17.25 MHz), and the 1st IF is converted to
450 kHz by the 2nd mixer and stripped of unwanted
components by ceramic filter CF1001. After passing

Circuit Description

through a limiter amplifier, the signal is demodulat-
ed 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-Ohm loudspeaker.
PLL Synthesizer
The 1st LO maintains stability from the PLL syn-
thesizer by using a 17.250 MHz reference signal from
crystal X1001. PLL synthesizer IC Q1029 1
(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 17.250 MHz reference
signal by 690 using the reference counter (25.0 kHz
comparison frequency). The phase detector compar-
ison frequency is configured to be eight times the
channel spacing (5 kHz). The VCO output is divid-
ed by the prescaler, swallow counter, and program-
mable counter. These two signals are compared by
the phase comparator and sent to the input of the
charge pump. A voltage proportional to their phase
difference is delivered to the low-pass filter circuit,
then fed back to the VCO as a voltage with phase
error, controlling and stabilizing the oscillating fre-
quency. This synthesizer also operates as a modula-
tor during transmit.
The RX-VCO is composed of Q1015 (for RX High
Band) or Q1042 (for RX Low Band) (2SK508) and
D1016, D1017 (HVU306A x 2) (for RX High Band) or
D1038, D1039 (HVU306A x 2) (for RX Low Band),
and oscillates between 47.4 MHz and 67.7 MHz ac-
cording to the programmed receiving frequency.
And the TX-VCO is composed of Q1016 (2SC4226-
R24) and D1012, D1013, D1014, D1015 (HVU300A x
4), and oscillates between 29.700 MHz and 50.000
MHz according to the programmed transmit fre-
quency. The VCO output passes through buffer am-
plifier Q1021 (2SC5107-O), and a portion is fed to
the buffer amplifier Q1023 (2SC4215Y) of the PLL
IC, and at the same time amplified by Q1027
(2SC5415E) to obtain stable output. The VCO DC
supply is regulated by Q1007 (2SC4154E). Synthe-
sizer output is fed to the 1st mixer by diode switch
D1022 (1SS321) during receive, and to drive ampli-
fier Q1004 (2SK3074) for transmit. The reference os-
cillator feeds the PLL synthesizer, and is composed
of crystal X1001 (17.25 MHz), the temperature com-
pensation circuit which includes D1028 (MC2850),
thermostats TH1001 and TH1002, and transmit (DCS)
modulation circuit D1025, D1029 (HVU300A x 2).
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