Standard Horizon HX471S, Service Manual

Page: 9 / 66

9
Circuit Description
UHF Band Transmission
Modulating audio from the CNTL UNIT passes through
deviation setting D/A converter
Q3010 to UHF MOD of
the VCO UNIT mounted on the RF UNIT. This signal is
applied to varactor
D4002 ( HSC277 ) in the tank circuit of
UHF VCO Q4002 ( EC3H07B ), which oscillates at the de-
sired UHF transmitting frequency. The modulated VCO
signal is buffered by amplifier
Q4006 and Q1010 and de-
livered through UHF T/R diode switch D1021 to the RF-
UNIT. The modulated low-level UHF transmit signal from
the VCO is passed through diode switch D1024 to amplifi-
er
Q1008 . The modulated UHF transmit signal from the
VCO is amplified by Q1016 and RF power amplifier Q101 9
up to 0.5 W (FRS). The RF output passes through TX diode
switch
D1030 . RF output is passed by T/R switch and low-
pass filter to suppress harmonics and spurious products
before output to the antenna at the antenna terminal.
PLL Frequency Synthesizer
PLL IC Q1013 on the RF UNIT consists of a data shift reg-
ister, reference frequency divider, phase comparator,
charge pump, intermittent operation circuit, and band se-
lector switch. Serial PLL data from the CPU is converted
into parallel data by the shift register in the PLL IC and is
latched into the comparative frequency divider and refer-
ence frequency divider to set a frequency dividing ratio
for each. An 11.7 MHz reference signal produced by
X1001
is input to REF pin 1 of the PLL IC. The internal reference
frequency divider divides the 11.7 MHz reference by 2,050
(or 1,640) to obtain a reference frequency of 5 kHz (or 6.25
kHz), which is applied to the phase comparator. Mean-
while, a sample of the output of VHF VCO
Q4004 or UHF
VCO Q4002 on the VCO UNIT, buffered by Q4006 , is in-
put to the PLL IC, where it is frequency-divided by the
internal comparative frequency divider to produce a com-
parative frequency also applied to the phase comparator.
The phase comparator compares the phase between the
reference frequency and comparative frequency to output
a pulse corresponding to the phase difference between
them. This pulse is input to the charge pump, and the out-
put from the charge pump passes through a loop filter com-
posed of L1018, R1054, C1096, and either R1055, C1110,
R1065 and C1113 for VHF, or R1051, C1107, R1064 and
C1112 for UHF, or R1056, C1111, R1066 and C1114 for MW
band, which convert the pulse into a corresponding
smoothed varactor control voltage (VCV). The VCV is ap-
plied to varactor
D4004 and D4013 ( 1SV325 ) in the VHF
VCO tank circuit, or to varactor D4001 ( HVC355B ) in the
UHF VCO tank circuit, or to varactor D4007 ( 1SV325 ) in
the MW band VCO to eliminate phase difference between
the reference frequency and comparative frequency, and
so locking the VCO oscillation frequency to the reference
crystal. The VCO frequency is determined by the frequen-
cy-dividing ratio sent from the CPU to the PLL IC. During
receiver power save operation, the PLL circuit operates
intermittently to reduce current consumption, for which
the intermittent operation control circuit reduces the lock-
up time.