Kenwood TK-782 Service Manual page 38

Transmitter System
" Outline
The transmitter circuit produces and amplifies the desired
frequency directly. It FM-modulates the carrier signal by
means of a varicap diode.
" VCO/PLL Circuit
The TK-782 has a VCO for the transmitter and a VCO for
the receiver in a sub-unit (A1). They are housed in a solid
shielded case and connected to the TX-RX unit (A/2) through
CN101. One of the VCOs is selected with an ST signal. A
filtered low-noise power supply is used for the VCOs and
varicap diodes.
The VCO for the transmitter is described below. It is de-
signed so that Q103 turns on with a prescribed frequency
when a reverse bias is applied to D102 and D107 by using
the control voltage (CV) through CN101. The control voltage
is changed by turning the trimmer capacitor (IC102). The
output from Q103 is applied to the buffer amplifier (Q106) to
generate a VCO output signal. This signal is used as a drive
input signal or a local signal of the first mixer. Since a signal
output from Q160 is input to the PLL IC, it passes through
CN101 and buffer amplifier (Q300) and goes to the PLL IC
(IC300). The modulation signal from CN101 is applied to D109
and passes through C125 and C126 to modulate the carrier.
The PLL IC uses a fractional N type synthesizer to improve
the C/N ratio and lock-up speed. The VCO output signal in-
put to the pin 5 of the PLL IC is divided to produce a compari-
son frequency according to a channel step. This signal is
compared with the reference frequency which is output from
the VCXO (X1). VCXO provides 16.8MHz, 2.5ppm ($30 to
+60%C) and guarantees stable performance when the tem-
perature changes. The output signal from the phase com-
parator passes through a charge pump and an external ac-
tive LPF (Q301, Q302) in the PLL IC to generate a DC VCO
control voltage CV. Serial data (DT, CK, EP) are output from
the CPU (IC511) and shift register (IC8) in the TX-RX unit (B/2)
to control the PLL IC. The PLL lock status is always moni-
tored by the CPU.
MIC IC711 IC504
AF AMP ,
AF AMP
IDC, LPF
TA75S01F
TC35453F
IC511
CPU
MIC KEY
30620M8A
INPUT
-2K9GP
CIRCUIT DESCRIPTION /
IC3 Q103
VCO
SUM AMP
2SK508NV 2SC4215
TA75W558FU
(K53)
X1 IC300
VCXO PLL
2SC4215
16.8MHz SA7025DK
Fig. 6 Transmitter system /
"
"
Q106 Q7 Q203
RF AMP BUFFER
RF AMP
2SC4215
2SC3357
(Y) (Y)
Q300
BUFFER
(Y)
TK-782
ANT
Q204 IC400
RF AMP POWER AMP
2SC3357 M68729
37