Kenwood TH-77A Service Manual page 12

IA-/7A/E
12
CIRCUIT DESCRIPTION
(6) PLL Circuit
The oscillator circuit consists of reference oscillators with inde-
pendent phase-locked loop (PLL) and voltage-controlled oscil-
lator (VCO) circuits for both the VHF and UHF bands. The oscil-
lator frequency is sent to the V/U PLL IC.
6-1 PLL
The 12.8-MHz crystal oscillator, X202, is oscillated by Q211.
The oscillation output is sent to 1C201 and Q212, and the out-
put signal of 212 is sent to C1. The reference oscillation fre-
quency is divided by IC1 (VHF) and IC201 (UHF) to produce a
reference frequency of 5 kHz or 6.25 kHz.
The VCO output of the comparison frequency is amplified by
Q2 (VHF) and Q202 (UHF), then divided by pulse swallow PLL
circuits IC1 and IC2. PLL synthesizers with 5-, 10-, 12.5-, 15-,
20-, and 25-kHz steps are established by comparing the phase
of the X201 crystal oscillator frequency with that of the divided
reference frequency.
6-2 VCO
The desired frequency is produced by direct oscillation by a
Colpitts oscillator circuit consisting of VCO-V (X58-3740-00),
VCO-U (X58-3760-00), and field-effect transistor (FET) 02. The
VCO control voltage is applied to varicap diodes D1 and D2 to
change the oscillation frequency. During reception, the T/R pin
is made high, and Q1 and D4 are turned on. The oscillation fre-
quency is then switched. VCO-U makes the T/R pin low during
reception and turns Q1 and D3 off to select the range of the os-
cillation frequency.
+5V
5CU
| x58-3760
Q3
RF
Amp
RF Amp
Q208 Switching
6-3 Unlock Detector Circuit
When the PLL circuit is in the unlock mode, the pulses output to
the LOCK pins (pin 13) of IC1 andiC201 are waveform-shaped
by R6 and C7 for the VHF band and R205 and C208 for the UHF
band. The UL pin is then amde high. The voltage at the UL pin is
detected by the microcomputer to select the transmitter or the
receiver and contro! the timing.
R6/R205
to p-com
ULU
C7/C 208 a].f
Fig. 9 Uniock Detector Circuit
fi fl a
uy,
-#—-
VHF
12.8MHz
to Xin
12.8
|
2] CPS 12.}8MHz
date
RST
; XOUT | 14
H Q202
OUT P
OO)
RF
' Amp
13 Unlock
OUT
Driver
Detection
or Mixer 1c201
M56760FP
MOD
OD MIC Amp
12
O
TXU
Sscu-6
Fig. 8 PLL circuit (UHF)