Kenwood TK-5810 Service Manual page 29

The VCO oscillation frequency is determined by two sys-
tems of operation switching terminals "STR" and "VCO1/2"
and two systems of voltage control terminals "C/V" and
"ASSIST".
The operation switching terminals, "STR" and "VCO1/2",
are controlled by the control lines (STR, VCO1/2) output
from the ASIC (IC708). When the STR logic is high and the
VCO1/2 logic is high, the RX VCO1 output a 1st local receive
signal. When the STR logic is high and the VCO1/2 logic is
low, the RX VCO2 output a 1st local receive signal. When
the STR logic is low, the TX VCO output a transmit carrier.
The voltage control terminals, "C/V" and "ASSIST", are
controlled by the PLL IC (IC400) and potentiometer IC (IC401)
and the output frequency changes continuously according
to the applied voltage. For the modulation input terminal,
"MOD", the output frequency changes according to the
applied voltage. This is used to modulate the VCO output.
"MOD" works only when "STR" is low.
The oscillation frequency is controlled by the voltage of
the input terminals of the VCO, obtained from the potenti-
ometer IC applying to varactor diodes (D416, D417, D421,
D411, D412, D422, D405, D406, and D407), and from the
phase detector applying to diodes (D413, D414, D408,
D409, D402, and D403).
5-3. Potentiometer IC (IC401)
The potentiometer IC (IC401) is connected to the VCO
voltage control terminal, "ASSIST", and quickly controls the
VCO oscillation frequency. However, its accuracy is low and
the VCO frequency cannot be matched accurately with the
desired transmit carrier or the 1st local receive signal.
The potentiometer IC is controlled by the ASIC (IC708)
through the 3-line "PCS", "DAT", "CLK" serial bus.
Q407
Q408
5C
SW
VCO 1/2
Q409
8CL
SW
Q404
RX VCO2
Q405 Q410
SW SW
RX VCO1
STR
Q406
SW
8C
IC402
DC/DC
CONVERTER
IC401
POTENTIO
METER
5C
PCS
CLK
DAT
CIRCUIT DESCRIPTION
8CL
5C
8CL
EP CLK DAT PLL
8CL
TX VCO
16.8MHz
Q411
X400
RIPPLE
3.3V
FILTER
VCXO
MB
15CL
VCO
ASSIST
TUNE
IC403
Fig. 11 PLL block diagram
TK-5810
5-4. Lockup Accelerator
The lockup accelerator is a circuit composed of a potenti-
ometer IC (IC401) and a DC amplifi er (IC403) to compel the
oscillation frequency of the VCO to be close to the target.
The potentiometer IC outputs the voltage which meets
the target frequency, and the signal is connected to one of
two input terminals of the VCO through the DC amplifi er.
5-5. PLL IC (IC400)
PLL IC compares the differences in phases of the VCO
oscillation frequency and the VCXO reference frequency, re-
turns the difference to the VCO CV terminal and realizes the
"Phase Locked Loop" for the return control. This allows
the VCO oscillation frequency to accurately match (lock) the
desired frequency.
When the frequency is controlled by the PLL, the fre-
quency convergence time increases as the frequency differ-
ence increases when the set frequency is changed. To sup-
plement this, the potentiometer IC is used before control by
the PLL IC to bring the VCO oscillation frequency close to
the desired frequency. As a result, the VCO CV voltage does
not change and is always stable at approximately 2V.
The desired frequency is set for the PLL IC by the ASIC
(IC708) through the 3-line "EP", "DAT", "CLK" serial bus.
Whether the PLL IC is locked or not is monitored by the
ASIC through the "UL" signal line. If the VCO is not the de-
sired frequency (unlock), the "UL" logic is low.
5-6. Local Switch (D419, D420)
The connection destination of the signal output from the
amplifier (Q418) is changed with the diode switch (D420)
that is controlled by the transmission power supply, 8T, and
the diode switch (D419) that is controlled by the receive
power supply, 8R.
If the 8T logic is high, it is connected to a transmit-side
pre-drive amplifi er (Q150). If the 8R logic is high, it passes
through the local amplifi er (Q203) and then is connected to
a receive-side mixer (IC200).
Q414,Q415
8CL
RX
VCO1,2
CV
CV
IC400 Q413
Q416
TX
VCO
MO
ASSIST 8CL
5C
DOUBLER
Q403
BG /5810H
( )
ASSIST
8C 8C 8T
Buff Buff
AMP AMP
D419,D420
Q418
T/R
SW
5C
8R
RF AMP
Q417
BG
( )
Pre-drive
AMP
(Q150)
Local AMP
(Q203)
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