Kenwood TKR-750 Service Manual page 20

TKR-750
4. PLL Frequency Synthesizer
The PLL frequency synthesizer circuit consists of the fol-
lowing circuits : 4-1 receiver PLL circuit, 4-2 transmitter PLL
circuit, and 4-3 unlocked detector circuit.
4-1. Receiver PLL
The receiver PLL circuit is located in VCO unit A3 (X58-
478) on TX-RX unit (X57-626 A/2), and consists of VCXO X2,
VCO's (Q350 and Q351), a single-chip PLL IC IC300, buffer
amplifier Q355, and high-frequency amplifier Q302.
The VCXO generates 16.8MHz. The frequency stability is
within ± 2.0ppm (Temperature range of –30 to +60°C). The
frequency tuning of the VCXO is done to apply a voltage to
pin 1 of the VCXO. The output of the VCXO is applied to pin
8 of the PLL IC through the pin 7 of the VCO.
The first local oscillator is an upper heterodyne local oscil-
lator, and the VCO oscillator frequency is 180.850 to
218.850MHz. Two VCOs cover the two bands : Q350 cov-
ers the lower band and Q351 VCO covers the upper band.
The oscillator frequency is controlled by applying the
VCO control voltage, obtained from the phase comparator to
the varactor diodes.
VCO unit (X58-478)
Q350
VCO A
Q354
SW SW
SW
SRR
VCO B
Q351
Q300 IC300
LPF PLL-IC
CP,DP
EPR,LDR
µ-com
IC604
Fig. 5 Receiver PLL
20
CIRCUIT DESCRIPTION
TX-RX unit
Q355
AMP
Q352
Q353
Q302
AMP VCXO
4-2. Transmitter PLL
The transmitter PLL circuit is located in VCO unit A2 (X58-
479) on TX-RX unit (X57-626 A/2), and consists of VCXO X3,
VCO's (Q350 and Q351), a single-chip PLL IC IC300, buffer
amplifier Q355, and high-frequency amplifier Q302.
The VCXO generates 16.8MHz. The frequency stability is
within ± 2.0ppm (Temperature range of –30 to +60°C). The
frequency tuning and modulation of the VCXO are done to
apply a voltage to pin 1 of the VCXO. The output of the
VCXO is applied to pin 8 of the PLL IC through the pin 15 of
the VCO.
The VCO oscillator frequency is 136.00 to 174.00MHz.
Two VCOs cover the two bands : Q350 covers the lower
band and Q351 VCO covers the upper band.
The oscillator frequency is controlled by applying the
VCO control voltage, obtained from the phase comparator to
the varactor diodes.
4-3. Unlock Detector Circuit
If a pulse signal appears at the LD pin of IC300, an unlock
condition occurs, causing the voltage applied to the pin of
the microprocessor to go low. The names of this pin are
LDT for TX PLL and LDR for RX PLL. When the micropro-
cessor detects this condition, the transmitter is disabled.
VCO unit (X58-479)
Q354
SW
STT
X2 Q300
LPF
Q350 Q355
VCO A AMP
MO
SW
Q352
MO
Q353
SW
MO
VCO B
Q351
IC300 Q302
PLL-IC AMP
CP,DP
EPT,LDT
µ-com
IC604
Fig. 6 Transmitter PLL
TX-RX unit
MO
X3
VCXO