Pll Frequency Synthesizer; Receiver Pll; Transmitter Pll; Unlock Detector Circuit - Kenwood TKR-750 Service Manual

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 (X57-695 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 a lower heterodyne local oscilla-
tor, and the VCO oscillator frequency is 180.850 to
218.850MHz. 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.
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
14
CIRCUIT DESCRIPTION
TX-RX unit
Q355
AMP
Q352
Q353
Q302 X2
AMP VCXO
TX VCO
(X58-479)
RX VCO
(X58-478)
Fig. 7 VCO oscillation frequency

4-2. Transmitter PLL

The transmitter PLL circuit is located in VCO unit A2 (X58-
479) on TX-RX unit (X57-695 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 7 of the VCO.
The VCO oscillator frequency is 136.000 to 174.000MHz.
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 microprocessor de-
tects this condition, the transmitter is disabled.
VCO unit (X58-479)
Q354
SW
STT
Q300
LPF
µ-com
IC604
Fig. 6 Transmitter PLL
Top view
(MHz)
VCO A : 136~152
VCO B : 152~174
VCO A : 136~152
+44.850
VCO B : 152~174
+44.850
TX-RX unit
Q355
Q350
VCO A AMP
MO
SW
Q352
MO
Q353
SW
MO
VCO B
Q351
Q302
IC300
PLL-IC AMP
CP,DP
EPT,LDT
MO
X3
VCXO