Pll Frequency Synthesizer; Vctcxo (X1); Vco; Local Switch - Kenwood NX-700 Service Manual

NX-700/700H

5. PLL Frequency Synthesizer

5-1. VCTCXO (X1)

VCTCXO (X1) generates a reference frequency of
19.2MHz for the PLL frequency synthesizer. This reference
frequency is applied to pin 9 of the PLL IC (IC3) and con-
nected to IF circuit as a 2nd local signal through Tripler. The
VCTCXO oscillation frequency is determined by DC voltage
of VC terminal. The VC voltage is fi xed to 1.65V by R1 and
R2, and supplied to VC terminal through IC1. Modulation
signal is also fed to VC terminal through IC1.
The frequency adjustment is achieved by switching
the ratio of dividing frequency that is not adjusted by the
DC voltage impressed to VC. The resolution of adjusting
frequency is approximately 8Hz. Because twice the VCO
output are input as for the input frequency of PLLIC, the
sending and receiving frequency can be adjusted by approxi-
mately 4Hz resolution.

5-2. VCO

There is a RX VCO and a TX VCO.
The TX VCO (Q10) generates a transmit carrier and the
RX VCO (Q9) generates a 1st local signal. For the VCO os-
cillation frequency, the transmit carrier is 136 to 174 MHz
and the 1st local receive signal is 194.05 to 232.05MHz.
The VCO oscillation frequency is determined by one sys-
tem of operation switching terminal "T/R" and two systems
of voltage control terminals "C/V" and "V-assist".
The operation switching terminal, "T/R", is controlled by
the control line (/T_R) output from the MCU (IC510). When
the /T_R logic is low, the VCO outputs the transmit carrier
and when it is high, it outputs a 1st local receive signal.
The voltage control terminals, "CV" and "V-assist", are
controlled by the PLL IC (IC3) and MCU (IC510) and the
output frequency changes continuously according to the
applied voltage. For the modulation input terminal, "VCO
MOD", the output frequency changes according to the ap-
plied voltage. This is used to modulate the VCO output.
"VCO MOD" works only when "/T_R" is low.
X1
19.2MHz
VC
TCXO
SDO1
SCK1
/PCS RF
20
CIRCUIT DESCRIPTION
Q9,Q10
D7,D8
IC3 D11~D16,D18
PLL Loop
VCO
Filter
CV
VCO MOD
IC5
150C
Q1
Ripple
Filter
IC4
ASSIST
RF
SW
D5,D6
5-3. PLL IC (IC3)
PLL IC compares the differences in phases of the VCO
oscillation frequency and the VCTCXO reference frequency,
returns 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 dif-
ference increases when the set frequency is changed. To
supplement this, the MCU is used before control by the PLL
IC to bring the VCO oscillation frequency close to the de-
sired frequency. As a result, the VCO CV voltage does not
change and is always stable at approx. 3.0V.
The desired frequency is set for the PLL IC by the MCU
(IC510) through the 3-line "SDO1", "SCK1", "PCS_RF" se-
rial bus. Whether the PLL IC is locked or not is monitored
by the MCU through the "PLD" signal line. If the VCO is
not the desired frequency (unlock), the "PLD" logic is low.
5-4. Doubler (Q6)
The doubler (Q6) extracts the twice harmonic component
from the signal from the VCO. This twice harmonic compo-
nents is then fed into PLL(IC3) through band pass fi lter.
Band pass fi lter is consists of two fi lter. One is for TX
(L7, 15, 18) and pass band is 272.0 to 348.0MHz. The other
is for RX 1st Local (L8, 12, 16) and pass band is 388.1 to
464.1MHz.
5-5. Local Switch (D101, D205)
The connection destination of the signal output from
the buffer amplifi er (Q14) is changed with the diode switch
(D101) that is controlled by the transmission power supply,
80T, and the diode switch (D205) that is controlled by the
receive power supply, 50R. If the 80T logic is high, it is con-
nected to a send-side pre-drive (Q101). If the 80T logic is
low, it is connected to a local amplifi er (Q208).
Q13 Q14
BUFF BUFF
AMP AMP
50C
Doubler 50C
Q6
L7,L15,L18
BPF (TX)
RF
SW
BPF (RX)
D19,D20
L8,L12,L16
Fig. 7
D101,D205
T/R
to TX stage
SW