Kenwood NXR-700 Service Manual page 10

NXR-700
2-4. Transmitter DDS circuit
The transmitter DDS circuit produces the transmitter
main PLL reference frequency signal 4.5MHz.
This circuit consists of IC307, IC202, CF201, Q210,
Q211, Q212 and Q213.
The 19.2MHz signal from the transmitter modulation
19.2MHz PLL circuit is amplifi ed by IC307 and supplied to
the IC202 reference signal pin.
IC202 produces the transmitter main PLL 4.5MHz refer-
5V #2
TX Mod
19.2MHz
19.2MHz
PLL circuit
IC307
2-5. Transmitter main PLL circuit
The transmitter main PLL circuit consists of the VCO
(Q102 and Q103), PLL IC (IC101) and divide-by-2 circuit
(IC811) and produces the transmitter frequency signal.
The VCO Q102 produces transmitter frequencies from
136.000MHz to 144.995MHz. (The transmitter frequency of
the VCO is from 272.000MHz to 289.990MHz.)
The VCO Q103 produces transmitter frequencies from
145.000MHz to 154.000MHz. (The transmitter frequency of
the VCO is from 290.000MHz to 308.000MHz.)
The signal produced by the VCO (Q102 or Q103) is fed
to the buffer amplifi er and is amplifi ed by Q106. The higher
harmonic wave is attenuated by LPF and returns to the PLL
IC (IC101).
IC101 divides the VCO oscillating frequency signal and
4.5MHz
TX DDS
circuit
10
CIRCUIT DESCRIPTION
5V #1
5V #1
4.5MHz
IC202
DDS IC
Q211
Fig. 4 Transmitter DDS circuit
IC102
OP-AMP
5V #2
IC101
PLL IC
9 4
1/R PD
Fref.=4.5MHz
R=45
1/N
Fpd=100kHz
6
Fig. 5 Transmitter main PLL circuit
ence frequency signal based on 19.2MHz on signal.
The spurious output by IC202 is attenuated by CF201
and LPF, 4.5MHz reference frequency signal is amplifi ed by
Q211, Q212, and Q213, and fed to the transmitter main PLL.
The comparison frequency of the transmitter main PLL is
100kHz and the PLL frequency step is 100kHz.
However, minute frequency step such as 2.5kHz and
3.125kHz because the DDS output frequency is variable.
5V #1
CF201
ATT
Q212
transmitter PLL reference signal (4.5MHz), and compares
the phase with the 100kHz comparison frequency.
The phase difference signal produced by the comparing
phase is converted to a DC voltage by a lag-lead type loop
fi lter.
The DC signal is applied to varicaps D101, D102, D107,
and D108 to lock the VCO oscillator frequency with the de-
sired oscillator frequency.
At the same time, the DC signal passes through the
IC102 operational amplifier for monitoring the transmitter
main PLL lock voltage.
The output from the VCO passes through the buffer am-
plifi er Q104. The divide-by-2 circuit (IC811) divides the fre-
quency and produces the transmitter frequency. The output
level of IC811 is about +6dBm (4mW).
5V #2
CVT
9V' 9V'
272~
D108
289.99MHz
Q102 Q104
9V'
290~
D102
308MHz
Q103
5V #2
ATT
Q106
5V #1
4.5MHz
ATT
Q213
3V
272~
136~
308MHz 154MHz
Driver
1/2
circuit
IC811
ATT
TX main
PLL circuit