Kenwood NX-203 Service Manual page 13

4-3. VOX
IC716 (2/2) amplifi es the audio signal captured in the mi-
crophone. The signal is then converted into the DC voltage,
rectifi ed by D706. The DC voltage activates the ASIC (IC108),
and the VOX starts.
4-4. Drive and Final Amplifi er
The signal from the T/R switch (D100 is on) is amplifi ed
by the drive amplifi er (Q102 and Q103) to 25~27dBm. The
output of the drive amplifi er is amplifi ed by the RF power
amplifi er (Q106) to 5.0W (1W when the power is low). The
RF power amplifi er is MOS FET. The output of the RF power
amplifier is then passed through the harmonic filter (LPF)
and antenna switch (D104, D105 are on) and applied to the
antenna terminal.
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 is con-
nected to the IF circuit as a 2nd local signal through the
Tripler (Q201). The VCTCXO oscillation frequency is deter-
mined by the DC voltage of the VC terminal. The VC voltage
is fixed to 1.65V by R59 and R60, and supplied to the VC
terminal through IC5. The modulation signal is also fed to
VC terminal through IC5.
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 the adjusting fre-
quency is approximately 8Hz. Because twice the VCO out-
put are input for the input frequency of PLL IC, the sending
and receiving frequency can be adjusted by approximately
4Hz resolution.
CIRCUIT DESCRIPTION
Q102 Q103
From
Drive Pre-Final
T/R SW
AMP AMP
(D100)
R136
+B
R138
R141
REF
VOL
(IC108)
Fig. 6 Drive and fi nal amplifi er and APC circuit
4-5. APC Circuit
The APC circuit always monitors the current flowing
through the RF power amplifi er (Q106) and keeps a constant
current. The voltage drop at R136, R138 and R141 is caused
by the current fl owing through the RF power amplifi er and
this voltage is applied to the differential amplifi er (IC100 1/2).
IC100 (2/2) compares the output voltage of IC100 (1/2) with
the reference voltage from IC108, and the output of IC100
(2/2) controls the VGG of Q102, Q103 and Q106 to make
the both voltages the same. The change of power high/low
is carried out by the change of the reference voltage. Q105,
Q107 and Q110 are turned on and Q104 and Q109 are
turned off in transmit and the APC circuit is active.
Q106 D104,D105
Final ANT
LPF
AMP SW
VDD VGG
IC100 IC100
(1/2) (2/2)
5-2. VCO
There is a RX VCO and a TX VCO.
The TX VCO (Q10) generates a transmit carrier and the
RX VCO (Q8) generates a 1st local signal. For the VCO oscil-
lation 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 "CV" and "ASSIST" .
The operation switching terminal, "T/R" , is controlled by
the control line (/T_R) output from the ASIC (IC108). 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 "ASSIST" , are
controlled by the PLL IC (IC3) and ASIC (IC108) 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
applied voltage. This is used to modulate the VCO output.
"VCO_MOD" works only when "/T_R" is low.
NX-203
ANT
13