Kenwood NX-300S 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 (D601 is on) is amplifi ed
by the drive amplifi er (Q601,Q602 and Q603) to 25~27dBm.
The output of the drive amplifi er is amplifi ed by the RF pow-
er amplifi er (Q606) 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 (D606, D607 are on) and applied to the
antenna terminal.
From
T/R SW
(D601)
5. PLL Frequency Synthesizer
5-1. VCTCXO (X500)
VCTCXO (X500) generates a reference frequency of
19.2MHz for the PLL frequency synthesizer. This reference
frequency is applied to pin 9 of the PLL IC (IC502) and con-
nected to the IF circuit as a 2nd local signal through the
Tripler (Q700). The VCTCXO oscillation frequency is deter-
mined by the DC voltage of the VC terminal. The VC voltage
is fi xed to 1.65V by R500 and R501, and supplied to the VC
terminal through IC501. The modulation signal is also fed to
the VC terminal through IC501.
The frequency adjustment is achieved by switching the
ratio of the dividing frequency that is not adjusted by the DC
voltage impressed to the VC. The resolution of the adjusting
frequency is approximately 4Hz.
CIRCUIT DESCRIPTION
Q601 Q602 Q603
Pre-Drive Drive Pre-Final
AMP AMP AMP
R642
+B
R645
R647
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 (Q606) and keeps a constant
current. The voltage drop at R642, R645 and R647 is caused
by the current fl owing through the RF power amplifi er and
this voltage is applied to the differential amplifi er (IC600 1/2).
IC600 (2/2) compares the output voltage of IC600 (1/2) with
the reference voltage from IC108, and the output of IC600
(2/2) controls the VGG of Q602, Q603 and Q606 to make
the both voltages the same. The change of power high/low
is carried out by the change of the reference voltage. Q607,
Q608 and Q610 are turned on and Q604 and Q605 are
turned off in transmit and the APC circuit is active.
Q606 D606,D607
Final ANT
LPF
AMP SW
VDD VGG
IC600 IC600
(1/2) (2/2)
5-2. VCO
There is a RX VCO and a TX VCO.
The TX VCO (Q509) generates a transmit carrier and the
RX VCO (Q508) generates a 1st local signal. For the VCO
oscillation frequency, the transmit carrier is 400 to 470MHz
and the 1st local signal is 341.95 to 411.95MHz.
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 (IC502) 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 ap-
plied voltage. This is used to modulate the VCO output.
"VCO_MOD" works only when "/T_R" is low.
NX-300(S)
ANT
13