Vox; Drive And Final Amplifier; Apc Circuit; Pll Frequency Synthesizer - Kenwood NX-210 Service Manual

NX-210

4-3. VOX

IC813 (2/2) amplifi es the audio signal captured in the mi-
crophone. The signal is then converted into the DC voltage,
rectifi ed by D803. The DC voltage activates the ASIC (IC308),
and the VOX starts.
4-4. Drive and Final Amplifi er
The signal from the T/R switch (D520 is on) is amplifi ed
by the drive amplifi er (Q516 and Q518) to 23~26dBm. The
output of the drive amplifi er is amplifi ed by the RF power
amplifi er (Q520) 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 (D529, D530 are on) and applied to the
antenna terminal.

5. PLL Frequency Synthesizer

5-1. VCTCXO (X501)
VCTCXO (X501) generates a reference frequency of
19.2MHz for the PLL frequency synthesizer. This reference
frequency is applied to pin 9 of the PLL IC (IC503) and is
connected to the IF circuit as a 2nd local signal through the
Tripler (Q506). 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 R501 and R502, and supplied to the VC
terminal through IC502. The modulation signal is also fed to
VC terminal through IC502.
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.
14
CIRCUIT DESCRIPTION
Q516 Q518
From
Drive Pre-Final
T/R SW
AMP AMP
(D520)
R685
+B
R689
R690
REF
VOL
(IC308)
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 (Q520) and keeps a constant
current. The voltage drop at R685, R689 and R690 is caused
by the current fl owing through the RF power amplifi er and
this voltage is applied to the differential amplifi er (IC511 1/2).
IC511 (2/2) compares the output voltage of IC511 (1/2) with
the reference voltage from IC308, and the output of IC511
(2/2) controls the VGG of Q516, Q518 and Q520 to make
the both voltages the same. The change of power high/low
is carried out by the change of the reference voltage. Q523,
Q525 and Q526 are turned on and Q519 and Q521 are
turned off in transmit and the APC circuit is active.
Q520 D529,D530
Final ANT
LPF
AMP SW
VDD VGG
IC511 IC511
(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 136 to 174 MHz
and the 1st local receive signal is 194.05 to 232.05MHz.
The VCO oscillation frequency is determined by one
system of operation switching terminal "/T_R" and two sys-
tems 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 (IC308).
When the /T_R logic is low, the VCO outputs the transmit
carrier and when it is high, it outputs a 1st local receive sig-
nal.
The voltage control terminals, "CV" and "ASSIST", are
controlled by the PLL IC (IC503) and ASIC (IC308) 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.
ANT