Vox; Drive And Final Amplifier; Apc Circuit; Pll Frequency Synthesizer - Kenwood NX-320 Series Service Manual

NX-320

4-3. VOX

The DSP detects the audio level. If the detected level be-
comes higher than the threshold level, the VOX starts.
4-4. Drive and Final amplifi er
The signal from the T/R switch (D413 is on) is amplifi ed
by the pre-drive amplifier (Q102) to 16~17dBm. It makes
it possible to spoil input to the drive amplifier by control-
ling the power supply of the pre-drive amplifier with the
PD switch (Q110). The output of the pre-drive amplifi er is
amplifi ed by the drive and fi nal amplifi ers (Q103, Q104) to
5.0W (1.0W when the power is low). Q103 and Q104 are
MOS FETs. The output of the fi nal amplifi er is then passed
through the harmonic fi lter (LPF) and antenna switch (D102,
D103 are on) and applied to the antenna terminal.

5. PLL Frequency Synthesizer

5-1. TCXO (X401)
The TCXO (X401) generates a reference frequency of
16.8MHz for the PLL frequency synthesizer. This reference
frequency is applied to pin 9 of the PLL IC (IC403) and is
connected to the IF circuit as a 2nd local signal through the
Tripler (Q301).
The frequency adjustment is achieved by switching the
ratio of dividing frequency. The resolution of the adjusting
frequency is approximately 4Hz.
5-2. VCO
There is a RX VCO and a TX VCO.
The TX VCO (Q402) generates a transmit carrier and the
RX VCO (Q401) generates a 1st local signal. For the VCO
oscillation frequency, the transmit carrier is 450 to 520MHz
(K,K2,K3) or 400 to 470MHz (K4,K5,K6,E,E2,E3) and the 1st
local receive signal is 400.05 to 470.05MHz (K,K2,K3) or
350.05 to 420.05MHz (K4,K5,K6,E,E2,E3).
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".
14
CIRCUIT DESCRIPTION
from Pre-Drive
T/R SW (D413)
VCC for Pre-Drive AMP
from PD SW (Q110)
R153
+B
R154
R155
Reference voltage
from IC610
Fig. 5 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 (Q104) and keeps a constant
current. The voltage drop at R153, R154 and R155 is caused
by the current fl owing through the RF power amplifi er and
this voltage is applied to the differential amplifier (IC101
1/2). IC101 (2/2) compares the output voltage of IC101 (1/2)
with the reference voltage from IC610, and the output of
IC101 (2/2) controls the VGG of Q103 and Q104 to make the
both current constant. The change of power high/low is car-
ried out by the change of the reference voltage. Q106, Q107
and Q108 are turned on and Q105 and Q109 are turned off
during transmission and the APC circuit is active.
Q102 Q103 Q104
Drive Final
AMP AMP AMP
IC101
(1/2)
The operation switching terminal, "T/R", is controlled by
the control line (/T_R) output from the ASIC (IC610). 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 (IC403) and ASIC (IC610) 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.
5-3. PLL IC (IC403)
The PLL IC compares the differences in phases of the
VCO oscillation frequency and the TCXO reference frequen-
cy, returns the difference to the VCO CV terminal and real-
izes the "Phase Locked Loop" for the return control. This
allows the VCO oscillation frequency to accurately match
(lock) the desired frequency.
ANT
D102,D103
ANT
LPF
SW
VGG
IC101
(2/2)