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NX-420
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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 (D412 is on) is amplifi ed
by the pre-drive amplifier (Q102) to
9
~10dBm. The output
of the pre-drive amplifi er is amplifi ed by the drive and fi nal
amplifi ers (Q104, Q106) to 3.0W (1.0W when the power is
low). Q104 and Q106 are MOS FETs. The output of the fi nal
amplifier is then passed through the harmonic filter (LPF)
and antenna switch (D102, D103 are on) and applied to the
antenna terminal.
4-5. APC circuit
The APC circuit always monitors the current flowing
through the RF power amplifier (Q106) and keeps a con-
stant current. The voltage drop at R132 and R133 is caused
by the current fl owing through the RF power amplifi er and
this voltage is applied to the differential amplifier (IC100
1/2). IC100 (2/2) compares the output voltage of IC100 (1/2)
with the reference voltage from IC607, and the output of
IC100 (2/2) controls the VGG of Q104 and Q100 to make
the both current constant. The change of power high/low is
carried out by the change of the reference voltage. IC106,
Q107,Q108 and Q108 are turned on and Q103 and Q105
are turned off during transmission and the APC circuit is ac-
tive.
Fig. 5 Drive and fi nal amplifi er and APC circuit
from
T/R SW (D412)
Drive
AMP
Pre-Drive
AMP
Q102
Q104
Final
AMP
Q106
ANT
SW
LPF
D102,D103
ANT
VGG
IC100
(1/2)
IC100
(2/2)
R132
R133
+B
Reference voltage
from IC607
RFAMP
Q101
VCC from
DC SW(Q504)
5. PLL Frequency Synthesizer
5-1. TCXO (X400)
TCXO (X400) generates a reference frequency of
16.8MHz for the PLL frequency synthesizer. This reference
frequency is applied to pin
9
of the PLL IC (IC400) and is
connected to the IF circuit as a 2nd osc through the Tripler
(Q300). 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 are two VCOs (VCO-1 and VCO-2).
VCO-1 (Q401) generates the 1st local signal for the re-
ceiver and a carrier for the transmitter.
VCO-2 (Q402) generates a carrier for the transmitter of TA
(Talk Around).
The oscillation frequency of the VCO is half of the carrier
or 1st local signal. The VCO oscillates from 403 to 435MHz
during transmit, and from 400.525 to 410.025MHz during
reception.
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 (IC607).
When the /T_R logic is low, VCO-2 is activated, and when
the /T_R logic is high, VCO-1 is activated.
The voltage control terminals, “CV” and “ASSIST”, are
controlled by the PLL IC (IC400) and ASIC (IC607) 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.
5-3. PLL IC (IC400)
The PLL IC compares the differences in phases of the
VCO oscillation frequency and the TCXO reference fre-
quency, returns the difference to the VCO CV terminal and
realizes the “Phase Locked Loop” for the return control. This
allows the VCO oscillation frequency to accurately match
(lock) the desired frequency.
CIRCUIT DESCRIPTION
Summary of Contents for NX-420 K
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