Kenwood UBZ-LJ8 Service Manual page 4

UBZ-LJ8
Transmitter System
■ Microphone Amplifier Circuit
The audio signal acquired from the microphone (MIC200)
passes through the MIC amplifier (IC204) to perform signal
amplification. An AGC (Automatic gain control) circuit is in-
cluded to maintain output audio signal level.
The amplified signal passes through the amplifier, pre-
emphasis circuit, limiter and Splatter filter contained inside
base band IC (IC209) and reaches the VCO as modulation sig-
nal. The splatter filter will eliminate the distortion compo-
nents outside the audio band.
■ Modulation Circuit
The output signal of the base band IC passes through VR1
for Max Deviation Adjustment and goes to the modulation
input of the VCO circuit for a variable reactance phase modu-
lation.
IC204
MIC AMP
MIC VR1
Fig. 6 Transmitter section configuration
IC204
MIC AMP
MIC
Q200
AGC
AMP
3
TX IN
Pre-
emphasis
IC209 : Base band IC
Fig. 7 Microphone amplifier circuit
■ VOX Circuit
After amplified by MIC amplifier (IC204), MIC signal
passes through VOX amplifier (Q201) to eliminate signal
which is out of frequency range 300Hz~2.5kHz.
The resulting signal goes into rectifier (D203) and converts
into DC level. This DC level is compared with preset value in
MPU. If the level is higher than preset, MPU will start trans-
mitting.
4
CIRCUIT DESCRIPTION
Q2,Q4,Q6 Q7
VCO RF AMP
Q14
RF AMP
ANT
Q8 ANT
RF AMP SW LPF
AMP Limiter
BPF
7
MOD
Splatter
filter
MIC VOX AMP
IC204
MIC AMP
■ Transmission Output Circuit
The VCO output is amplified by a high-frequency amplifier
circuit (Q14, Q7 and Q8). The resulting signal will pass
through a high-frequency antenna switch (D8), a low pass fil-
ter before being transmitted by the antenna.
The output power is about 500mW with 4.5V DC supply at
battery terminal.
■ Low Pass Filter Circuit
A three-stage low-pass filter is located between the an-
tenna and the antenna switch circuit. It helps to remove the
harmonic components that is generated by transmission cir-
cuit.
3T
VCO
Q14
Fig. 9 Transmission output circuit
PLL System
■ VCO Circuit
The VCO circuit is housed in a shielding case.
The VCO circuit consists of a oscillation transistor (Q2), a
variable capacitor diode (D2) for frequency control, a variable
VCO
capacitor diode (D3) for variable reactance modulation, trans-
mit/receive frequency switch diode (D1), control switch (Q1),
ripple filter (Q3) and buffer amplifier (Q4).
The oscillation frequency is mainly determined by C12,
C13, D2 and L3 (TX) or L3 and L2 (RX). D3 is used for modu-
lation.
In RX mode, the TX/RX switch line goes low; switch Q1
and D1 turn off. The inductor L2 will be connected to oscilla-
tion circuit and Q2 will produce the first local frequency for
reception purpose. (Receiving channel frequency subtracted
by 21.7MHz equals to the receive frequency.)
In TX mode, the TX/RX switch line goes high; switch Q1
and D1 turn on. The inductor L2 is bypassed to ground
through C2, D1 and C3. Q2 oscillation frequency becomes
about 446MHz and the VCO will output the frequency which
is needed for transmission.
The ripple filter (Q3) produces a 2.4V voltage supply for
the oscillation transistor (Q2). D4 is used for speeding up the
ripple filter operation when 3C line turns on.
Q201
D203
4
Fig. 8 VOX circuit
3T 3T B
D8
Q7 Q8
VOX
IC208
MPU
ANT
LPF