Kenwood TK-880 Service Manual page 28

AF Signal System
The detection signal (DEO) from the TX-RX unit goes to
the audio processor (IC504) of the control unit. The signal
passes through a filter in the audio processor to adjust the
gain, and is output to IC502. IC502 sums the AF signal and
the DTMF signal and returns the resulting signal to the TX-
RX unit. The signal (AFO) sent to the TX-RX unit is input to
the D/A converter (IC5). The AFO output level is adjusted by
the D/A converter. The signal output from the D/A converter
is added with the BEEP signal (BPO) and the resulting signal
is input to the audio power amplifier (IC10). The AF signal
from IC10 switches between the internal speaker and
speaker jack (J1) output.
CONTORL UNIT
IC504 IC504
AUDIO
SUM
DEO
PROCE. AMP
DTMF
FIg. 4 AF signal system
Squelch Circuit
The detection output from the FM IF IC (IC11) is ampli-
fied by IC2 and the signal (DEO) is sent to the control unit.
The signal passes through a high-pass filter and a noise am-
plifier (Q503) in the control unit to detect noise. A voltage is
applied to the CPU (IC511). The CPU controls squelch ac-
cording to the voltage (ASQ) level. The signal from the RSSI
pin of IC11 is monitored. The electric field strength of the
receive signal can be known before the ASQ voltage is input
to the CPU, and the scan stop speed is improved.
CONTROL UNIT
IC2
IC11 AMP
DEO
HPF
IF
SYSTEM
RSSI
Fig. 5 Squelch circuit
MIC IC711 IC504
AF AMP,
AF AMP
IDC, LPF
TA75S01F
TC35453F
IC511
CPU
MIC KEY
30620M8
INPUT
-394GP
CIRCUIT DESCRIPTION
IC5 IC10 SP
AFO
D/A
AF PA
CONV.
BPO
IC503 Q503
AMP NOISE AMP D509 IC511
DET
CPU
IC3 Q103 Q106
VCO RF AMP
SUM AMP
2SK508NV 2SC4226
TA75W558FU
(K53) (R24)
X1 IC300 Q300
BUFFER
VCXO PLL
2SC4215
16.8MHz SA7025DK
(Y)
Fig. 6 Transmitter system
Transmitter System
Outline
The transmitter circuit produces and amplifies the de-
sired frequency directly. It FM-modulates the carrier signal
by means of a varicap diode.
VCO/PLL Circuit
The TK-880 has a VCO for the transmitter and a VCO for
the receiver in a sub-unit (A-1). They are housed in a solid
shielded case and connected to the TX-RX unit through
CN101. One of the VCOs is selected with an ST signal. A
filtered low-noise power supply is used for the VCOs and
varicap diodes.
The VCO for the transmitter is described below. It is de-
signed so that Q103 turns on with a prescribed frequency
when a reverse bias is applied to D102 and D104 by using
the control voltage (CV) through CN101. The control voltage
is changed by turning the trimmer capacitor (IC109). The
output from Q103 is applied to the buffer amplifier (Q106) to
generate a VCO output signal. This signal is used as a drive
input signal or a local signal of the first mixer. Since a signal
output from Q160 is input to the PLL IC, it passes through
CN101 and buffer amplifier (Q300) and goes to the PLL IC
(IC300). The modulation signal from CN101 is applied to
D105 and passes through C112 and C113 to modulate the
carrier.
The PLL IC uses a fractional N type synthesizer to im-
prove the C/N ratio and lock-up speed. The VCO output sig-
nal input to the pin 5 of the PLL IC is divided to produce a
comparison frequency according to a channel step. This sig-
nal is compared with the reference frequency which is out-
put from the VCXO (X1). VCXO provides 16.8MHz, 2.5ppm
(–30 to +60°C) and guarantees stable performance when
the temperature changes. The output signal from the phase
comparator passes through a charge pump and an external
active LPF (Q301, Q302) in the PLL IC to generate a DC VCO
control voltage CV. Serial data (DT, CK, EP) are output from
the CPU (IC511) and shift register (IC8) in the control unit to
control the PLL IC. The PLL lock status is always monitored
by the CPU.
Q7 Q202 Q204
BUFFER
RF AMP
RF AMP
2SC5110 2SC4093
2SC3357
(O)
(R27)
TK-880
ANT
Q205 IC400
RF AMP POWER AMP
2SC2954 : E M57729H : E
2SC3357 : E3 M57729L : E3
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