Circuit Description - Kenwood TK-980 Service Manual

Frequency Configuration
The TX-RX unit (A/2) incorporates a VCO, based on a frac-
tional N type PLL synthesizer system, that allows a channel
step of 12.5kHz to be selected. The incoming signal from
the antenna is mixed with a first local oscillation frequency
to produce a first intermediate frequency of 44.85MHz.
The signal is then mixed with a second local oscillation
frequency of 44.395MHz to produce a second intermediate
frequency of 455kHz. This is called a double-conversion
system. The transmit signal frequency is generated by the
PLL VCO, and modulated by the signal from the micro-
phone. It is then amplified by TX amplifier and PA amplifier,
and sent to the antenna.
TK-980 TK-981
TX 806~825MHz TX 896~902MHz
851~870MHz 935~941MHz
RX 851~870MHz RX 935~941MHz
ANT
1st MCF
MIX 44.85MHz
ANT RF
SW AMP
TK-980
806.15~
825.15MHz
TK-981
890.15~
896.15MHz
TX
PA
AMP AMP
TK-980
806~825MHz
851~870MHz
TK-981
896~902MHz
935~941MHz
Fig. 1 Frequency configuration
Receiver System
Outline
An incoming RF signal from the antenna terminal passes
through the antenna switch (D208, D209, and D210 are off)
and then the bandpass filter (L203). The signal is amplified
by RF amplifier Q201, and passes through the bandpass fil-
ter (L207) again. The resulting signal goes to the first mixer
(Q203), where it is mixed with the first local oscillator signal
output from the frequency synthesizer to produce the first
IF 44.85MHz.
ANT
D208~210
ANT
SW

CIRCUIT DESCRIPTION

CF 455kHz
IF SYSTEM AFO
44.395MHz
MD
PLL/VCO
VCXO MB
16.8MHz
L203 Q201 L207 Q203
BPF RF AMP BPF 1st MIX
1st local
OSC (PLL)
Fig. 2 Receiver system
AF Signal System
The detection signal (DEO) from the TX-RX unit (A/2)
goes to the audio processor (IC504) of the TX-RX unit (B/2).
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 (A/2). The signal (AFO) sent to the TX-RX
unit (A/2) is input to the D/A converter (IC5). The AFO out-
put 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 inter-
nal speaker and speaker jack (J1) output.
TX-RX UNIT (B/2)
IC504 IC504
AUDIO SUM
DEO
PROCE. AMP
DTMF
Fig. 3 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 TX-RX unit (B/
2). The signal passes through a high-pass filter and a noise
amplifier (Q503) in the TX-RX unit (B/2) to detect noise. A
voltage is applied to the CPU (IC511). The CPU controls
squelch according 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 im-
proved.
TX-RX UNIT (B/2)
IC2
IC11 AMP
DEO
IF
SYSTEM
RSSI
Fig. 4 Squelch circuit
XF1 Q15
MCF 1st IF
IC11
MIX,DET,IF
DET DET
X2
CF1
2nd
local OSC
TK-980/981
IC5 IC10
AFO
D/A
AF PA
CONV.
BPO
IC503 Q503
AMP NOISE AMP D509 IC511
HPF DET
IC2
AF AMP
DEO
SP
CPU
25