25
TK-980/981
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.
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.
■
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.
Fig. 2
Receiver system
CIRCUIT DESCRIPTION
ANT
SW
RF
AMP
1st
MIX
TX
AMP
PA
AMP
CF 455kHz
MCF
44.85MHz
IF SYSTEM
TK-980
806.15~
825.15MHz
TK-981
890.15~
896.15MHz
PLL/VCO
VCXO
16.8MHz
TK-980
TX 806~825MHz
851~870MHz
RX 851~870MHz
TK-981
TX 896~902MHz
935~941MHz
RX 935~941MHz
TK-980
806~825MHz
851~870MHz
TK-981
896~902MHz
935~941MHz
44.395MHz
AFO
MD
MB
ANT
ANT
L203
BPF
L207
BPF
Q201
RF AMP
Q15
1st IF
IC2
AF AMP
Q203
1st MIX
XF1
MCF
D208~210
ANT
SW
CF1
IC11
MIX,DET,IF
X2
2nd
local OSC
1st local
OSC (PLL)
DEO
DET DET
AUDIO
PROCE.
SUM
AMP
D/A
CONV.
IC504
IC504
IC5
AF PA
IC10
SP
DTMF
BPO
AFO
DEO
TX-RX UNIT (B/2)
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.
IC2
AMP
IC503
AMP
Q503
NOISE AMP D509
IC11
IC511
DEO
RSSI
HPF
DET
CPU
IF
SYSTEM
TX-RX UNIT (B/2)
Fig. 4
Squelch circuit
