Selcall Operation - Kenwood TK-90 Service Manual

SELCALL Operation

The SELCALL signal is passed through FSK IC for de-
modulation NJM2211M (IC865). When the SELCALL func-
tion is on, Q859 is turned ON by MCU signal and power is
supplied to the SELCALL circuit. The receive detection signal
is always connected to the SELCALL circuit input section
buffer (IC863), and is output to the FSK demodulator (IC865,
2 pin).
Receive (NJM2211M Operation)
The FSK signal mean frequency, f0 (2210Hz), is the center
of the shift frequency (2295Hz and 2125Hz). When changing
the value in the adjustment mode, the SELADJ signal is out-
put from the MCU and IC865 becomes the mean frequency
setting mode. NJM2211M is a PLL IC, and locks within the
range of f0± ∆f. While locked, the LOW level signal is output
to the FSL terminal. The output, in the range of f0+∆f (High:
5V) and f0–∆f (Low: 0V) is sent to the FSD terminal. The FSD
signal while IC865 is locked (FSL=Low) is analyzed by the
MCU. The lock range ∆f is determined by the circuit con-
stant, and is set at approximately 170Hz.
IC863
DET
IC865
NJM2211M
IC864
FSL
SELADJ
MCU
Fig. 14 SELCALL operation (RX)
NQ
Transmission
Transmission is implemented by shifting the carrier fre-
quency (CAR). The DDS IC (IC702) has 2 registers (FREQ0
and FREQ1), each of which can retain different frequency
data. Selection of these frequencies is possible by changing
the applied voltage at the FSELECT terminal (10 pin). The
MCU writes data at 2125Hz and 2295Hz to DDS. The FSEL
data is changed according to the FSK signal, generating the
FSK transmission signal.
In the case of SELCALL, the FSK shift volume is fixed at a
value of 170Hz. In the case of RTTY operation, the FSEL ter-
minal is connected to the external terminal (RTK) via the ana-
log switch IC (IC851), and shifts the frequency according to
the signal from RTK.
Fig. 16 SELCALL operation (TX)=L= NS
CIRCUIT DESCRIPTION /
VR851
Center frequency f0
adjustment
FSD
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