Tait TM8000 Series, Service Manual

Page: 64 / 244

64 Circuit Descriptions TM8100 Mobile Radio Service Manual
May 2004 © Tait Electronics Limited
Serial Peripheral
Interface
The control-head board uses an SPI (serial peripheral interface) to
control the display on the LCD, turn back-lighting on and off, control
the
STATUS
LEDs, and read the status of the keys. The interface consists
of the following four lines at the control-head connector:
■
pin 11: data out
CH SPI DO
■
pin 12: latch line
CH LE
■
pin 15: data in
CH SPI DI
■
pin 16: clock
CH SPI CLK
The handling of the data is described below.
Data Input and Output
Data that are input to the control head are clocked through the LCD
driver and daisy-chained to a shift register. Once all the data have been
clocked in, the latch line is driven low. On this falling edge all the
outputs (LEDs and LCD segments) are driven to their new state. When
the latch line is driven high, the state of each key is latched into another
shift register. The data are then clocked out back to the radio body so
that the radio can respond accordingly.
LCD Driver
An oscillator is used to run the LCD. It oscillates at about 60 Hz and
employs a Schmitt trigger and D flip-flop to ensure a 50% duty cycle to
the LCD. A reset circuit is required because the reset from the main
board is not routed to the control head. The reset circuit also employs a
Schmitt trigger.
Volume Control
The volume-control potentiometer is linear and passes the DC voltage
signal
VOL WIP DC
to the radio body. The signal is read by an ADC on the
main board, and the volume is adjusted accordingly.
3.2 Transmitter
Broadband Exciter
The transmitter circuitry is given on sheet 3 of the circuit diagram for
the main board. The broadband exciter is a common element in all the
variants of the TM8100 mobile radio, as it operates across all frequencies
from 66 MHz to 940 MHz. It is made up of Q300 and Q303, which
amplify the signal provided by the frequency synthesizer from its level of
7 to 9 dBm up to 24.5 dBm for the frequency band from 66 to 530 MHz
and slightly less than this for the bands covering 530 MHz to 940 MHz.
The exciter operates in full saturation, thereby maintaining a constant
output power independent of the varying input power level supplied by
the synthesizer (7 to 9 dBm).