Tait TB7100, Installation And Operation Manual

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26 Functional Description TB7100 Installation and Operation Manual
© Tait Electronics Limited December 2005
3.2 Transmitter Operation
Parts of Transmitter
Board
The main circuit parts of the transmitter board are:
■
transmitter
■
frequency synthesizer
■
CODEC (coder-decoder) and audio circuitry
■
power supply
■
interface circuitry
Software plays a prominent role in the functioning of the board.
When describing the operation of the radio the software must be included
with the above. This is considered further below.
These functional parts are described in detail below.
3.2.1 Audio Processing and Signalling
Microphone Input
The input to the transmitter path begins at either the SI board or the
PROG
/
MIC
connector of the UI board. Only electret-type microphones are
supported. The audio input is then applied to tap point T8 on the
transmitter board (the tap point is user-selectable).
Analog Processing
of Microphone
Signal
The CODEC (AD6521) performs microphone selection and amplification.
The microphone amplifier consists of an amplifier with a fixed gain of 16 dB
followed by a programmable-gain amplifier with 0 dB to 22dB gain.
The amplified microphone signal is converted to a digital stream by a 16-bit
ADC with integral anti-alias filtering (0.1 to 3.2 kHz). The digital stream is
transported to the DSP for further audio processing.
Figure 3.3 Transmitter high-level block diagram
Analog-to-digital conversion
LPF
ANT
PLL
Loop
filter
Loop
filter
Frequency
control
Triple-point
Equalisation
Audio
filtering
Pre-
emphasis
Optional
processing
Data and signalling
encoders
ALC
Mic
PGA
Dir.
Coup.
Fin
Drv
/Ex
Pwr
Crtl
Ramp
control
TCXO:
13.000 MHz
VCO
VCXO
Bias
CUSTOM
LOGIC
BLOCK
HARDWARE
BLOCK
HARDWARE BLOCK
Digital-to-analog conversion
KEY
PROG/MIC connector
DSP
BLOCK
SYSTEM connector
Tap point T8