22 Digital Circuitry TB9100 Reciter Service Manual
© Tait Electronics Limited January 2006
The digital downconverter (DDC) digitally downconverts the desired image
(9.9MHz) to baseband. This is achieved by digital mixing with a
numerically controlled oscillator (NCO). The mixing process is done using
in-phase and quadrature methods to achieve image rejection, and allows
channel filtering to be applied before the signal is passed to the digital signal
processor (DSP) for demodulation. The digital channel filtering also
decimates the sample rate down to 50kSPS (kilosamples per second) for the
DSP.
3.2 Digital Signal Processor (DSP)
The DSP is responsible for software processing of digitized signals in the
receiver and transmitter. The processing word width is 16-bit fixed point.
There are 96 kilobytes of on-chip program memory, and 64 kilobytes of
on-chip data memory. Although no external memory is used, the external
memory interface is connected to the DDC for initialization and
configuration.
3.2.1 Transmit Functions
The DSP performs the following transmit functions:
■ CTCSS and DCS sub-audible signal generation
■ CWID generation
■ pip tone generation
■ audio filtering: including removal of sub-audible components, pre-
emphasis and low pass filtering
■ signal path switching
■ signal level adjustment
■ peak FM deviation limiting
■ FM signal generation by controlling the dual point modulator
■ calibration parameter estimation
■ line level monitoring.
3.2.2 Receive Functions
The DSP performs the following receive functions:
■ detection of CTCSS and DCS signalling
■ audio filtering: including removal of sub-audible components, de-
emphasis and low pass filtering
■ signal path switching
■ signal level adjustment
■ FM demodulation of the base band signal
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