54 General Description TM8100 Mobile Radio Service Manual
May 2004 © Tait Electronics Limited
Control-head Board The control-head board does not include a microprocessor. A
synchronous bi-directional serial interface provides communication of
key status, LCD and LED-indicator data between the radio body and the
control head. On the control-head board the serial data are converted to
or from parallel form by a number of shift registers for the keys and
indicators. For the LCD, the serial data are fed to a driver IC that
converts the serial data to a form suitable for the LCD itself. The keys
are scanned and the LCD and LED indicators updated approximately
every 50 ms.
2.4 Operation in Receive Mode
Receive Path This subsection describes the functioning of the radio in receive mode.
Detailed descriptions of the circuitry involved are given in Section 3
(on page 63). As shown in Figure 2.14, the receive path exists in the
hardware, custom-logic and DSP domains. From a functional point of
view there are three major parts:
■ RF hardware
■ digital base-band processing
■ audio processing and signalling
These functional parts are described in detail below.
RF Hardware
Front-end Circuitry
and First IF
The front-end hardware amplifies and image-filters the received RF
spectrum, then down-converts the desired channel frequency to a first
intermediate frequency IF1 of 45.1 MHz (UHF) or 21.4 MHz (VHF)
where coarse channel filtering is performed. The first LO (local
oscillator) signal is obtained from the frequency synthesizer and is
injected on the low side of the desired channel frequency for all bands.
In receive mode the modulation to the frequency synthesizer is muted.
See Subsection 2.5 (on page 57) for a description of the synthesizer. The
output of the first IF (intermediate frequency) stage is then down-
converted using an image-reject mixer to a low IF of 64 kHz.
Quadrature
Demodulator
The LO for the image-reject mixer (quadrature demodulator) is
synthesized and uses the TCXO (temperature-compensated crystal
oscillator) as a reference. This ensures good centring of the IF filters and
more consistent group-delay performance. The quadrature demodulator
device has an internal frequency division of 2 so the second LO operates
at 2 x (IF1 + 64 kHz). The quadrature output from this mixer is fed to
a pair of ADCs (analogue-to-digital converters) with high dynamic range
where it is oversampled at 256 kHz and fed to the custom logic device.
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