Tait TM8000 Series, Руководство по обслуживанию

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58 General Description TM8100 Mobile Radio Service Manual
May 2004 © Tait Electronics Limited
DSP Audio Processing
The output of the automatic level control provides the input to the DSP
audio-processing chain at a sample rate of 8 kHz. Optional processing
such as encryption or companding is done first if applicable. Pre-
emphasis, if required, is then applied. The pre-emphasised signal is hard-
limited to prevent over-deviation, and filtered to remove high-frequency
components. The sample rate is then interpolated up to 48 kHz and
scaled to be suitable for the frequency synthesizer.
Data and Signalling
Encoders
The data and signalling encoders inject their signals into various points
within the audio-processing chain. The injection point depends on the
bandwidth of the encoders and whether pre-emphasis is required.
Frequency Synthesizer
Main Parts of
Synthesizer
As shown in Figure 2.14 , the frequency synthesizer consists of two main
parts:
■
FCL (frequency control loop)
■
RF PLL (phase-locked loop)
The FCL and RF PLL are described briefly below. Detailed descriptions
of the circuitry are given in
Section 3 (on page 63) . Note that patents
are pending for several aspects of the synthesizer design.
Frequency Control
Loop
The FCL consists of the following:
■
TCXO
■
mixer
■
loop filter
■
VCXO (voltage-controlled crystal oscillator)
■
frequency control block
The FCL provides the reference frequency for the RF PLL. It generates
a high-stability reference frequency that can be both modulated and
offset in fine resolution steps.
RF PLL
The RF PLL consists of the following:
■
RF PLL device
■
loop filter
■
VCO (voltage-controlled oscillator)
■
VCO output switch
The RF PLL has fast-locking capability but coarse frequency resolution.
The above combination of control loops creates improved frequency
generation and acquisition capabilities.