Guardian G25AMK005, Техническое руководство

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3-8 G U A R D I A N V H F 1 1 0 W M O B I L E
3.3.1.7 Transceiver Interface
The transceiver DAC has four output ports, two of which modulate the carrier. One of the two channels maintains
carrier frequency accuracy. On transmit channel changes, the controller provides the DSP with two fractional values
used to scale the two signals output from the DAC. The controller provides the DSP with an additional integer value
at one second intervals, and is added to one of the DAC output signals to control carrier frequency accuracy. The
modulation interface receives modulation data samples at 48 ksps, independent of the transmit mode. When the
radio is operating as a transmitter, the transceiver interface controls the operation of the DAC via ESSI 1 on the
DSP. Data is written to the DAC at 96 ksps.
3.3.2 DSP Receive Chain
The radio receive chain hardware consists of an RF transceiver board, analog to digital converter (ADC), a
Motorola® DSP 56302 or DSP 56309, and an audio CODEC. The Receive DSP Chain block diagram is shown in
Figure 1-3. The major signal receive functions of the DSP receive chain are described in the following paragraphs.
Figure 3-3: Receive DSP Chain
3.3.2.1 Transceiver
The RF transceiver board performs mixing and filtering of the received signal to produce a 455 kHz, 25 kHz
bandwidth (BW), IF signal. The output signal from the transceiver is digitized by the ADC and fed to the DSP. The
bulk of signal processing is performed by the DSP. An ISR that implements the transceiver/ADC/DAC interface is
called at a rate of 96 kHz in receive modes. The ISR reads ADC output data, stores the values in a circular buffer,
and controls data transmission to the DAC.
3.3.2.2 FM Demodulator
The FM demodulator converts the FM output of the transceiver to a real-valued, baseband signal. FM demodulation
is implemented by a discriminator task. Sub-sampling the 455 kHz IF at 96 kHz folds down the signal to 25 kHz. A
mixing function mixes the sampled IF data in the input buffer before filtering. For 12.5 kHz channels, a second
CVSD DES
Module
Audio
CODEC
Transceiver
Audio
Processing
Module
FM
Demodulator
Project 25
Voice Module
Analog FM
Module
Signal
Detectors