Guardian G25AMK005, Техническое руководство

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3-14 G U A R D I A N V H F 1 1 0 W M O B I L E
3.5.5 Receive Physical Link Layer
This software uses common FM demodulation software, and mode-dependent receiver physical layer software
modules.
3.5.5.1 Receive ADC and DAC Interface
In receive modes SSI port 1 is used in a duplex manner to allow the ADC to be read continuously at 96 kHz and the
DAC written up to 48 ksps for AGC and reference oscillator adjustment. In receive modes, the SSI port is clocked
at 1536 kHz from an external clock source, using a 16-bit cycle. At every cycle a value is read out of the ADC. At
every other cycle a value may be written to one of the DAC channels to control the AGC and reference oscillator.
3.5.5.2 FM Demodulator
This software provides a common interface to the 96 ksps IF signal at SSI port 1 and produces 48 ksps of frequency
deviation data for use in all modes. The SSI port uses an external clock at 1536 kHz. The software implements an
FM demodulator function using a quadrature mix with a 24 kHz (Fs/4) local oscillator, dual I and Q channel filters,
and a frequency estimator. The channel filtering function is programmable-dependent on the channel spacing used.
The channel filtering provides the adjacent channel filtering in addition to that provided by the hardware to achieve
the radio adjacent channel rejection performance.
3.5.5.3 Receive CAI Physical Link Layer
This software takes the 48 ksps of frequency deviation data and outputs a 9.6 kbps data stream. The software
implements an integrate and dump filter and data slicer as described in the CAI. The integrate and dump filter is
controlled by a clock recovery function that selects one of ten possible phases for output to the slicer. The slicer
incorporates an averager with a time constant of at least 100 bits, to correct for dc offsets in the received signal.
In parallel with the above, a FIR correlator searching for the CAI fixed framing sequence of 24 symbols operates on
the filtered 48 ksps of frequency deviation data. The correlator operates at 10 samples per symbol. The correlator
phase with the highest correlation peak selects the clock phase for use in the integrate and dump filter and slicer.
This correlator operates continuously when searching for CAI traffic and occasionally when tracking an CAI signal
when subsequent frame syncs are expected. The correlator trigger is used to provide a framing signal for the
subsequent CAI link layer processing.
3.5.5.4 Receive Analog FM Physical Link Layer
This software takes the 48 ksps frequency deviation data and outputs 8 ksps of audio to the receive audio processing.
Software signal processing implements a high-pass filter to remove CTCSS tones and de-emphasis if required. The
gain of the signal path is adjusted to cope with the different deviations used on different channel bandwidths. The
signal processing signal path is controlled by squelch signals. The software includes audio expanding to reverse the
transmit companding.
3.5.5.5 Receive CVSD Physical Link Layer
This software takes the 48 ksps frequency deviation data and outputs 12/16 kbps serial data. The software uses a
data filter, a slicer, and a clock recovery function.
3.5.6 DES Encryption
The DSP software implements DES encryption of traffic in the CAI and CVSD modes.
3.5.6.1 DES Kernel
The DSP software implements the DES encryption kernel as described in FIPS 46-2, encrypting 64 data bits using a
56-bit key. It uses output feedback operation or a single bit cipher feedback operation.