Guardian G25AMK005, Техническое руководство

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G U A R D I A N V H F 1 1 0 W M O B I L E 3-9
filter is applied to the IQ data stream. Calculating the angular difference between consecutive IQ pairs demodulates
the received signal.
3.3.2.2.1 Analog to Digital Converter
Because the signal BW is much less than the 455 kHz carrier frequency, the ADC sub-samples the 455 kHz IF
producing a frequency translation as part of the sampling process. The ADC sampling rate is 96 ksps.
3.3.2.2.2 25 kHz Frequency Translation
The 25 kHz frequency translation converts the signal image into a baseband signal, centered at 0 Hz. The DSP
implements the digital equivalent of a mixer to perform frequency translation.
3.3.2.2.3 IF Filtering
The complex baseband signal is sent through two linear phase FIR filters. The first IF filter is used as a decimate by
two, polyphase, FIR filters and applied to the 96 ksps, complex, baseband output of the 25 kHz mixer. This filter
removes some of the out-of-band noise produced by the nonlinear analog components of the transceiver. CVSD
DES and analog wide modes have a 25 kHz BW and the first IF filter is the only filtering performed for these
modes. Project 25 and analog narrow modes have a 12.5 kHz BW. The second IF filter provides the filtering
required for these modes and is applied to the 48 ksps output of the first IF filter.
3.3.2.3 Signal Detectors
The radio uses three signal detectors to detect the presence or absence of a modulated signal in the tuned channel.
These signal detectors search for Project 25, analog FM (including noise, CTCSS, and DCS), and CVSD DES
signals.
3.3.2.3.1 Project 25 Detector
The Project 25 frame detector detects a Project 25 signal by searching for the frame synchronization (FS) signal and
network identifier (NID) embedded in the preamble of every Project 25 data unit. The detector uses this information
to perform bit recovery and packet identification. It processes and buffers the binary data for use by the Project 25
Voice Module. Once a target signal is detected, the radio disables squelch using an enable transmit function, so the
user can monitor the signal. If the detected signal is lost, squelch is enabled by the shutdown active receive
function.
3.3.2.3.2 Analog FM Detector
The analog FM detector uses a function to decimate the incoming data stream by six, to run the noise detector,
CTCSS single-tone detector, CTCSS multi-tone detector, and the DCS multi-code detector. A noise squelch detect
function detects the appearance of a carrier by searching for a drop in power in a frequency band just above the
audio band. In the analog noise detector, the input data is scaled and high-pass filtered, then rectified and scaled
again. Then the data is low-pass filtered. The output of the low-pass filter is used to determine whether or not a
signal is present.
The detector has two states, searching and locked. If the detector state is searching and the detected power drops
below the lower squelch threshold, the detector state transitions to locked. Conversely, if the detector state is locked
and the detected power rises above the upper squelch threshold, the detector state transitions to searching. Upper
and lower threshold values are BW dependent and can be adjusted at run time.
The multiple-value DCS detector searches for a 134.4 bps bit stream in the sub-audible frequency band used for
DCS codes. If found, the code is extracted and appropriate state variables updated. Code extraction is performed in
two steps: input data is converted to a binary bit stream, and then data extraction and code comparisons are
performed.
The conversion of the input data to a binary bit stream starts with the 8 ksps input data sent through a decimate by
six, FIR filter to produce a 1.33 ksps, real valued data stream. This filter removes any signal energy outside of the
sub-audible frequency band. The data is then split into two paths. The lower path estimates the dc content of the
signal with a narrow low-pass infinite impulse response (IIR) filter. Subtracting the lower path signal from the