User Manual Part 1

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In Project 25 Digital Mode, the carrier is modulated with four discrete deviation levels.
These levels are ± 600 Hz and ± 1800 Hz. Digitized voice is created using an IMBE™ vocoder.
(IMBE™ voice coding technology embodied in this product is protected by intellectual property
rights including patent rights of Digital Voice Systems, Inc.)
Receive Mode
The signal is processed in the same way as an analog mode transmission until after the
squelch function is performed. If a signal is detected to be present, the DSP resamples the signal
from 20 kHz to 24 kHz. This is done so that the sample rate is an integer multiple (5x) of the data
rate of the digital modulation which is 4800 symbols/sec (9600 bits/sec).
The resampled signal is then processed by a demodulator routine to extract the digital
information. The resulting bit stream (9600 bps) is sent to a routine that performs unframing,
error-correction, and voice decoding. The result of these operations is a reconstructed voice signal
sampled at 8 kHz. The sampled voice signal is sent to a D/A in the CODEC to produce an analog
signal for output to the audio power amplifier and speaker.
Transmit Mode
The microphone signal is processed as in the analog mode until it reaches the DSP. At this
point the audio signal is processed by a voice encoding routine to digitize the information. The
resulting samples are then converted to a bit stream that is placed into the proper framing
structure and error protected. The resulting bit stream has a bit rate of 9600 Hz.
This bit stream in then encoded, two bits at a time, into a digital level corresponding to one
of the four allowable frequency deviations. This produces 16-bit symbols with a rate of 4800 Hz.
The symbols are resampled to a rate of 48 kHz and filtered to comply with channel bandwidth
requirements. The filtered signal is then sent to a D/A in the CODEC to produce the analog
modulation signal for the VCO. The modulated VCO signal is then mixed up to the final transmit
frequency and then sent to the RF PA for transmission.
5.2 VHF BOARDS
5.2.1 VHF RF BOARD
The RF Board is not field serviceable. It must be replaced as a unit with a new board.
5.2.1.1 RECEIVER
Front End Bandpass Filter
A harmonic filter is followed by a PIN diode transmit/receive switch. Following this switch
a fixed tuned bandpass filter is used at the front-end of the receiver. This filter provides first
image rejection with minimal loss in order to provide the desired receiver sensitivity. Following
the filter a variable attenuator is used to increase the dynamic range of the receiver when
receiving high level signals.