5100 ES Series Portable Radio Service Manual 5-3
Circuit Description
5.1.3 Project 25 Digital Mode
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.
5.1.3.1
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.
5.1.3.2
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
UHF Low RF Board
Note
The RF Board is not field serviceable. It must be replaced as a unit with a new board.
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