CIRCUIT DESCRIPTION
5-2
The signal is then mixed with the second local
oscillator frequency to create a second IF signal. The
second IF signal is then sampled and downconverted
to baseband. The baseband signal is then decimated to
a lower sample rate that is selectable at 20 kHz. This
signal is then routed via a serial interface from the IF
chip to the DSP on the logic board.
On the logic board the DSP digitally filters the
input signal and performs frequency discrimination to
obtain the message signal. The DSP first performs a
carrier- detection squelch function on the radio. If a
signal is determined to be present, the audio portion of
the signal is resampled and then filtered appropriately.
The filtered signal is then routed back to a D/A in the
CODEC to produce an analog signal for output to the
audio power amplifier and then the speaker. Any
detected signaling information is decoded and the
resulting information is sent to the microcontroller.
Transmit Mode
The signal from the microphone is amplified and
then routed to the CODEC chip where it is first digi-
tized and then sent to the DSP. The DSP performs the
required filtering, adds the desired signaling, converts
the sample rate and then sends the resulting signal
back to a D/A in the CODEC to produce the analog
modulation signals for the VCOs. The modulated
VCO signal is then sent to the RF power amplifier and
transmitted.
5.1.5 PROJECT 25 DIGITAL MODE
Introduction
In Project 25 Digital Mode, the carrier is modu-
lated with four discrete deviation levels. These levels
are ± 600 Hz and ± 1800 Hz. Digitized voice is created
using an IMBE™ vocoder.
Receive Mode
The signal is processed in the same way as an
analog mode transmission until after the squelch func-
tion 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 recon-
structed 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 ampli-
fier 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 UHF RF BOARD (VERSION C)
NOTE: The following describes the new revised
Version C RF board described in Section 1.12.1. The
RF Board is not field serviceable. It must be replaced
as a unit with a new board.
5.2.1 RECEIVER
Front End Bandpass Filter
A harmonic filter is followed by a PIN diode
transmit/receive switch. Following the switch, two
fixed tuned bandpass filters are used in the front-end
of the receiver. Depending on the desired receive band,
the appropriate filter is selected using RF switches
(U37/U41). This filter provides first-image rejection
with minimal loss in order to provide the desired level
IMBE™ voice coding technology embodied in this product
is protected by intellectual property rights including patent
rights of Digital Voice Systems, Inc.
UHF RF BOARD (VERSION C)
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