Service Manual Part 2

CIRCUIT DESCRIPTION
4-2
September 2001
Part No. 001-5100-001
Transmit Mode
The signal from the microphone is amplified by
the audio PA and is then routed to the ADSIC chip
where it is first digitized at a 16 ksps rate and then sent
to the DSP. The DSP performs the required filtering,
adds the desired signaling, converts the sample rate to
48 ksps and then sends the resulting signal back to a
D/A in the ADSIC to produce the analog modulation
signal for the VCO. The modulated VCO signal is then
sent to the RF PA for transmission.
4.1.3 PROJECT 25 DIGITAL MODE
Introduction
In Project 25 Digital Mode, the carrier is modu-
lated with 4 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 ADSIC 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 ADSIC 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.
4.1.4 RF BOARD
NOTE: The RF Board is not field serviceable. It must
be replaced as a unit with a new board.
The receiver front end consists of a preselector,
RF amplifier, second preselector, and mixer. Both
preselectors on the VHF and UHF board are varactor-
tuned, two-pole filters controlled by the microcon-
troller unit through the D/A IC. The 800 MHz board
uses stripline technology for the preselector. The RF
amplifier is a dual-gate gallium-arsenide IC. The
mixer is a double-balanced, transformer-coupled
active mixer. Injection is provided by the VCO
through an injection filter. See Table 4-1 for local
oscillator (LO) and first IF information.
The frequency generation function is performed
by three ICs and associated circuitry. The reference
oscillator provides a frequency standard to the synthe-
sizer/prescaler IC which controls the VCO IC. The
VCO IC actually generates the first LO and transmit
injection signals and buffers them to the required
power level. The synthesizer/prescaler circuit module
incorporates frequency division and comparison
circuitry to keep the VCO signals stable. The synthe-
sizer/prescaler IC is controlled by the microcontroller
through a serial bus. Most of the synthesizer circuitry
is enclosed in rigid metal on the RF Board to reduce
microphonic effects.
Table 4-1 LO and First IF Frequencies
VHF UHF 800 MHz
LO Frequency
range
181.15 -
219.15 MHz
329.65 -
446.65 MHz
776.65 -
796.65 MHz
First IF
Frequency
45.15 MHz 73.35 MHz 73.35 MHz