Service manual

23
SECTION VII. BASIC THEORY OF OPERATION
A. Introduction to This Section
The following theory will help isolate the problem
to a particular board. Using circuit board replacement
as the basic service approach will maximize working-
time of the radio.
B. General Overview
The ASTRO Digital SABER radio is a wideband,
synthesized, fixed-tuned radio available in the
VHF, UHF, and 800MHz bands. All ASTRO Digital
SABER radios are capable of both analog operation
and ASTRO mode (digital) in 12.5kHz or 25kHz
bandwidths.
The ASTRO Digital SABER radio consists of four
major assemblies. They are:
Controller Board - contains the microcontrol unit
(MCU) and its associated memory and memory
management integrated circuit (IC), the digital sig-
nal processor and its associated memories and
support IC, the audio power amplifier, and a
switching regulator.
RF Board - contains all transmit, receive, and fre-
quency generation circuitry including the digital
receiver back-end IC and the reference oscillator.
Display/Keypad Assemblies - contain the inter-
nal microphone and speaker, a two-line liquid
crystal display (LCD), and a 3 x 6 keypad.
Control Top - contains switches for volume and
mode selection, push-to-talk (PTT), monitor, and
several function-selectable switches.
C. Analog Mode of Operation
When the radio is
receiving
, the signal comes from
the antenna/antenna-switch connector to the RF
board, passes through the RX/TX switch and the
receiver front end. The signal is then filtered, ampli-
fied, and mixed with the first local-oscillator signal
generated by the voltage-controlled oscillator (VCO).
The resulting intermediate frequency (IF) signal is fed
to the IF circuitry, where it is again filtered and ampli-
fied. This amplified signal is passed to the digital
back-end IC, where it is mixed with the second local
oscillator to create the second IF at 450kHz. It is then
converted to a digital bit stream and mixed a third time
to produce a baseband signal. This signal is passed to
the controller board through a current-driven differen-
tial output. On the controller board, the digital-
signal-processor-support IC digitally filters and dis-
criminates the signal, and passes it to the digital-signal
processor (DSP). The DSP decodes the information in
the signal and identifies the appropriate destination for
it. For a voice signal, the DSP will route the digital
voice data to the DSP-support IC for conversion to an
analog signal. The DSP-support IC will then present
the signal to the audio power amplifier, which drives
the speaker. For signalling information, the
DSP will decode the message and pass it to the
microcontrol unit.
When the radio is
transmitting
, microphone audio
is passed from the audio power amplifier (PA) to the
DSP-support IC, where the signal is digitized. The
DSP-support IC passes digital data to the DSP, where
pre-emphasis and low-pass (splatter) filtering are
done. The DSP returns this signal to the DSP-support
IC, where it is reconverted into an analog signal and
scaled for application to the voltage-controlled oscilla-
tor as a modulation signal. Transmitted signalling
information is accepted by the DSP from the micro-
control unit, coded appropriately, and passed to the
DSP-support IC, which handles it the same as a voice
signal. Modulation information is passed to the synthe-
sizer along the modulation line. A modulated carrier is
provided to the RF PA, which transmits the signal
under dynamic power control.
D. ASTRO Mode of Operation
In the ASTRO mode (digital mode) of operation,
the transmitted or received signal is limited to a dis-
crete set of deviation levels, instead of continuously
varying. The receiver handles an ASTRO-mode signal
identically to an analog-mode signal up to the point
where the DSP decodes the received data. In the
ASTRO receive mode, the DSP uses a specifically
defined algorithm to recover information. In the
ASTRO transmit mode, microphone audio is pro-
cessed identically to an analog mode with the
exception of the algorithm the DSP uses to encode the
information. This algorithm will result in deviation lev-
els that are limited to discrete levels.
E. RF Board Basic Theory of Operation
The receiver front end consists of a preselector,
an RF amplifier, a second preselector, and a mixer.
Both preselectors in the VHF and UHF radios are var-
actor-tuned, two-pole filters controlled by the
microcontrol unit through the digital/analog (D/A) IC.
On the 800MHz receiver front end, these filters are
fixed-tuned. The RF amplifier is a dual-gate, gallium-
arsenide based IC. The mixer is a double-balanced,