Basic Theory of Operation
7
Transmitted signalling information is accepted by the DSP from the microcontrol unit,
coded appropriately, and passed to the D/A converter, which handles it the same as a voice
signal. Modulation information is passed to the synthesizer along the modulation line. A
modulated carrier is provided to the RF PA, which transmits the signal under dynamic
power control.
ASTRO Mode of
Operation
In the ASTRO mode (digital mode) of operation, the transmitted or received signal is
limited to a discrete 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 processed identically to an analog mode
with the exception of the algorithm the DSP uses to encode the information. This algorithm
will result in deviation levels that are limited to discrete levels.
RF Basic Theory
of Operation
The receiver front end consists of a preselector, an RF amplifier, a second preselector, and
a mixer. Both preselectors are varactor-tuned, two-pole filters, controlled by the
microcontroller unit through the D/A IC. The RF amplifier is a dual-gate, gallium-arsenide-
based IC. The mixer is a double-balanced, active mixer, coupled by transformers. Injection
is provided by the VCO through an injection filter. See Table 2 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 synthesizer/prescaler IC, which
controls the VCOB IC. The VCOB IC actually generates the first LO and transmit-injection
Figure 3. RF Block Diagram (Power and Control Omitted)
Reference
Oscillator
FracN
U201
MOD
IN
Loop
Filter
LPF
FL200
EPIC
Y200
DAC
U203
VCO
VCOBIC
U250
VCO
Crystal
Filter
FL3
Mixer
U1
2ND
LO
Sample
Clk
To RX_SSI
From TX_SSI
Preselector
Filter
FL2
Attenuator
PCIC
U102
Power
Module
Q101
TX
Buffer
Q216
TX Driver
Amplifier
U101
RF Power
Detector
D103
Directional
Coupler
U106
Antenna
Switch
Preselector
Filter
FL1
RX LNA
Q1
To
Antenna
Harmonic
Filter
ABACUS III U401
MAEPF-27322-O
VCO
3
3
Table 2. Local Oscillator and First IF Frequencies
700 MHz 800 MHz
LO Frequency Range -- --
First IF Frequency 109.65 MHz 109.65 MHz
To Next Page
Loading...
