Astro Mode Of Operation; Rf Basic Theory Of Operation; Figure 3. Rf Block Diagram (Power And Control Omitted); Table 2.Local Oscillator And First If Frequencies - Motorola XTS 2500 Basic Service Manual

From TX_SSI
ASTRO Mode of
Operation
RF Basic Theory
of Operation
Reference
Oscillator
EPIC
Y200
FracN
U201
3
MOD
IN
LPF
DAC
U203 FL200
ABACUS III U401
3
To RX_SSI
Sample 2ND
Clk LO

Figure 3. RF Block Diagram (Power and Control Omitted)

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.
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.
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.
Table 2. Local Oscillator and First IF Frequencies
LO Frequency Range
First IF Frequency
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
TX TX Driver Power
VCOBIC
Buffer Amplifier Module
U250
Q216 U101 Q101
Loop
VCO
Filter
VCO
PCIC
U102
VCO
Crystal Preselector
Mixer
Filter Filter
U1
FL3 FL2
700 MHz
--
109.65 MHz
Basic Theory of Operation
Directional
Coupler
U106
RF Power
Detector
Antenna
D103
Switch
RX LNA Attenuator Preselector
Q1 Filter
FL1
MAEPF-27322-O
800 MHz
--
109.65 MHz
To
Antenna
Harmonic
Filter
7