Basic Theory of Operation
8
transmitter power amplifier, which transmits the signal under
dynamic power control.
ASTRO Mode of
Operation
In the ASTRO (digital) mode of operation, the transmitted or received
signal is limited to a discrete set of frequency deviation levels. 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 different algorithm to recover
data.
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. Using this algorithm, transmitter
FM deviation is limited to discrete levels.
Transceiver (XCVR)
Board Basic Theory
of Operation
The receiver front end consists of a preselector filter, low-noise
amplifier, a second preselector, and a mixer. Both preselectors are
varactor-tuned band pass filters, controlled by the microcontroller. See
Table 2 for local oscillator (LO) and first IF information.
The frequency generation function is performed by three ICs, three
VCOs, and associated circuitry. The reference oscillator IC provides a
frequency standard to the synthesizer. The fractional-N synthesizer
turns on one of three external VCOs, and tunes it to the RX LO or TX
carrier frequency. The VCO buffer and a transistor amplify the signal
to the required power level. The synthesizer is controlled by the
microcontrol unit through a serial peripheral interface (SPI) bus. Most
of the synthesizer circuitry is enclosed in rigid metal cans on the XCVR
board to reduce interference and microphonic effects.
The receiver back end consists of a bandpass crystal filter, input and
output impedance matching networks, and the digital back-end IC.
Final filtering is done digitally in the DSP.
The Abacus III digital back-end IC contains a low-noise amplifier, a
mixer, a variable gain amplifier with integral anti-alias filter, a
bandpass, sigma delta, analog-to-digital converter, and a decimation
filter with a programmable decimation factor. The Abacus III also
contains an automatic gain control (AGC) circuit to provide 25 dB of
continuous gain adjustments. For the second LO, the Abacus III has an
internal, integer-N frequency synthesizer, and an external, discrete
loop filter and voltage-controlled oscillator (VCO). The output of the
Abacus III is digital data on the RX_SSI bus.
The transmitter power amplifier (PA) consists of a driver amplifier IC
and a discrete final-stage. Transmit power is controlled by a power
control IC (PCIC) that monitors the output of a directional coupler
Table 2. Local Oscillator and First IF Frequencies
700/800 MHz
LO Frequency Range 741.35-885.65 MHz
First IF Frequency 109.65 MHz
To Next Page
Loading...
