Motorola ASTRO Digital XTS 3000 Basic Service Manual - Page 38

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Motorola ASTRO Digital XTS 3000 Basic Service Manual

ASTRO Mode

of Operation

RF Board Basic

Theory of

Operation

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 oscillator as a modulation signal.

Transmitted signalling information is accepted by the DSP from the

microcontrol 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 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. On the 800MHz receiver front end, both preselectors

are varactor-tuned, two-pole, ﬁxed-tuned ﬁlters. 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 13 for local oscillator (LO) and ﬁrst IF information.

Table 13 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 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 synthesizer/prescaler IC is controlled by the microcontrol unit through a

serial bus. Most of the synthesizer circuitry is enclosed in rigid metal cans on

the RF board to reduce microphonic effects.

The receiver back end consists of a two-pole crystal ﬁlter, an IF ampliﬁer, a

second two-pole crystal ﬁlter, and the digital back-end IC. The two-pole ﬁlters

are wide enough to accommodate 5kHz modulation. Final IF ﬁltering is done

digitally in the DSP-support IC.

800MHz

732.65-796.65MHz

73.35MHz