Controller Section - Motorola ASTRO Digital XTL 5000 Detailed Service Manual

3-62
3.7.4.12 Receiver VCOs
Receiver first local-oscillator frequencies are generated from two VCOs in U6755. U6755 is not
serviceable and should be replaced if determined to be non-functional. For receiver frequencies in
the range of 764 to 776 MHz, high-side, first local-oscillator injection of 837.35 to 849.35 MHz (Fc +
73.35 MHz) is generated when Aux3 (pin 2) of U6751 is active high (3.0 Vdc). This 3.0 volts is
applied to transistor switch Q6756 allowing the superfiltered 8.2-V supply to be connected to pin 2
(SWBPOSC1), bias for the first VCO of U6755. For receiver frequencies in the range of 851 to
870 MHz, low-side, first LO injection of 777.65 to 796.65 MHz (Fc - 73.35 MHz) is generated when
Aux4 (pin 3) of U6751 is active high (3.0 Vdc). This 3.0 volts is applied to transistor switch Q6760
allowing the superfiltered 8.2-V supply to be connected to pin 17 (SWBPOSC2), bias of the second
VCO of U6755. Pin 20 (RFOUT) of U6755 is common to both oscillators and couples the oscillator's
output signal to the first stage of the receiver injection string. Frequency steering is accomplished
through pin 8 (CONT_V) of U6755.
3.7.4.13 Prescaler Feedback
RF feedback for the synthesizer loop is provided by prescaler amplifier Q6761. Feedback from both
the transmitter and receiver injection strings are coupled to this amplifier through resistor networks
that both balance and attenuate the levels prior to amplification by Q6761. The output of Q6761 is
coupled to U6751 at pin 32 (PREIN), which is the prescaler input for the synthesizer.
3.8

Controller Section

The controller section consists of a daughtercard module and associated circuitry to which it
interfaces. It is the central interface between the various subsystems of the radio. Its main task is to
interpret user input, provide user feedback, and schedule events in the radio operation, which
include programming ICs, performing digital signal processing (DSP) on baseband audio and data,
and sending messages to the control head display.
components of the controller section.
The DSP section of the microprocessor performs digital vocoder (voice coder-decoder) functions
previously performed by analog circuitry. This includes all tone signaling, trunking signaling, and
conventional analog voice processing.
All analog signal processing is accomplished digitally using the Patriot IC microprocessor (U100).
This microprocessor consists of a microcontroller, as well as a DSP. In addition, it provides a digital
voice plus data capability utilizing IMBE voice-compression algorithms. Vocoder is the general term
used to refer to these DSP-based systems.
May 25, 2005
Theory of Operation: Controller Section
Figure 3-43 on page 3-63 illustrates the
6881096C74-B