Chapter 3 Theory Of Operation; Rf Board; General - Motorola ASTRO Digital Spectra Service Manual

Chapter 3 Theory of Operation

3.1

RF Board

This section provides a detailed circuit description of the ASTRO RF board for VHF, UHF and
800 MHz models. This board contains the common synthesizer circuits (synthesizer section) and
dual IF receiver and demodulation circuits (receiver back-end). When reading the theory of
operation, refer to your appropriate schematic and component location diagrams located in
7. Schematics, Component Location Diagrams, and Parts
will help isolate the problem. However, first use the ASTRO Digital Spectra and Digital Spectra Plus
Mobile Radios Basic Service Manual (68P81076C20) to troubleshoot the problem to a particular
board.
3.1.1

General

The synthesizer section includes the prescaler IC (U601), the synthesizer IC (U602), and the
reference oscillator (U600). The prescaler and the synthesizer chips are completely controlled by the
serial data bus.
The prescaler IC (see
• Multi-dual modulus prescaler
• 5-V regulator
• Super filter 8.6-V regulator
• Fixed divide-by-8 circuit for the reference oscillator
• Programmable divide-by-N and charge pump phase detector to support the second injection
VCO
The synthesizer IC (see
• Reference divider
• Phase modulator
• Dual-bandwidth adaptive filter
• Ramp generator
• Sample-and-hold phase detector
• Programmable loop divider
• Auxiliary output bits for system control
Figure 3-1) provides the following:
Figure 3-2) provides:
Lists". This detailed Theory of Operation
"Chapter