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 “Chapter
7. Schematics, Component Location Diagrams and Parts Lists”. This detailed Theory of Operation
will help isolate the problem. However, first use the ASTRO Digital Spectra and Digital Spectra Plus
Mobile Radios Basic Service Manual (6881076C20) 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 Figure 3-1 on page 3-2) provides the following:
• 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 Figure 3-2 on page 3-2) provides:
• Reference divider
• Phase modulator
• Dual-bandwidth adaptive filter
• Ramp generator
• Sample-and-hold phase detector
• Programmable loop divider
• Auxiliary output bits for system control
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