TB9100 Reciter Service Manual Digital Circuitry 27
© Tait Electronics Limited January 2006
The RISC controls the frequency generation subsystem. It detects an
external reference source and selects internal or external reference. It also
programs, and handles out-of-lock signals for, the following synthesizers:
■ 12.8MHz external reference synthesizer
■ 40MHz digital clock synthesizer
■ receiver synthesizer
■ exciter synthesizer.
The RISC communicates with the control panel via I
2
C bus to:
■ read the button states
■ read the microphone PTT state
■ control the LEDs
■ turn the speaker amplifier on and off
■ turn the microphone amplifier on and off.
The following signals go via the control panel for signal conditioning:
■ CSS communication via the RS-232 interface
■ fan-good indication (front panel fans).
Note that the volume control on the control panel is analog only and is not
controlled by the RISC.
The RISC communicates with the other modules in the subrack via I
2
C in
order to:
■ verify that they are present
■ write configuration settings
■ read their current status.
The RISC subsystem communicates with the network PCB via SPI to:
■ set input and output gains
■ mute and unmute outputs
■ read digital inputs
■ write digital outputs.
The Rx Gate and Tx Relay lines go via the network PCB for signal
conditioning.
3.4 40MHz Digital Clock
The 40MHz synthesized digital clock is situated on the digital PCB. It is
used to drive the entire digital circuitry.
The 40MHz frequency synthesizer is implemented using an Integer_N-
based phase locked loop (PLL) IC. The PLL is a negative feedback loop,
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