Download Sector Phase Control Unit

NLC Timing System
Phase Control Unit
Overview
This unit is part of the redundant phase transmission system. It takes a fiber optic input from
each of the two timing channels. Phase shifters are used to adjust the relative phases of the
received signals to match each other, and to match the phase measured from the electron beam.
A switch/sample-and-hold system allows the same electronics to be used for both phase
comparisons, thus eliminating component variation effects.
When one of the fiber optic timing channels drops out, the system switches immediately to the
other channel, with minimal change in phase. The system retransmits the optical signal.
Functional Description
Fiber Input A: An optical detector at the module’s input converts the fiber optic light signals
from to electrical signals. The detector’s output is split. One splitter output drives an RF
detector, whose output is sampled by an Analog to Digital Converter. The RF level reading is
stored in the local microprocessor’s memory space.
The other splitter output feeds one side of a mixer in the local 10 kHz bandwidth Phase Lock
Loop (PLL).
The mixer’s output drives two items (1) a Comparator whose output is sampled by the local
microprocessor through a “glue logic” function block, and (2) a Voltage Controlled Oscillator
(VCO) whose nominal output is 357 MHz.
The VCO’s output is split two ways: (1) one output drives the PLL’s mixer, (2) the other output
goes through a Phase Shifter which is controlled by a Digital to Analog Converter under control
of the local microprocessor. The Phase Shifter’s output is routed to an RF switching matrix.
Fiber Input B: This function block is identical to the Fiber Input A block described above,
with the exception that the output drives a different input on the RF switching matrix.
The RF switching matrix’s output drives one input of a mixer in the 100 kHz bandwidth output
Phase Lock Loop. The other input of the mixer comes from the electrical output of an optical
detector which monitors the PCU module’s optical output. The mixer’s output is sampled at a
rate determined by the Eight Channel Alarm Clock (ECAC) delay output generator. The
Sample and Hold circuit output drives the 357 MHz Voltage Controlled Crystal Oscillator
(VCXO).
At the output of the VCXO there is a Fiducial Generator function block, driven by the ECAC.
The Fiducial Generator’s output drives a Laser Diode, whose output is routed through a
directional coupler to the PCU module’s optical output.
A local microprocessor controls the ECAC, the ADC’s and DAC’s, and communicates with the
host computer via the VXI interface.
PCU.doc
05/07/17
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