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Beam Position Monitors sense the passage of the beam when it induces a signal in the pickup electrodes mounted in the beam waveguide. This induced signal is of low current but can be 10's of volts for several nanoseconds, in synch with the beam's arrival. This signal is put directly on to a high quality coax to the receiving electronics.
In addition to this readback pulse, the stripline BPM system generates a calibration pulse that is sent to the striplines to correct for signal gain variation and drift. This is performed between beam pulses.
LCLS Stripline BPM Processor Engineering Specification (DRAFT)
LCLS Cavity BPM CalibrationLCLS BPM Data Acquisition System WPPB40
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LCLS-II cavity BPM electronics in Building 913
LCLS Cavity BPM Receivers Network Information
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