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Multiplexing Beam Position Monitors (BPM) are widely used for their
simplicity and inherent drift cancellation property. These systems
successively feed the signals of (typically four RF) pickups through one
single detector channel. The beam position is calculated from the
demultiplexed (base band) signal. However, as shown by this contribution,
transverse beam motion results in positional aliasing errors due to the
finite multiplexing frequency. Fast horizontal motion, for example, can
alias into an apparent, slow vertical position change. A thorough
analysis is presented and the impact of essential parameters such as the
multiplexing rate and the scanning pattern/sequence of classical 4-button
pickups is discussed.
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