LR - BPM Log Ratio Beam Position Monitor

Beam position measurement
Optimal for single-pass short bunches,
Linacs, transfer lines, first turns
Fast-cycling synchrotrons, boosters
Beam charge range > 500
Beam-based center determination

The Log-Ratio Beam Position Monitor (LR-BPM) is an electronics module for fast analog processing of beam pickups signals.


  • Input signals parallel processing allows single-pass position measurement
  • Bunches at any repetition rate up to 500 MHz.
  • Individual bunches can be distinguished from one another up to 5 MHz repetition.
  • L-band, S-band, X-band beams can be processed provided bunch groups are short (
  • ±2V X and Y outputs are held until the next bunch when Sample & Hold mode (optional) is activated
  • Provides log signal from each pickup electrode for computer analysis, with 5 MHz bandwidth.
  • Beam-based center determination accessory equalizes pickup signals to simulate beam on center
  • Log-Ratio BPM is plug-compatible with Bergoz' multiplexed BPM
  • LR-BPM may be custom-built on daughter card for installation on user's DSP mother boards
  • Cables length matching not critical: pickup signals don't need to be in phase


Operating principle:


Based on the pioneering work of Robert E. Shafer at Los Alamos Laboratory, the Log-Ratio BPM derives beam position from logarithm of the ratio of opposite pickup signals: Log(A/B). Position measured by this method is more linear, over a wider range, than difference-over-sum.

Schema LR-BPM 

Beam-based center determination:


Differences between parallel processing channels can be near-eliminated using the Beam-based center determination accessory. It equalizes opposite pickup signals to simulate beam on center. The zero offsets can thus be measured at regular intervals under user's control and deducted from further readings. Signals entering the equalizer must be in phase. It is controlled by the LR-BPM module, via the input cables. Center determination and beam position measurement can be interleaved.