Log-Ratio BPM Electronics

Non-intercepting beam position measurement

Optimal for single-pass short bunches


Beam charge range

> 50 dB

Repetition frequency

≤ 500 MHz


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.

Signal processing

Signals from the pickups are stretched to produce bursts. This is essential to measure the single pass of a bunch.
Four parallel logarithmic amplifiers detect the burst envelopes.
Amplifiers’ response is log of amplitude.
Logs of opposite pickups are subtracted.
If pickups are rotated, axes are translated to obtain X and Y positions.
The process is all-analog, wideband.











  • 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 500MHz. Individual bunches can be distinguished from one another up to 5 MHz repetition.
  • ±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 5MHz bandwidth.
  • Cables length matching not critical: pickup signals don’t need to be in phase

Beam intensity range


Bunch/group train

Fmax = 500MHz

Output frequency

<5MHz rep rate: individual position is measured
>5MHz rep rate: average position is measured with 5MHz bandwidth


X and Y: -2V…0...+2V, 40mA max
Sum of logs: 0...+2V, 40mA max

 X and Y gain

1.5V = ½ of aperture radius for orthogonal pickups
1.0V = ½ of aperture radius for rotated pickups