Beam Position Monitor Electronics using DC Coupled Demodulating Logarithmic Amplifiers

An electronics circuit operating up to 120 MHz suitable for Beam Position Monitor signal processing is described Two different channels process signals from the electrodes. Each channel IS realized with two cascaded DC coupled demodulat-ing logarithrr,ic amplifiers, providing an output voltage propor-tional to the logarithm of the input signal amplitude. The outputs from the two channels are processed by differential and summmg amplifiers. The difference output produces a voltage proportional to the beam displacement between the electrodes, but both the difference and sum outputs are digitized in order to allow for a software correction of the gain and offset mis-matches. The electronics show better characteristics than previ-ous implementations utilizing log-amp circuits. The dynamic range has been increased, keeping the linearity error smaller than 1 % over a 65 dB input signal range. The noise characteris-tics have been improved providing good resolution at low cur-rents. The RF burst response has also been tested showing good characteristics for use on a Linac or Transfer Line. One prototype, working ai 60 MHz, has been built and is planned for use on one or more machines at the SSC. I