A High-Resolution S-band Down-Converting Digital Phase Detector for SASE FEL Use

Each of the rf phase detectors in the Advanced Photon Source linac [1] consists of a module that down converts from S-band to 20 MHz followed by an analog I/Q detector. Phase is calculated from one digitized sample per pulse each of I and Q. The resulting data have excellent long-term stability but are noisy enough so that a number of samples must be averaged to get a usable reading. The more recent requirement to support a selfamplified spontaneous emission (SASE) free-electron laser (FEL) has presented the need to accurately resolve the relative phase of a single pulse. We replaced analog detection with digital sampling and replaced internal intermediate frequency (IF) reference oscillators with a lower-noise external oscillator in order to control the two largest components of noise. The implementation of a central, ultra-low-noise reference oscillator and a distribution system capable of maintaining the low phase noise is described, together with the results obtained to date. The principal remaining technical issue is determining the processing power required as a function of measurement channels per processor, measured pulse repetition rate, intrapulse data bandwidth, and digital filter characteristics. The options and tradeoffs involved and the present status are discussed.