Low phase noise PLL based LO generation module for femtosecond precision RF field detection

The Low Level RF (LLRF) system for the new generation of Free-Electron Laser (FEL) machines, contains many specialized devices with superior performance. One of the crucial LLRF subsystems is the Local Oscillator (LO) generation. The LO signal is mixed with the cavity probe signal, which carries the information about amplitude and phase of electromagnetic field in cavity. The resulting IF signal is processed by the LLRF control system feedback scheme. Lowering the phase noise of the LO signal allows to have more precise information about the accelerating field. Therefore the main effort in the LO generation design is focused on ultra low phase noise devices. Another requirements for LO generation devices are phase drift minimization, reliability and small physical dimensions allowing to use such devices in modular hardware standards like xTCA. In this paper the PLL architecture of the 1354 MHz LO generation module is described and the compromise between low phase noise and compact package is discussed. The phase noise contribution of different frequency dividers, amplifiers and VCOs is presented. A prototype of the module was designed and elaborated in laboratory. A residual integrated RMS phase jitter of the LO signal of less than 100 fs has been achieved.