Signal Processing for High Precision Phase Measurements*

High precision phase measurement is important for many areas of accelerator operation. In a heterodyne digital receiver, one source of phase error is the thermal variation of the input stage. We have developed a technique to calibrate this drift. A CW calibration signal is sent through the same components together with the RF signal to measure and compensate the component drift. At intermediate frequency (IF), we use FPGA based digital signal processing to measure and reconstruct the RF signal after applying appropriate correction. Using this technique, we can measure the phase of a 2856 MHz signal with an accuracy of 15 milli-degrees. We describe how this approach is applied to the femtosecond timing distribution system.