I am using a Costas loop for carrier recovery with QAM16 data. The carrier is only 2khz. The I/Q output of the Costas loop seems to track (the original sin/cos of the modulating carrier's Frequency & Phase) steadily for a long period (minutes) and then the Phase moves off, normally in +/- 45 or 90 degrees in one or both of the phases. Any thoughts on why this occurs or how I can fix this issue?
Thanks...Tom