Brian Corey Spurious Phase Calibration Signals: How To Find Them and How To Cure Them ABSTRACT This memo describes some of the causes of spurious phase calibration signals in geodetic VLBI systems, tests that can be performed at the stations to detect the signals, and measures that can be taken to reduce their levels. The discussion assumes a Mark III/IV calibration system with a ground unit in the control room driving an antenna unit in the receiver; the antenna unit generates narrow pulses spaced 1 microsecond apart, which are injected into the RF signal path via a directional coupler. Most of the discussion is also applicable to other systems, with some minor modifications.