Foreign object detection in a wireless power transfer system

A wireless power transmitter (101) comprises a driver (201) generating a drive signal for a transmitter coil (103) thereby generating a power transfer signal during power transfer intervals of a repeating time frame comprising at least one power transfer interval and one foreign object detection interval. A test generator (211) generates a test drive signal for a test coil (209) thereby generating an electromagnetic test signal during foreign object 5 detection intervals. A foreign object detector (207) performs a foreign object detection test in response to a measured parameter for the test drive signal during the foreign object detection intervals. An adapter (213) generates a varying test drive signal for the test coil (209) during an adaptation interval, and determines a test drive signal parameter value in response to the varying test drive signal. The test generator (211) sets a signal property of the test drive 10 signal during at least one foreign object detection interval in response to the test drive signal parameter value.