Recharge tuning techniques for an implantable device

Techniques are disclosed for tuning a frequency at which an external device transcutaneously transfers energy. The transferred energy may be used to charge a rechargeable power source of an implantable medical device (IMD) and/or to power the IMD directly. One embodiment relates to a charging system that may comprise a circuit to drive a primary coil of an external device at a drive frequency and a control circuit to tune the drive frequency based on a characteristic of a monitored signal that is associated with the primary coil. The characteristic is not present when the primary coil is being driven at a resonant frequency of the system. In a specific example, the characteristic comprises a stub pulse and the control circuit is configured to tune the drive frequency based on at least one of a relative timing and a width of the stub pulse.