An untethered device is configured to inductively couple to a source device, which generates a varying magnetic field having a fixed frequency. The untethered device includes a tunable resonant circuit having a resonance frequency and configured to generate a supply voltage for the untethered device in response to the varying magnetic field. The untethered device further includes a frequency control circuit coupled to the tunable resonant circuit and configured to produce a control signal using signals developed within the tunable resonant circuit. The control signal is useable to adjust the resonance frequency of the tunable resonant circuit to the fixed frequency of the source device.