US Patent 10055032 Active pen signal detection

In a method of active pen signal detection, a signal is received from an active pen, wherein the active pen is configured for use with a capacitive sensing input device. At least one of downconverting and aliasing is performed on the signal to achieve a resulting signal. Presence of one of a plurality of possible resulting signal frequencies present in the resulting signal is detected. The detected resulting signal frequency is associated with a particular one of a plurality of possible transmission frequencies of the signal, and each of the plurality of transmission frequencies of the signal represents information about operation of the active pen.