A rotary magnetic encoder assembly that has a freewheeling rotatable exciter magnet onboard that excites a magnetic field sensor region of an encoder chip when magnetic interaction between the exciter magnet and rotating encoder shaft causes the exciter magnet to rotate. In one embodiment, a drive magnet carried by the shaft magnetically couples with the exciter magnet because the medium therebetween has low magnetic permeability enabling rotation substantially in unison with the shaft. The exciter magnet is disposed in an onboard retainer pocket that accurately locates the magnet relative to the sensor region of the encoder chip. In one preferred embodiment, the exciter magnet retainer pocket is disposed onboard the encoder chip, such as by being formed as part of the package body of the chip that can be integrally formed or as part of a module that is mountable on the chip.