The perturbations may be provided in {HEAD ~ the form} of {TAIL ~ a low amplitude alternating current ingredient} in the sensor drive signal substantially at a fixed frequency 
of
The perturbations may be provided in { HEAD ~ the form } of { TAIL ~ a low amplitude alternating current ingredient } in the sensor drive signal substantially at a fixed frequency
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH