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