the invention features a theft resistant security system for motor vehicles . the system has receptacle with a coupling coil . a saw device associated with the receptacle for relative movement thereto is also provided . the saw device has a code and its own coupling coil for interaction with the receptacle coupling coil . a testing mechanism is connected to the receptacle for determining whether the code of the saw device is valid for operation of the motor vehicle . problems of crosstalk and wave reflection may produce spurious code pulses in systems of this type . the invention provides techniques for eliminating or reducing these unwanted pulses .