of voltage from the higher voltage 184 to the lower voltage 186 through the reference voltage 190 the envelope detector 172 then provides a voltage to the comparator that is compared to the voltage on reference voltage resistor 192 so that the control microprocessor 152 is signalled that a crossover or increased impedance has taken place that is tantamount to the touchdown the control microprocessor 152 driving the motor drive then causes the motor drive to stop looking more particularly at the details of the circuit it can be seen that a voltage limiter in the form of a resistor 202 and diodes 204 and 206 are seen this voltage limiting circuit is connected to the signal sample that is received across resistor 158 the shunt diodes 204 and 206 will short out and prevent inputs from high level voltages and at the same time allow the system to function a summing resistor 208 is connected to the input of an op amp 210 at its minus side the op amp is an active diode detector the op amp incorporates a feedback resistor connected thereto in the form of resistor 212 and active diodes 214 and 216 diode 216 is a rectifier diode while diode 214.