Eric Foster
New member
A common misconception is that the voltage flyback pulse has a direct bearing on the signal from a metal target. PI theory shows that it doesn't. The voltage pulse occurs simply because the coil becomes almost open circuit at the end of the TX drive and the collapsing magnetic field tries to maintain the current at the level it was just before cut off. This voltage rises till it exceeds the avalanche voltage of the Mosfet and then stays at this level until the magnetic energy has dissipated such that the voltage falls below the avalanche rating, and then the remaining energy is absorbed by the damping resistor. If you insert a small resistor in the ground end of the coil, the coil current waveform can be observed, which exactly matches the magnetic field waveform. There is no spike, but just a smooth ramp down of the current and field. Now, the ramp down is important in that it must be significantly faster than the target time constant, otherwise signal will be lost. Theory states that provided the ramp down is faster than 1/10th of the object TC, then it is as if the field were removed instantaneously. So, objects with a short TC need a fast ramp down and objects with a long TC can get away with a slower ramp down. Also, for maximum signal you have to factor in the duration of the TX pulse. This too has to be significantly different to the object TC to achieve maximum signal, in this case longer. Again the factor of 10 comes in, but having it 5x longer will still get you around 90% of the signal. Looking at the collapsing magnetic field from the target