Pulse transformers are used to isolate low voltage signals, provide impedance matching between circuits, and voltage conversion. The key parameters used to specify pulse transformers are: the turns ratio, isolation voltage, and the Et product. The Et Product is a simple calculation used to determine the optimum size (or area) of a waveform that will pass with minimal levels of signal degradation. The ‘on’ period of a square wave has two properties, voltage and time. If the input voltage (in Volts) is multiplied by the pulse width (in microseconds), the result is the Et product, in volt-microseconds. When selecting a transformer, the Et value should be as close as possible to the calculated value. If the Et product of the transformer is too low, not all of the signal will pass due to the saturation of the transformer core. If the Et product is too high, there will be some roll off at the leading and trailing edges due to parasitic inductance and capacitance. In some applications, however, this is acceptable at the benefit of better satisfying other parameters.