Duty cycle meter - direct duty cycle measurement

The duty cycle of a square wave signal is normally measured by means of a pulse counter or an oscilloscope. However, this can be simplified considerably by using two VMOS-F ETs and a voltmeter. The FETs are switched in turn by the input pulses. The R2/C2 network combi- nation provides an average DC level corresponding to the input waveform: Uav = TH • Ub. The meter reading can be interpreted as follows: The indication of the duty cycle can be expressed as a percentage (link A). For link B, a voltmeter with a centre zero is preferable. A DVM would also do the trick, but not quite as well. The voltage level at the input of the meter will be half the supply voltage when the duty cycle is 50%. Since the other side of the meter is connected to half the supply voltage (via voltage divider R3/R4) there will be no current flow through the meter (hence a zero reading). The duty cycle can be read directly in % if the scale is divided into 1 ... 10 (Ub = 10 V) and the centre point (5) mark...