Wave Shaping

Pulse-Width Discriminator

The circuit shown in the figure above prevents short-duration pulses from affecting output B. The normal states for input A and output B are negative voltages supplied by saturated operational amplifiers. In this operating mode, the potential at A₁ is approximately -8.2 volts. This value is unaffected by changes in the power supply or the operational amplifiers, since it is maintained by the zener diode circuit. When A becomes positive, A₁ changes to +4.2 volts. This shift from -8.2 to +4.2 volts causes A₂ and A₃ to increase exponentially, since diode D₁ blocks the current. When A returns to its normal state, diode D₁ conducts and quickly returns A₂ to its normal state.

If A remains positive for longer than 10 milliseconds, A₃ will build up to a value that is positive with respect to the voltage at A₄ and trigger output B to a positive voltage. The positive feedback circuit around the operational amplifier and the blocking action of diode D₂ keeps B latched at a positive value even though A₂ returns to its normal state. Output B remains positive until A₄ increases to a positive value greater than the voltage at A₃. One hundred milliseconds are required before B resets to its normal state. When B becomes negative, diode D₃ conducts and returns A₄ to its normal state. The circuit is then ready to detect another input pulse at input terminal A. The adjustment for setting the minimum detectable pulse width is made with the 10-kilohm trimmer potentiometer (R₂).