4/6/2017 Lesson 17 EET 150 CHARACTERISTICS OF PULSE AND SQUARE WAVES Learning Objectives In this lesson you will: compare idea waveforms to actual waveforms define pulse rise and fall times see pulse and square wave signal not based at zero volts define the parameters pulse width and duty cycle see pulse tilt, undershoot, overshoot, and ringing 1 4/6/2017 Ideal versus Actual Pulse Waveforms Idea waveforms are a theoretical concept Function generators produce non-ideal waves that approximate theoretical shapes The Comparison Ideal (Theoretical) Actual (Generated) Levels can change Instantaneously Levels change requires a finite time Waveforms have no harmonic distortion Harmonic distortion adds small amounts of other frequencies Waveforms have unlimited amplitude Amplitudes limited to capabilities of generator Pulse Waveform Characteristics An Ideal Pulse e d Rising Falling u t Edge Edge i l p m Amplitude A t1 t2 Pulse Time Width Ideal pulse width, tp = t2 – t1 2 4/6/2017 Pulse Width An Actual Pulse Definition : Pulse width, tp, is the time difference between the 50% amplitudes of the rising and falling edges. e A d u t i l p A/2 m A t1 t2 t p Time Pulse Characteristics Positive and Negative Pulses Positive pulses increase from baseline voltage e d u t i l p m V baseline >0 A V baseline =0 Time 3 4/6/2017 Pulse Characteristics Positive and Negative Pulses Negative pulses decrease from baseline voltage e d u Negative pulse with t i l p positive base voltage m A + Negative pulse with negative base voltage Time e d u t i l p m A - Pulse Characteristics Pulse rise-time and fall-time 90% e d u t i 10% l p m A tr tf Rise-time, tr, = time required for rising edge of pulse to go from 10% to 90% of amplitude Fall-time, tf, = time required for falling edge of pulse to go from 90%Ti mtoe 10% of amplitude 4 4/6/2017 Pulse Characteristics Pulse Tilt V V V 1 2 2 V1 V V is the average of max V2 and min values V V %Tilt 1 2 100% V Pulse Characteristics Overshoot and Ringing Pulse W ith Ringing 4 4 Overshoot occurs on rising and falling edges 3 when the waveform 2 exceeds the desired value. 2 e(t) e d Amplitude u 1 t i l Ringing occurs on p rising and falling edges m 0 A when the waveform alternates about the 1 1 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 desired value. 0 t 0.17 Time Time 5 4/6/2017 Pulse Characteristics Overshoot and Ringing Watch for the overshoot and ring on this graph Pulse Frequency Pulse frequency is also known as pulse repetition rate (PRR) PRR is period in which the pulse pattern repeats 1 PRR T e d u t T = Period (Seconds) i l p m PRR = Pulse repetition A rate (Hz) or pulses/second T Time 6 4/6/2017 Pulse Duty Cycle Percent of the period that pulse is at its high level T = period tp = pulse width tp pulse width Duty Cycle 100% period t Duty Cycle p 100% T T Pulse Duty Cycle Example tp = 0.003 S e d u t i l p m e A d u t i l p T=0.01 S m Time A 0.003 S Duty Cycle 100% 0.01 S Duty Cycle 30%Time 7 4/6/2017 Variable Duty Cycle 60%30%70% 90% T 0 ≤ tp ≤ T Characteristics of Pulse and Square Waves End Lesson 17 EET 150 Coming Next: Function Generator Controls and Operation e d u t i l p m A Time 8 .