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
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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
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Pulse Width An Actual Pulse
Definition : Pulse width, tp, is the time difference between the 50% amplitudes of the rising and falling edges.
A e 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
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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 e d u t i
l 90% p m A
10%
tr tf Time
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% to 10% of amplitude
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Pulse Characteristics Pulse Tilt V V V 1 2 2 V1 V V is the average of max V
e 2
d and min values u t i l p m A V V %Tilt 1 2 100% V Time
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) Amplitude 1 Ringing occurs on rising and falling edges 0 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
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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 = Period (Seconds) t i l p
m PRR = Pulse repetition
A rate (Hz) or pulses/second
T Time
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Pulse Duty Cycle
Percent of the period that pulse is at its high level
T = period
tp = pulse width e d
u t t p
i pulse width l
p Duty Cycle 100%
m period A
t Duty Cycle p 100% T Time T
Pulse Duty Cycle
Example
tp = 0.003 S e d u t i l p m A
T=0.01 S Time
0.003 S Duty Cycle 100% 0.01 S Duty Cycle 30%
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Variable Duty Cycle e d u t i l
p 60%30%70% 90% m A
T Time
0 ≤ tp ≤ T
Characteristics of Pulse and Square Waves End Lesson 17 EET 150 Coming Next: Function Generator Controls and Operation
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