= Pulse width
{title2=$\tau$}
<Pulse width> is the time separation of specified crossings on a pulse's leading and trailing edges. The threshold convention is essential when the edges are not instantaneous. Keysight's primary pulse-generator documentation uses the rising and next falling 50-percent crossings, and allows a duty-cycle setting as an alternative control.<Pulse Width in Keysight BenchVue Function Generator documentation>[Pulse Width, 50-percent definition and duty-cycle setting]{ref}
For low and high levels $V_L$ and $V_H$, define the fractional threshold as $V_L+a(V_H-V_L)$ with $0<a<1$. A positive pulse's width at that level is $\tau(a)=t_\downarrow(a)-t_\uparrow(a)$. The half-height convention is related to <full width at half maximum> for an isolated positive pulse above a defined baseline; it is not necessarily the duration of the flat top.
For an original asymmetric trapezoid, let a full linear rise take $t_r=2\,\mathrm{\mu s}$, the plateau last $H=8\,\mathrm{\mu s}$, and a full linear fall take $t_f=4\,\mathrm{\mu s}$. These ramp durations refer to the full low-to-high excursions, not the commonly quoted 10–90 percent rise interval. The crossing times give
$$
\tau(a)=H+(1-a)(t_r+t_f).
$$
The 50-percent width is $11\,\mathrm{\mu s}$, while the first departure from the low level to final return lasts $14\,\mathrm{\mu s}$. At a 90-percent threshold the width is only $8.6\,\mathrm{\mu s}$. All describe the same waveform under different conventions.
For a repeating pulse train, <duty cycle> compares the specified width to the repetition period. Finite ramps mean that width times peak <amplitude> alone need not give the waveform integral or <energy>. Electrical loading and bandwidth also change the delivered crossings; the <pulse generator> setting must be checked at the actual load.
A measurement records both edges on an oscilloscope with a common level definition. <Pulse-width uncertainty from correlated edge timing> depends on $\operatorname{Var}(t_\downarrow-t_\uparrow)$, including their <covariance>. A common trigger-time translation can cancel from width while remaining in absolute arrival time. Ringing with several crossings, hysteresis, negative pulses, or multiple peaks needs an explicit crossing-selection convention rather than an unspecified single duration.
Back to article page
