A&D Series 57ZZ Manual do Utilizador descarregar pdf (Página 30)

vertically, then taking the measurement along the

center vertical graticule line having the smaller divi-

sions makes for the best voltage measurements (see

Figure 40). The more screen area you use, the more

accurately you can read from the screen.

Many oscilloscopes have on-screen cursors that let

you take waveform measurements automatically on-

screen, without having to count graticule marks. A

cursor is simply a line that you can move across the

screen. Two horizontal cursor lines can be moved up

and down to bracket a waveform’s amplitude for

voltage measurements, and two vertical lines move

right and left for time measurements. A readout

shows the voltage or time at the positions of the

cursors.

Time and Frequency Measurements

You take time measurements using the horizontal

scale of the oscilloscope. Time measurements

include measuring the period, pulse width, and

timing of pulses. Frequency is the reciprocal of the

period, so once you know the period, the frequency

is one divided by the period. Like voltage measure-

ments, time measurements are more accurate when

you adjust the portion of the signal to be measured to

cover a large area of the screen. Taking time measure-

ments along the center horizontal graticule line,

having smaller divisions, makes for the best time

measurements (see Figure 41).

Pulse and Rise Time Measurements

In many applications, the details of a pulse’s shape

are important. Pulses can become distorted and

cause a digital circuit to malfunction, and the timing

of pulses in a pulse train is often significant.

Standard pulse measurements are pulse width and

pulse rise time. Rise time is the amount of time a

pulse takes to go from the low to high voltage. By

convention, the rise time is measured from 10% to

90% of the full voltage of the pulse. This eliminates

any irregularities at the pulse’s transition corners.

This also explains why most oscilloscopes have 10%

and 90% markings on their screen. Pulse width is

the amount of time the pulse takes to go from low to

high and back to low again. By convention, the pulse

width is measured at 50% of full voltage. See Figure

42 for these measurement points.

Pulse measurements often require fine-tuning the

triggering. To become an expert at capturing pulses,

you should learn how to use trigger holdoff and how

to set the digitizing oscilloscope to capture pretrigger

data, as described earlier in The Controls section.

Horizontal magnification is another useful feature for

measuring pulses, since it allows you to see fine

details of a fast pulse.

Figure 41. Measure time on the center horizontal graticule line.

Figure 42. Rise time and pulse width measurement points.

Figure 40. Measure voltage on the center vertical graticule line.

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