A&D Series 57ZZ Manual do Utilizador descarregar pdf (Página 31)

Phase Shift Measurements

The horizontal control section may have an XY

mode that lets you display an input signal rather

than the time base on the horizontal axis. This mode

of operation opens up a whole new area of phase-

shift measurement techniques.

The phase of a wave is the amount of time that

passes from the beginning of a cycle to the beginning

of the next cycle, measured in degrees. Phase shift

describes the difference in timing between two

otherwise identical periodic signals.

One method for measuring phase shift is to use XY

mode. This involves connecting one signal to the

vertical system as usual and then another signal to

the horizontal system. (This method only works if

both signals are sinusoidal.) This set up is called an

XY measurement because both the X and Y axis are

tracing voltages. The waveform resulting from this

arrangement is called a Lissajous pattern (named for

French physicist Jules Antoine Lissajous and

pronounced LEE-sa-zhoo). From the shape of the

Lissajous pattern, you can tell the phase difference

between the two signals. You can also tell their

frequency ratio. Figure 43 shows Lissajous patterns

for various frequency ratios and phase shifts.

The XY measurement mode originated with analog

oscilloscopes. Due to their relatively low sample

density, DSOs may have difficulty creating real-time

XY displays. Some DSOs create an XY image by

accumulating data points over time, then displaying

the composite. Digital Phosphor Oscilloscopes, on

the other hand, are able to acquire and display a

genuine XY mode image in real-time, using a contin-

uous stream of digitized data. DPOs can also display

an XYZ image with intensified areas.

Figure 43. Lissajous patterns.

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A&D Series 57ZZ Manual do Utilizador Página 31