What is phase measurement on an oscilloscope
Using an oscilloscope to measure the phase difference between two sinusoidal voltages is of practical significance. A counter can measure frequency and time, but it cannot directly measure the phase relationship between sinusoidal voltages. The use of oscilloscopes to measure the phase of many methods, the following, only a few commonly used simple methods.
1. Double-trace method
Dual-trace method is a dual-trace oscilloscope on the fluorescent screen to directly compare the waveform of the two measured voltages to measure their phase relationship. Measurement, the phase ahead of the signal into the YB channel, the other signal into the YA channel. Select YB trigger. Adjust the "t/div" switch so that one cycle of the measured waveform occupies exactly 8div on the horizontal scale, so that the phase angle of one cycle 360° is divided into 8 equal parts, and each 1div is equivalent to 45°. Read the difference T between the overshooting waveform and the lagging waveform in the horizontal axis, and calculate the phase difference φ according to the following formula:
φ = 45°/div × T (div)
If T == 1.5div, then φ = 45°/div × 1.5div = 67.5°.
2. Li Shayu graphical method of phase measurement
Place the X-axis selection of the oscilloscope at the X-axis input position, connect the signal u1 to the Y-axis input of the oscilloscope and the signal u2 to the X-axis input of the oscilloscope. Adjust the relevant knob on the oscilloscope panel appropriately, so that the fluorescent screen shows a suitable size of the ellipse (in special cases, it may be a positive circle or a diagonal line).
It can be seen, set the Y-axis deflection plate on the signal u1 lead in the X-axis deflection plate on the signal u21/8 cycle, set the initial phase of u2 is zero, i.e., φ2 = 0, so when the u2 is zero, u1 is a larger value. Such as the "0" point in the figure. At this time, the point of light on the fluorescent screen is also correspondingly located in the "0" point. With the change of time, u1 rises, u2 also rises, the fluorescent screen on the point of light to the upper right. When the 1/8 cycle, u1, u2 were reached "1" point, at this time, u1 reached its maximum value, u2 is a larger value, the fluorescent screen is located in the corresponding point of light "1". This continues, the point of light on the fluorescent screen will trace a clockwise rotation of the ellipse. If u1 lags behind u2, a counterclockwise rotating ellipse is formed. Of course, this is only in the signal frequency is very low (such as a few hertz), and in the short afterglow of the fluorescent screen will be clearly seen on the fluorescent screen point of light clockwise or anti-clockwise rotation phenomenon. From the above, the shape of the ellipse is seen with the two sinusoidal signal voltage u1, u2 phase difference is different. Therefore, according to the shape of the ellipse to determine the two sinusoidal signals between the phase difference Δφ. In Figure 5-13 set A is the ellipse and the Y-axis intersection of the longitudinal coordinates, B is the ellipse on the coordinates of the maximum value. From the figure, A is the instantaneous voltage corresponding to u1 at t=0, i.e.
A=Um1sinφ1
B is the amplitude corresponding to u1, i.e.
B=Um1
Thus A/B=(Um1sinφ1)/Um1=sinφ1
to express. In the actual test for the convenience of reading, often read 2A, 2B (or 2C, 2D), according to the formula
Δφ = arcsin (2A/2B) or Δφ = arcsin (2C/2D)
to calculate the phase difference.
