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# Employ correlation to measure phase difference

Posted: 30 Dec 2015     Print Version

Keywords:oscilloscope  algorithm  PLL generated clocks  FPGA  Correlation

Measuring the phase difference between two periodical signals is often required for a science such as meteorology, computing, and communications. An oscilloscope offers a quick and simple way to make such measurements. Unfortunately, an oscilloscope's noise, bandwidth, and timing resolution limit its measurement accuracy.

The oscilloscope's sampling rate sets its timing resolution. For example for a 100MHz signal, each degree in phase translates into 27 ps. Clearly, for a one-degree phase measurement accuracy, the sampling time of the oscilloscope must be less than this number. That translates into a sampling rate higher than 36GHz, which is beyond the reach of the majority of oscilloscopes. To demonstrate this measurement we used an Analog Arts SA985 USB oscilloscope, which has a sampling rate of 100GHz and a bandwidth of 1GHz. You can perform this measurement with any oscilloscope that meets the timing requirements of your application. Even with the proper oscilloscope, you must use special techniques to get accurate phase measurements.

An oscilloscope's timing markers (figure 1) offer the simplest technique to measure the phase between two signals. The time difference between two corresponding points on the signals represents the phase in units of time. Multiplying the ratio of this value to the period of the signals calculates the phase in degrees. The precision of the measurement is highly dependent on the oscilloscope's noise and the triggering uncertainties.

 Figure 1: Using timer markers let you measure the phase difference between two signals.

Traditionally, Lissajous patterns (figure 2) have been used to measure the phase between two sine waves. Making precise measurements from Lissajous plots is, however, simply not possible. Furthermore, for signals other than sine-waves, these patterns are difficult to interpret.

 Figure 2: A basic Lissajous Pattern for measuring the pahse difference between two sine waves.

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