How to calculate phase shift with the oscilloscope

Written by j.t. barett
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How to calculate phase shift with the oscilloscope
An oscilloscope can show phase shift between two signals. (Stockbyte/Stockbyte/Getty Images)

While phase shift is not always a bad thing, it's an inescapable fact of life that electronic circuits delay signals, shifting their phase. The amount of phase shift in circuits differs from design to design. You can choose a circuit whose phase shift is less than others and you can visually measure it with an oscilloscope. By measuring the circuit's input signal with the scope's channel 1 and its output with channel 2, you can compare the signals and calculate the phase shift produced by the circuit.

Skill level:
Moderate

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Things you need

  • Sine wave oscillator
  • BNC T-connector
  • BNC cables
  • Circuit to test
  • Two-channel oscilloscope
  • Oscilloscope probe
  • Calculator

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Instructions

  1. 1

    Connect a BNC T-connector to the sine wave oscillator's output. This in effect gives the oscillator two outputs. Connect a BNC cable between one end of the T-connector and the oscilloscope's channel 1 input. Connect another BNC cable between the other end of the T-connector and the input of the circuit you wish to test. Attach an oscilloscope probe to the scope's channel 2 input. Connect the probe to the circuit's output.

  2. 2

    Turn the oscillator, circuit and oscilloscope on. Adjust the oscillator so its frequency and amplitude fall into the circuit's operating range. Do not overdrive the circuit with the oscillator.

  3. 3

    Turn the oscilloscope's horizontal sweep rate control until you see two or three sine wave cycles on the oscilloscope's display. Adjust the vertical position and sensitivity controls until you can clearly see sine waves from both channels, with channel 1 on top and 2 on the bottom. If necessary, set the scope's sweep trigger to channel 1 to stabilise the sine wave on the display.

  4. 4

    Find a peak on the top sine wave and follow that point straight down to the horizontal time-division marks in the centre of the scope's display. Find the corresponding peak on channel 2 and its time-division mark. Count the major divisions between the two time-division marks, including any fraction of a division. This is the phase shift of the two signals.

  5. 5

    Look at the top sine wave again. Count the major divisions for one complete cycle, including any fraction of a division. Divide the difference between the channel 1 and channel 2 signals by the divisions for one complete cycle. Multiply by 2 times pi (3.1416) to get a phase shift in radians. For degrees, multiply by 360 instead of 2 times pi.

Tips and warnings

  • Some digital oscilloscopes can measure and calculate the time difference between two points on the screen automatically. You select the time measurement function and place two vertical cursors at the beginning and ending points on the screen for an interval you want to measure. The scope will display the time difference as a number on the screen.

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