Changing the Time Base

• When a signal is viewed on the CRO display the period of the signal can be measured by counting the number of horizontal divisions that the signal "covers" on the display and multiplying by the scale of each division.
  • • i.e if the signal "covers" 4 divisions at 5 milliseconds per division,
      the signal period is 20 milliseconds and hence
      the frequency is 50Hz.
• Now because the signal being viewed can be a large range of frequencies we need to adjust the viewing scale on the display in order to achieve the optimum viewing size of the signal.
  • • i.e if we attempted to view a 10Hz sine wave and a 10kHz sine wave on the same scale one or both of the signals would not be properly visible on the display.
      A graphical example of this.

How Do We Change The Time Base?

• Well firstly we consider the diagram below which shows the Time Base Control labelled with Sweep time/div.

• Now we can see that the control has settings labelled from 0.2 uS per division to 0.2 S per division.
• The time base is changed by simply rotating the control to the desired setting. Some graphical examples of this.

Things To Remember About The Time Base Control.

1. If we are viewing two signals on the display, they cannot be viewed such that each signal is on a different horizontal scale. So be aware of the setting of the Time Base.
2. The red knob ( the Variable Time Base Control ) that lies within the Time Base Control must be rotated completely clockwise until a "click" is heard to ensure calibration and correct readings. (Be Warned - there will be turkeys who play around with these knobs just to ruin your results and get a laugh, so always be sure to check this. )

The Variable Time Base Control

• This control as mentioned above is located in the centre of the Time Base Control and is used to vary the horizontal scale ouside the limits of the Time Base Control. It is most useful to check the general shape of a signal.

  

• ALWAYS be sure that this control is not left in the uncalibrated ( calibration relies on the knob being rotated completely clockwise until a "click" is heard ) position when making measurements of signal amplitude.

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Revised: 15 September 2004