# Measuring Time Constants on the TDS4013B Using Cursors

Start by getting an image that shows the output going from its initial state to an equilibrium.  The image below shows both the input (blue) and the output (cyan).  Note that one oscillation fills a large part of the screen – this will increase the accuracy of the measurement.  Note also (for all of these images) that I hit the “Menu Off” button to get an uncluttered image.  You should do the same before you take screenshots.

Screenshot of first order circuit.  Rising edge of input is at center of screen.  Input=blue, output=cyan.

We wish to find the time constant of the output.

Start by turning the cursors on (“Cursor” button is at upper right of scope).  We’ll start by measuring the change in voltage using the Horizontal bars (H bars).  Select “H bars” and move one of the bars to the final (equilibrium) voltage with the knob that is next to the “Cursor” button (this is the max voltage in our case).  Now choose the other cursor (the choice of cursors is toggled by hitting the “Select” button next to the knob), and set it to the initial (or bottom) voltage.  This is shown below.

Image showing horizontal cursors measuring difference between initial and final voltages.

The label at the upper right tells us that the difference between the two cursors is 21.1 V and that the selected cursor is at -9.1 V.

Since the difference between final and initial voltages is 21.1 V, we know that the time constant occurs when the output has gone through 63% of this total or 13.3V.  Now we need to find how long it took the voltage to change by 13.3 V.

We select the “Cursor” button again, and then select “V Bars.”  Put the first cursor where the input voltage changes (this will be our “t=0”).  Set the other cursor such that the delta V is 13.3 V.  This is shown below.   The time constant is the difference between the two cursors.

Image showing vertical cursors measuring difference between initial time, and time at which voltage has gone through 63% of total change in voltage.

In this case the time constant is 7.84 mS.  This is close to the expected time constant – the circuit being used had R=10kΩ and C=0.8μF,for a time constant of

Note: one common error is measuring off of the wrong channel.  When you have the cursors up on the screen, the text associated with them should be in the same color as the channel you want to measure.  If you are measuring a different channel, then just hit the appropriate channel button – this changes the measurement to that channel and also changes the color of the text on the screen.

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