9 Important Things to Know When Making Sensitive Measurements with Oscilloscopes

We've routinely posted on how you can characterize your total measurement system to gain important "situational awareness" when using an oscilloscope to make sensitive measurements. The knowledge gained from these tests helps you properly interpret your measurement results so that you can deduce what is actually going on with your circuit, versus what is an artifact of the measurement system. Listed here are nine important things you should know before making sensitive measurements with your oscilloscope, with links to blog posts that instruct you how to test them.

Transmission Lines for Oscilloscope Users, Part 3

Figure 1: The Thevenin equivalent circuit model can be used
to characterize a voltage source with respect to the 
interconnect cable and oscilloscope input termination.

In Part 2, we demonstrated how to calculate the instantaneous impedance of a transmission line. However, any measurement made using an oscilloscope should consider not only the transmission line, but the source, the transmission line and the oscilloscope as a system. Therefore, characterizing your source, as well as knowing the effects of your oscilloscope input impedance, is important to developing the “situational awareness” needed to interpret measurements properly.

Two terms needed for us to characterize the source are the Thevenin source voltage and the Thevenin source resistance. Once we know these, we have all the pieces we need to fully understand what is happening with our measurements. This is true whether the signal source is a Cal terminal or a device-under-test.

Transmission Lines (Part V): Reverse-Engineering the DUT

Figure 1: Every DUT can be thought of as a Thevenin
voltage source with some internal resistance

There are always two primary elements of any test and/or measurement application: the oscilloscope and the device under test (DUT). Getting valid measurement results depends, first and foremost, on the oscilloscope's capabilities given the task at hand. It also depends on what we'll call "situational awareness," or the operator's understanding of the oscilloscope and of the characteristics of the DUT.