Thierauf Design & Consulting: Signal Integrity Design, Analysis & Training

People have been asking how Figure C.7 in Understanding Signal Integrity: A Laboratory Manual was created. This graphic shows the difference between using the ground spring on the end of an oscilloscope probe verses using a 6” (15cm) long wire.

You can see in Figure C.7 (on the right, reproduced from the text) that with the wire lead the probe doesn’t act as you’d expect from theory (which was shown in Figure C.6). Some frequencies are displayed by the oscilloscope with larger amplitudes than the actual measurement (the Y axis 100% mark), but others are displayed with less than what was actually present. Pulses (which are made from many harmonic frequencies) will be distorted, and their shape and the amount of ringing will change as the ground lead is repositioned or changed in length.Capture22.JPG

So, how was the plot taken, and how can you measure the characteristics of your own probe?

I used the tracking generator from my RF spectrum analyzer as a source because I know its output is flat across the frequency range I wanted to measure. You cans also use a sine wave generator provided you know its output is flat (the amplitude doesn’t change as you adjust the frequency).

 

Because you want to determine the response when your probe is connected to your oscilloscope, the best way to make the measurement is to connect the probe to your oscilloscope and attach the probe tip and ground lead to the generator output. Don’t use the spectrum analyzer input because its frequency response isn’t the same as the frequency response of your oscilloscope. I set the tracking generator output amplitude to a convenient value and measured the amplitude on the scope when I changed the frequency in 10MHz increments from 1MHz to 300MHz. The results are displayed in Figure C.7. With this setup you can see how the frequency response changes as the ground wire is repositioned, or when you make it longer or shorter.

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