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9-April-2007

Noise Measurement

To accurately measure noise (or high frequency signals) with your oscilloscope it is necessary to reduce the radiated noise that is coupled to your probe. The noise is coupled to the loop formed by the tip and ground wire. The loop area can be reduced to zero by removing the ground wire and using the probe shield as your ground connection. To connect the probe to the PCB I use either a coax jack that the probe plugs into or two pins that I can rest the probe between.

The coax jacks I use are made by Johnson --- part numbers 129-0701-301 (horizontal) and 129-0701-201(vertical). Both are available from Digi • Mouse. Johnson 129-0701-201 is shown in the top right photo. Verify the size of your probe sleeve before you order the jacks.

The test pins I use are Vector K24A. The pins are spaced so that the ground sleeve of the probe rests against a ground pin and the tip rests against the signal pin. The Vector pins are available from Digi • Mouse. (bottom right photo)

If it is not possible to design your board with either a coax jack or test pins the loop area can be reduced by coiling the excess ground wire around the probe.