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The Secret World Of Oscilloscope Probes

Ever wondered how scope probes really work? Most textbooks treat scope probes as a combination of a resistive divider in combination with capacitors to provide an extended frequency response. But as will be revealed, the reality is that they are much more complex in principle. Read on.

By Doug Ford

The oscilloscope is an essential tool for anyone working in electronics. Whether you’re working in electronics service, production, testing R&D or in your home workshop, you need an oscilloscope.

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If you listen to a bunch of technical people chatting about their scopes, they’ll talk about their bandwidth or whether they have colour displays, depth of memory or portability but the probes rarely get mentioned.

In fact, most users don’t think about their probes until they hear the sickening crunch underfoot which tells them they shouldn’t have left them dangling off the bench onto the floor.

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Fig.1: Circuit and response of x10 probe (“Conventional” explanation).

There are many varieties of “specialist” probes: active-FET probes, differential-floating probes, current-sensing probes are just some we could mention.

They all have their uses but by far the most common is the “times ten” (x10) passive voltage probe. Typically, you’re given two of them free with every oscilloscope.

But how much do you really know about these probes?

A few hours of Googling will yield countless explanations about basic operation (voltage division and capacitance compensation) but you are unlikely to find explanations which show the probe’s transmission-line properties.

Nor will you find any adequate description of the design differences between inexpensive 40MHz probes and much dearer 350MHz probes.

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