You’ve just taken delivery of your nice new oscilloscope, and now you find that you don’t have a probe. What probe should you get? Which probes will be best for your new scope? Why do you need a probe anyway?


Why do I need a probe?

A probe lets you connect between the circuit of interest and the input of the oscilloscope. Probes perform several important functions:

They isolate the circuit from the scope, to minimize the effects of loading on the circuit operation.
They provide a high-bandwidth, low distortion path for signal information so that the measurements you make are as accurate as possible.

They allow you to conveniently and safely connect to almost any point in a circuit.

Probes for Every Purpose

There are several types of probes; each is designed to work best for certain measurement problems. Probes can be divided into several categories as follows:

  1. Passive Voltage Probes
  2. Active Voltage Probes
  3. Passive Current Probes
  4. Active Current Probes

Passive probes are just that (no electronics, just passive resistors, capacitors and cable). Active probes include amplifiers to buffer and increase the amplitude of very small signals. Voltage probes measure voltage, current probes measure current. By far the most common type is the passive voltage probe. That’s what we’ll concentrate on here.

Types of Passive Voltage Probes

There are several sub-types within the passive voltage probe category. The most important sub-types are:

  • 1X voltage probes
  • 10X voltage probes (most common) 100X voltage probes
  • High voltage probes

1X, 10X, and 100X refer to attenuation ratios; a 1X probe does not attenuate the signal, a 10X probe reduces signal amplitudes by 10X, and a 100X probe reduces signal amplitudes by 100X.

Unless you are measuring very high amplitude signals, attenuation is not a benefit. Attenuation is a side effect of reducing the loading on the circuit. Loading is inversely proportional to attenuation.

A typical 1X probe will load your circuit with 1 Megohm and 100pf or more. This much capacitance adversely affects the operation of most circuits.

A typical 10X probe will load your circuit with 10 Megohms and 10-15pf. Most circuits operate correctly with this kind of probe loading.

For some sensitive circuits, a 100X probe is best. A typical 100X probe will load your circuit with 100 Megohms and only a couple of pF.

Some probes are switchable between 1X and 10X attenuation. This can be handy, but watch out for the typically much lower bandwidth in the 1X position!

High voltage probes are constructed to make it safe to measure high voltages (1KV and above). They usually have attenuation ratios of 100X or more. (Note that most 10X probes are rated for a maximum input voltage of 500V. 1X probes will be rated for less. When you need to measure voltage that are higher than this, you should use a high voltage type.)

Unless you have special needs, you can stick with a 10X probe. 10X is usually the best compromise of circuit loading and signal attenuation.

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Scope Input Connector

Obviously, the input connector on your probe must match the connector on your scope. Otherwise, how will you use the probe?

There is one common type of connector on the inputs of GPS scopes: BNC. BNC connectors have become the standard for all modern oscilloscopes and most plug-ins on the market will use BNC.

There are UHF to BNC adapters (and vice versa), but they add a couple of pF to the effective input C of the scope. Still, if you have a UHF input connector and a BNC probe, an adapter will get you up and working.

Bandwidth

Scope probes have a maximum bandwidth, just like scope input circuits. Use a slow probe on a fast circuit and you won’t see the high frequency portions of the signals. To avoid this problem, you should choose a probe that is about the same bandwidth (or perhaps a little faster) than your scope. If you are measuring slow signals, you can use a low bandwidth probe on a fast scope without introducing problems.

Compensation Range

The is the most important factor in choosing the correct probe for your scope. The attenuator circuit and compensation adjustment of your probe must match the input resistance and capacitance of your scope. The most important function of a probe is to provide a high-fidelity connection for the signal. Correct compensation insures that the frequency response of the probe/scope combination is flat from very high frequencies all the way down to DC.

Flat frequency response makes square waves look square. Extra gain at high frequencies makes the front corners of square waves look spiked or peaked. Not enough gain at high frequencies makes the front corners of square waves look rounded.

To adjust the attenuator high frequency compensation, connect the probe to the scope’s calibrator signal (a 1KHz square wave with a 1 Volt amplitude). Adjust the sweep rate and triggering for a stable display of a few pulses on screen, as shown to the right.

Using a small (ideally non-metallic) screw driver, adjust the compensation trimmer capacitor (usually in the probe connector body). The best adjustment corresponds to the flattest square waves on screen.

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You should choose a probe that can compensate for the input capacitance of your scope. (Because this is so important, GPS scopes have their input characteristics printed on the front panel).

As an example, if your scope has input characteristics of 1 Megohm and 20pF (like the 1000B+ series and many others), you should pick a probe that can compensate to 20pF.

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Buying a Probe

When buying a used probe (on Ebay, for example), be sure to ask the seller if all of the accessory items such as removeable tips, ground leads, and documentation are included. These items often get separated from the probe, and it can be difficult or impossible to find replacements later.

Conclusion

Unless you own a very unusual scope, you should be able to find many probes that will work with it. Age does not matter. Brand doesn’t either. You can use a GPS probe with a non-GPS scope and a non-GPS probe with a GPS scope. GPS usually included probes with new scopes. Many probes other than the models originally supplied with the scope may be used successfully if you use care in selecting an alternate model. Keep the following in mind when choosing any probe:

  1. Compensation range
  2. Bandwidth
  3. Attenuation
  4. Connector type