Oscilloscope probes

This article sheds some light on the different kinds of probes for the oscilloscope, along with some safety precautions to take when using them.

Passive probe

The passive probe is the most commonly used type of probe. Oftentimes when you purchase an oscilloscope, there is one such probe included for each channel.

The head of this probe consists of a ground clip and a hat in which a retractable hook is to be found. By pulling back the hat, you can hook up the probe to a pin or a piece of bare wire to measure it. If desired, the hat with the hook can be removed altogether, allowing you to probe directly with the tip of the probe.

For most basic measurements, these probes will suffice. However, there are limitations that must be taken into account when working with fragile circuits, high voltages or high frequencies. Furthermore, this probe can only measure voltage, no current.

The specified voltages at which the probe is rated for only applies to DC voltage. At higher frequencies the maximum permissible voltage decreases. You will find this curve in the manual of the probe.

A passive probe. On the left: the ground clip, on the right: the BNC connector.

Bandwidth and scaling

In order to pick up less electromagnetic noise, the passive probe, unlike multimeter probes, consists out of a coax. As a side-effect this adds some capacitance to the cable which limits the highest measurable frequency by the probe. It is therefore key to pay attention to the bandwidth of the probe, hence this information is always stated in the product description.

Almost all probes have a scale factor switch which allows to attenuate the measured signal by a factor of 10. Some probes can attenuate the signal up to 100x. This is useful when the range of the oscilloscope is insufficient for the measurement, or to reduce the load on the DUT. The probe and the oscilloscope together have a non-negligible impedance that can significantly affect the measurement in certain cases.

However, setting a scale factor can affect the image of the measured signal when the input capacity of the probe is improperly matched. Therefore, oscilloscopes have a test clip on the front on which a test signal in the form of a square wave is generated. By measuring this signal with the probe before the actual measurement, the input capacity of the probe can be set correctly with the included plastic screwdriver, such that the measured signal looks like a proper square wave.

After a scale factor has been applied, the oscilloscope can scale back the values by specifying the used scaling factor in the menu of the channel in use.

Ground clip

The ground clip serves as a reference point for the measured signal. This cable is kept short to prevent creating a loop on which noise is accumulated by means of picking up electromagnetic signals from the surroundings. The smaller this loop is, or the closer the ground is to the tip of the probe, the less interference will be picked up.

The ground of any arbitrary channel is equal to the ground of all other channels. This entails that it is possible to measure a signal with the tip of one probe, referenced against the ground clip of a second probe. Hence pay close attention to always hooking up ground clips to the same potential to avoid shorts.

Furthermore, the grounds are not only hooked up to each other, but to the mains earth of the oscilloscope as well. Anything touched with a ground clip is implicitly grounded to mains earth. This arises the danger of accidentally shorting circuits by means of an unintended ground loop.

If one needs to measure in-between two various points (such as the mains phases, or between two arbitrary points in an amplifier circuit), there are various ways to overcome this problem:

• One could tape off the ground line of the mains plug of the oscilloscope. Not only is this impractical, it is especially dangerous as well because the ground (and inherently its chassis) may be subject to high voltages.
• One could use two probes, both referenced against earth. By digitally taking the difference of the two channels with the MATH function, you can use your probes as a crude differential probe. This method can be used, but not all oscilloscopes have the MATH function. Also, this method requires using two channels simultaneously which is the maximum number of channels on cheaper DSOs. However, this is method is equally unsuited for probing on the mains.
• One uses a differential probe, which is specifically created for this purpose. Read more about this probe further below.

Order your passive probes here

Active probe

Active probes are priced much higher and are commonly found in the segment of faster scopes (starting at roughly 1 GHz). Active probes have a much lower capacity, so the bandwidth is considerably higher compared to passive probes. An active probe exerts a smaller load upon the DUT compared to a passive probe, which is useful for measuring very sensitive circuits.

An active probe with miscellaneous probe connectors.

Because an active probe has active components, it must be powered externally. Active probes are however quite niche and often used in combination with higher end oscilloscopes. The manufacturer of such oscilloscopes often manufacture their own active probes for their respective series. The connection of the active probe is therefore often a non-standard hybrid of BNC with additional pins underneath, specific to that range of oscilloscopes. These pins also provide power to the probe.

A disadvantage of active probes is that they are several orders of magnitude more expensive than passive probes. They are very specialised probes, made by oscilloscope manufacturers for their high-end series. Also, the voltage range is very small, often between 5 and 30 V. They must be handled with extreme care. But thanks to active probes, it is possible to do measurements that are completely out of the league of passive probes.

Order your active probes here

Differential probe

With a differential probe one can measure between any two points, just like with a multimeter. This is possible because the negative probe is not directly mains earth referenced, but more of a reference point for the positive probe.

A differential probe with various pairs of probe heads: sharp point, clamp and hook. The USB cable serves as a power cable.

A differential probe consists of two probes connected to a body, which is connected to the oscilloscope via a BNC connector. In the body the difference (hence the 'differential') is taken between the signals of the probes, so they are stripped of their common mode. The body must be powered, either through a supplied power adapter or nowadays through the USB port of the oscilloscope. If the probe has a scale switch it can be found on the body.

A common misconception is that the differential probe is completely isolated from the output to the oscilloscope. This is not the case. The common mode rejection is limited with respect to the ground of the oscilloscope. In practise this rarely causes any problems, because many differential probes provide hundreds or even thousands of volts of isolation voltage.

Differential probes are somewhat more limited in bandwidth compared to passive probes, but in the past few years well affordable 100 MHz differential probes have entered the assortment of Eleshop. The main disadvantages of the differential probe compared to passive probes are the higher cost and the limited shielding against electromagnetic interference (because of the long measuring cables, which are best twisted to keep the loop small).

Order your differential probes here

Current probe

Another type of probe is the current probe, which can measure current instead of voltage. Not all probes are suitable for measuring DC currents. Also, the bandwidth of current probes are limited.

A current probe.

Like the differential probe, this one consists of a special probe connected to a body. The probe is often a singular probe in the shape of a clamp, which is clamped around a cable or a bare wire to be measured. This way you do not need to break open the circuit for measurement, unlike with a multimeter. But there are also high frequency current probes which are shaped in a closed loop, intended to be used for EMC purposes.

The measured current is translated in the body to a voltage to be read on the oscilloscope. The scale is not always 1:1, the exact scale used is often specified on the body near the attenuation switch or in the manual.

The measured values can be scaled back on the oscilloscope, just like with a passive probe. Oftentimes the unit of the channel can also be adjusted to Amperes instead of Volts.

The current probe must also be powered externally. Many newer models are powered by USB, for example through the USB port of the oscilloscope or with a phone charger.

Order your current probes here

Which type of probe do I need?

A current probe is required to measure current. Argue first if it is necessary to measure on the oscilloscope instead or with a multimeter. Not all current probes can handle DC current.

For most voltage measurements a standard passive probe will do. If you perform measurements where you work with reference points other than ground or earth, buy a differential probe. Such scenarios may involve power supplies and amplifier circuits, for example. Also buy a differential probe if you want to measure on the mains.

Do you work with fast oscilloscopes starting from 1 GHz? And is the signal you want to measure not well received on a coax connection? Then active probes are a must have.