Probes.

A probe - also known as a test probe or measurement probe - connects the oscilloscope to the device under test (DUT) as part of the measurement setup. There are different probes for different test applications: A current probe (or current clamp) is used to measure current, for example, while digital signals can be recorded and displayed with the aid of a logic probe. This requires a suitable interface to an MSO (Mixed Signal Oscilloscope). In addition to the pure contact between the measuring device and the object to be measured, the probe has the task of measuring all signals with the highest possible accuracy.

It is therefore important to minimize the additional influence that is naturally caused by each probe and not to stress the signal source, i.e. the measurement object. Active probes typically place less load on the measurement object than passive probes. A high measuring resistance prevents unwanted interference or mains noise from being detected. The high resistance enables ideal voltage pick-up at the probe and thus an optimum connection to the oscilloscope.

The differential probe is a special type of active probe. The differential probe measures the difference between two voltage potentials without connecting the oscilloscope ground to the circuit under examination. This is an effective method of circumventing the ground problem with oscilloscopes, especially with signals whose reference potential is not 0 V (e.g. the symmetrical high-speed signals of a bus system). Differential probes are able to separate common mode noise from the signal component to be analyzed in fast low-voltage applications. This is important because digital signals are falling further and further below the usual noise limits in integrated circuits.

A passive probe contains only passive components and does not require a power supply for operation. It is suitable for measuring signals with bandwidths below 600 MHz. An active probe is usually required for broadband signals. Passive probes are generally inexpensive, robust and easy to use. They represent a relatively high capacitive load and a relatively low resistive load. Passive probes include, for example, low-impedance resistance divider probes and high-voltage probes.

Passive probes typically place a greater load on the measurement object than active probes. They are very well suited for qualitative measurements, for example for checking clocking signals and for troubleshooting. For quantitative measurements such as output voltage ripple and rise times, on the other hand, a higher measurement accuracy is achieved with an active probe. The combination of passive voltage probe and current probe is an ideal solution for power measurements.

Passive probes usually have an attenuation factor, for example ten times (10X) or one hundred times (100X), to reduce the circuit load at the tip of the probe. The circuit load increases with the frequency level and/or with signal sources with a higher impedance. The 10X attenuated probe improves the accuracy of the measurements, but reduces the amplitude of the signal at the oscilloscope input by a factor of 10.

Passive probes are suitable as general-purpose probes for universal applications. However, signals with very fast rise times and high signal clocking rates require probes for higher speeds and with a lower circuit load: active probes and differential probes are therefore the better solution for measuring high-speed and differential signals.

A high-voltage probe is used for measurements on devices and systems that work with voltages of up to 20,000 volts, for example in power generation and distribution, electric motors, power converters, switching power supplies and in telecommunications applications. A balanced high-voltage probe has two test conductors, while an unbalanced high-voltage probe uses just one test conductor. The probe measures the voltage by measuring the voltage potential between two points and transmitting the difference value to the oscilloscope.

A current probe is a simple way to measure an AC/DC current with the oscilloscope. The current probe detects the current flowing through a conductor and converts it into a voltage that is measured and displayed by the oscilloscope. The most common method is to measure the magnetic field of the current-carrying conductor. Depending on the application, other measurement techniques are used for current measurement, including a Rogowskicoil. A current probe has the advantage of galvanic isolation between the probe and the device under test. Due to the high sensitivity, small currents can also be measured with a large dynamic range. The probe can be placed anywhere in the current path without interrupting the circuit. The low insertion impedance ensures precise measurements.

A Rogowski coil or Rogowski probe is able to measure large alternating currents in a wide range, from mA to kA. The upper range of the measurable current is limited either by the maximum input voltage of the oscilloscope or by the voltage breakdown limit of the coil. The Rogowski coil is therefore very flexible to use. The high bandwidth makes it possible to measure even rapidly changing current signals. This means that higher-order harmonics in systems with high switching frequencies or signal shapes with fast rise/fall times can be analyzed. One of the main applications of Rogowski coils is the non-invasive measurement of high AC currents, for example in electric drives.

A current probe (also known as a current clamp) is used together with an oscilloscope to measure the current flowing through a conductor. The current probe converts the current into a voltage, which is measured and displayed by the oscilloscope. The most common method is to measure the magnetic field of the current-carrying conductor. Depending on the application, other measuring techniques – for example, a Rogowski coil – can also be used for current measurement. DC probes are always active. Pure AC probes are usually passive. Their function is based on the transformer effect or the Hall effect.

It is only possible to select the right probe by taking the application into consideration, because each measuring task and each device under test have specific requirements. The bandwidth of the probe is one of the fundamental characteristics that must be taken into account. The bandwidth indicates the maximum signal frequency up to which the probe can be used. It should be at least 3 to 5 times the highest signal frequency that you want to measure.

Probes with voltage dividers make it possible to measure signals with higher voltages, for example with a scaling factor ratio of 10:1 or 100:1. There are also models with a switchable scaling factor. It should be noted that the inherent noise of the oscilloscope has a greater effect on the measurement as the scaling factor increases. On the other hand, although less noise is seen with a smaller scaling factor, the measuring point is subjected to a greater load, which can distort the measurement signal.

In principle, the following applies: The oscilloscope is only one part of the measuring system. The connected probe has a decisive influence on the signal integrity. If a 1 GHz oscilloscope is used in conjunction with a probe that supports a bandwidth of 500 MHz, the oscilloscope bandwidth is not fully utilized. Precision measurements start at the tip of the probe. The "right" probe is ideally matched to the oscilloscope and the device under test and enables the signal to be fed cleanly into the oscilloscope. In addition, it should not alter the signal in order to ensure maximum signal integrity and measurement accuracy. To obtain precise results, a probe with the lowest possible load should be selected.

Another selection criterion is the form factor: Small form factor probes provide easier access to today's densely packed circuits.

As soon as a probe is connected to the device under test, it becomes part of the circuit, because part of the electrical energy of the circuit flows through the probe. Therefore, no probe can pick up a signal completely without feedback. This phenomenon is known as load. There are three types of load: resistive (ohmic), capacitive and inductive load. If the device under test is subjected to too high a load, amplitude measurement errors can occur and the signal shape can be distorted.

Resistive load: The input resistance of the probe should be at least ten times as high as the output resistance of the signal source so that the amplitude is reduced by less than 10%. Capacitive load: This leads to longer rise times and a lower bandwidth. To reduce the capacitive load, select a probe with at least five times the bandwidth of the signal. The inductive load is caused by effects of the grounding cable and appears as ringing/harmonics in the signal. Therefore, use the shortest possible test conductors.

Switchable probes are special probes for oscilloscopes that have a function for changing the attenuation factor. For example, a switch can be used to change between the levels 1:1 and 10:1. Switchable probes are particularly useful in applications where it is necessary to switch frequently between different signal levels or when versatile measurement options are required without having to use multiple probes. If the oscilloscope does not recognize the probe division automatically, the user must make the corresponding settings on the oscilloscope manually.

Automatic probe detection is a function of modern oscilloscopes that enables the device to automatically recognize which probe is connected. By identifying the type of probe connected (e.g. 1:1, 10:1, active probe), the oscilloscope adjusts its settings, such as the specific attenuation factor, accordingly. This reduces the error rate and simplifies operation. In addition to the attenuation factor, other information such as bandwidth, impedance and maximum input voltage can also be automatically recognized and taken into account. Some systems also perform automatic calibrations and compensations to ensure that the measurements are precise and the signal quality is optimal.