What Is a Probe?

Essentially, a physical and electronic connection needs to be established between the test point or signal source and the oscilloscope. According to measurement requirements,

This connection can be very simple, such as using a longer cable; Or it can be very complicated

An oscilloscope probe connects a signal source to a certain device or network on the oscilloscope input

Regardless of what the probe actually is, it must provide a sufficiently convenient and high-quality connection between the signal source and the oscilloscope input. The probe acts as a bridge between the oscilloscope and the object being measured; without this bridge, the oscilloscope cannot read information about the object being measured

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To perform oscilloscope measurements, the probe must first be physically connected to the test point. To achieve this, most probes are used to

one or two meters less length related cable. This probe cable allows the oscilloscope to remain fixed on a trolley or workbench while the probe moves between test points of the circuit under test. However, this does affect convenience to some extent. The probe cable reduces the probe bandwidth; The longer the cable, the greater the drop

Besides cable length, most probes also have a probe head or handle and a probe tip. The probe head can be fixed in place

Physically connecting the probe to the test point also establishes an electrical connection between the probe tip and the oscilloscope input. To obtain usable measurement results, when connecting the probe to the circuit, its impact on circuit operation must be minimized, and the signal from the probe tip must be transmitted to the oscilloscope input with sufficient fidelity via the probe head and cable

The probe in Figure 1 is a device that forms both physical and electrical connections between the oscilloscope and the test point

Figure 2 Most probes consist of a probe head, probe cable, compensation device, or other signal regulation network

What is the ideal probe?

Absolute Signal Fidelity An ideal probe should transmit any signal from the probe tip to the oscilloscope input with absolute signal fidelity. In other words, the signal occurring at the probe tip should be realistically reproduced on the oscilloscope input

To achieve absolute fidelity, from the probe tip to the oscilloscope input probe circuits must have zero attenuation, infinite bandwidth, and operate at all frequencies Achieve linear phase. These ideal requirements are not only impossible in practice implementation is also not feasible. For example, when processing audio frequency signals no one needs a probe or oscilloscope with infinite bandwidth. At 500MHz, it can cover most high-speed digital, TV, and other typical when applying oscilloscopes, infinite bandwidth is also not required

easy and convenient connection, zero signal source load, and noise resistance are also the qualities an ideal probe should have

easy and convenient connections: For microcircuits, such as high-density surface packaging technology(SMT), through the miniaturized probe head and for SMT various probe tip adapters designed for the device can improve ease and convenience. However, these probes are too small for practical use in industrial power circuits, where high-voltage lines and large lines are commonly used. Power applications require probes with higher safety margins and physically larger ones

From these physical connection examples, it is clear that there is no ideal probe specification or configuration suitable for all applications. For this reason, the industry has designed various probe specifications and configurations to meet the physical connection requirements of different applications

zero signal source load: The circuit behind the test point can be considered a signal source model. Any external device connected to the test point, such as a probe, will show additional load on the signal source behind the test point

in the slave circuit (signal source) When absorbing signal current, external devices operate as loads. This load or absorbed signal current will change the test point the subsequent circuit, thereby changing the signal seen at the test point.

An ideal probe results in zero signal source load. In other words, it does not absorb any signal current from the signal source. This means that to make the absorbed current zero, the probe must have infinite impedance, essentially representing the test point as open-circuit

In practice, probes with zero signal source load cannot be achieved. This is because probes must absorb a small amount of signal current to form signal voltage at the oscilloscope input. As a result, a certain signal source load is expected to be generated when using the probe. However, our goal should always be to minimize the number of loads by selecting the probe correctly

comprehensive noise immunity: Fluorescent lamps and fan motors are two of the two main sources of electrical noise in our surrounding environment. These noise sources cause noise in nearby electrical cables and circuits, which in turn increases noise in the signal. Because it is easily affected by induced noise, a simple wire is the suboptimal choice for an oscilloscope probe

An ideal oscilloscope probe should be able to comprehensively counter all noise sources. As a result, compared to the noise generated by the signal at the test point, the signal transmitted to the oscilloscope would not generate more noise

In practice, using shielding allows the probe to achieve advanced noise immunity for most commonly used signal levels. However, noise can still be a problem for certain low-level signals. In practice, common-mode noise can cause problems for differential measurements

An ideal probe results in zero signal source load. In other words, it does not absorb any signal current from the signal source. This means that to make the absorbed current zero, the probe must have infinite impedance, essentially representing the test point as open-circuit

In practice, probes with zero signal source load cannot be achieved. This is because probes must absorb a small amount of signal current to form signal voltage at the oscilloscope input. As a result, a certain signal source load is expected to be generated when using the probe. However, our goal should always be to minimize the number of loads by selecting the probe correctly

comprehensive noise resistance

Fluorescent lamps and fan motors are two of the many sources of electronic noise present in our environment. These noise sources cause noise in nearby electrical cables and circuits, which in turn increases noise in the signal. Because it is easily affected by induced noise, a simple wire is the suboptimal choice for oscilloscope probes

An ideal oscilloscope probe should be able to fully counter all noise sources. As a result, the signal transmitted to the oscilloscope does not generate more noise compared to the noise generated by the signal at the test point

In practice, using shielding allows the probe to achieve advanced noise immunity for most commonly used signal levels. However, for some low-level signals, noise can still be a problem. In practice, common-mode noise can cause problems for differential measurement, which we will discuss later

Figure 3 The industry provides various probes for different application technologies and measurement needs

a. detectionSMTEquipment

b.High-Voltage Probe

c.Clamp-Type Current Probe

Reprinted from Tektronix Technology "Questions About Probes"