What is an oscilloscope probe? Principle of oscilloscope probe. Use of oscilloscope probe

Advantages and disadvantages often coexist, and this is also true for active single-ended probes. The ability to measure signals with higher bandwidths is an advantage, but since an active amplifier needs to be integrated, its cost is higher than that of passive probes. An active single-ended probe with a bandwidth of several GHz can cost tens of thousands of RMB. In addition, since the signal input range of a high-bandwidth amplifier is very limited, its dynamic range is limited. Generally, the dynamic range of an active single-ended probe is only within a few volts, and the maximum voltage that the probe can withstand is only tens of volts.

Compared with the passive transmission line probe mentioned above, the active single-ended probe can also be used in the measurement environment of low impedance and high frequency signals, and because its input impedance is higher than that of the passive transmission line probe, its loading effect is smaller. In addition, the R&S active single-ended probe can also be connected to the RT-ZA9 (N-type conversion connector, USB power supply) accessory, and then used on the RF signal source and spectrum analyzer to test signals in special environments, such as detection points that cannot be connected with traditional 50-ohm coaxial cables, or when a high-impedance probe is needed to detect the signal spectrum of the test point.

Figure 10 R&S RT-ZS series single-ended active probe connected to RT-ZA9 N-type converter

In addition to active single-ended probes, active differential probes are another important type of active probes. We can understand the difference between the two probes literally. The front end of the active single-ended probe has two connection points: signal point and ground. As the name suggests, the active differential probe is mainly used to test differential signals. The front end of the probe has three connection points: signal positive, signal negative, and ground.

Figure 11 Active single-ended probe head (left) and active differential probe head (right)

The schematic diagram of the active differential probe is as follows:

Figure 12 Schematic diagram of active differential probe

Compared with the active single-ended probe, the biggest difference is the use of a differential amplifier. The active differential probe also has the characteristics of low parasitic capacitance and high bandwidth, but the difference is that the active differential probe has a high common mode rejection ratio (CMRR) and has a strong ability to suppress common mode noise. The active differential probe is mainly used to test differential signals, that is, to test the relative voltage difference between two signals (generally positive and negative signals with a phase difference of 180 degrees), which is independent of the ground.

Figure 13 Schematic diagram of differential signal test principle

The figure above shows the principle of using an active differential probe to test differential signals. The red waveform in the figure shows the differential signal Vin+, and the blue waveform shows the differential signal Vin-. The two have the same amplitude and a phase difference of 180 degrees. Vin+ and Vin- are detected by the positive and negative detection points of the differential probe, amplified by the differential amplifier, and then transmitted to the oscilloscope, and finally the green differential waveform is obtained.

Here we will introduce several concepts so that everyone can better understand the common mode rejection ratio CMRR.

Common Mode: The signal components with the same amplitude and phase at both ends of the differential signal are expressed as Vcm = (Vin+ + Vin-)/2.

Since ideally Vin+ and Vin- have the same amplitude and opposite phase, the sum of the two should be zero. However, in actual working environment, noise interference Vnoise will be superimposed on Vin+ and Vin-. Since Vin+ and Vin- are in the same environment, the noise superimposed on them is often the same, so from the CM expression, we can know that: CM = Vnoise.

Differential Mode: The different signal components at both ends of the differential signal, expressed as Vdm = Vin+ - Vin-.

Common Mode Rejection: The ability of a differential amplifier to suppress common mode signals, that is, one of the main capabilities of a differential amplifier is to suppress and eliminate Vnoise. If the gain of the common mode voltage Vcm after the differential amplifier is Acm, and the gain of the differential mode voltage Vdm after the differential amplifier is Adm, then we can use the common mode rejection ratio (Common Mode Rejection Ratio) or CMRR to represent the common mode rejection capability, and its expression is:

CMRR = Adm / Acm

For example, as shown in the figure below: the amplitude of the differential mode signal Vdm is 1V, and after passing through the differential amplifier, the amplitude is still 1V, that is, Adm = 1. The amplitude of the common mode signal Vcm is 4.5V, and after passing through the differential amplifier, the amplitude is suppressed to 0.45V, that is, Acm=0.1. Therefore, CMRR = 1 / 0.1 = 10:1 = 20dB.

Figure 14 Differential signal test example

For an ideal differential amplifier, we hope that it can completely suppress the common-mode signal, thereby eliminating the influence of noise Vnoise on the differential signal measurement. For general differential signal measurements, a CMRR of 20dB is sufficient, and the CMRR of the R&S RT-ZD40 can reach 50dB, which is an excellent performance.

Figure 15 R&S RT-ZD40 active differential probe

It is worth mentioning that both R&S active single-ended probes and active differential probes are equipped with MicroButton multi-function buttons and ProbeMeter probe meter functions. Among them, MicroButton is a miniature button located at the front of the active probe. Users can easily press the button during testing to perform specific controls on the oscilloscope (customizable), such as: automatic settings, default settings, single run, continuous run, etc.

Figure 16 MicroButton multi-function button

ProbeMeter is a 16-bit DC voltmeter integrated in the front end of the active probe, which can be used to test the DC voltage directly at the probe point. This is completely different from other manufacturers' solutions of using probes to capture waveforms and then transmitting them to oscilloscopes to measure the waveforms to obtain DC values. Obviously, ProbeMeter eliminates the distortion effect of probe transmission, thus achieving a high accuracy of 0.1%. When using a differential probe, you can use this function to quickly and easily view the single-ended, common-mode, and differential-mode voltage values.

Figure 17 ProbeMeter probe voltmeter

Active differential probes can be used to measure most small-amplitude differential signals, but for high-voltage differential signals with amplitudes of hundreds or even thousands of amplitudes, active differential probes are not up to the task. At this point, we can only resort to the help of high-voltage differential probes, which have a higher dynamic range and can withstand higher voltages than general differential probes.

Figure 18 R&S RT-ZD01 ±1400V high voltage differential probe

High-voltage differential probes are expensive compared to passive high-voltage probes. Therefore, some users may choose to cut off the power ground wire of the oscilloscope when testing high-voltage differential signals to make the oscilloscope "float" for testing. This is very dangerous and must be avoided. We will explain this in detail in the second part.

Strictly speaking, current probes are also a type of active probe. Almost all current probes require power supply during use. Current probes are mainly divided into three categories: AC (can only test alternating current), DC (can only test direct current), and AC+DC. Currently, most current probes have the measurement function of AC+DC.

The principle of the current probe is as follows, mainly utilizing the electromagnetic effect (AC measurement) and the Hall effect (DC measurement).

Figure 19 Schematic diagram of AC+DC current probe

When AC current passes through the wire and the front closed jaws of the current probe, a corresponding magnetic field is generated, and the strength of the magnetic field is directly induced to the coil of the current probe. The probe is like a current transformer, and the system directly measures the induced current.

If it is DC or low-frequency current, when the current clamp is closed, a magnetic field will appear near the current conductor. The magnetic field deflects the electrons in the Hall sensor and generates a voltage at the output of the Hall sensor. Based on this voltage, the system generates an anti-phase (compensation) current to the coil of the current probe to make the magnetic field in the current clamp zero and prevent magnetic saturation. The system measures the actual current value based on the anti-phase current.

The selection of current probe is mainly based on its measurement bandwidth, range and jaw diameter.

MSO digital logic probes are often used in digital logic testing. Compared with general 8-bit analog probes, digital logic probes display the captured voltage on the screen as a digital signal of 0, 1 transition (1 bit) according to the decision gate line level set by the oscilloscope. Users can judge the performance of logic circuits based on the logic levels and relationships of multiple digital signals.

Figure 20 R&S RTO-B1 digital logic probe

EMI near-field probe is another special type of probe. It actually uses the antenna receiving principle to capture the electromagnetic field interference radiated from the space on the circuit board, especially for diagnosing EMI electromagnetic interference in system integration.

What is an oscilloscope probe? Principle of oscilloscope probe. Use of oscilloscope probe

Figure 21 Schematic diagram of EMI near-field probe

In addition to the various probes introduced above, there are also optical probes, temperature sensor probes and other types of sensor probes. In principle, any sensor that can convert various physical quantities into voltage signals and has the ability to be interconnected with an oscilloscope can be used as an oscilloscope probe. Users can choose the appropriate probe type according to the specific use environment and needs.