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

The oscilloscope probe is crucial to the accuracy and correctness of the measurement results. It is an electronic component that connects the circuit under test to the input of the oscilloscope. The simplest probe is a wire that connects the circuit under test to the input of the electronic oscilloscope. The complex probe is composed of resistors and capacitors and active devices. Simple probes are not shielded and are easily interfered by external electromagnetic fields. In addition, the equivalent capacitance itself is large, which increases the load of the circuit under test and distorts the measured signal.

What is an oscilloscope probe?

Essentially, an oscilloscope probe is a physical and electronic connection between a test point or signal source and an oscilloscope; in fact, an oscilloscope probe is a device or network that connects the signal source to the oscilloscope input. It must provide a sufficiently convenient and high-quality connection between the signal source and the oscilloscope input. There are three key issues in the adequacy of the connection: physical connection, effect on circuit operation, and signal transmission.

Oscilloscope Probe Evolution Over the past 50 years, various oscilloscope probe interface designs have continued to evolve to meet the increased instrument bandwidth speed and measurement performance requirements. In the earliest days, banana plugs and UHF type connectors were commonly used. In the 1960s, ordinary BNC type connectors became the common probe interface type because BNC is smaller and has higher frequencies. BNC probe interfaces are still used in test and measurement instrument designs, and the current higher quality BNC type connectors provide maximum available bandwidth capabilities close to 4GHz. Later, some manufacturers proposed a workaround for the ordinary BNC type probe interface design. While using the BNC connector, an analog coded scale factor detection pin is additionally provided as part of the mechanical and electronic interface design, allowing compatible oscilloscopes to automatically detect and change the vertical attenuation range of the oscilloscope display.

Types and working principles of oscilloscope probes

Most people pay more attention to the use of the oscilloscope itself, but ignore the selection of the probe. In fact, the probe is the intermediate link between the measured signal and the oscilloscope. If the signal is distorted at the probe, then no matter how good the oscilloscope is, it will be useless. In fact, the design of the probe is much more difficult than the oscilloscope, because the inside of the oscilloscope can be well shielded and does not need to be frequently disassembled. In addition to meeting the requirements of detection convenience, the probe must also ensure at least the same bandwidth as the oscilloscope, which is much more difficult. Therefore, when the earliest high-bandwidth real-time oscilloscope first appeared, there was no corresponding probe, and the probe came out after a while.

To choose the right probe, the first thing to do is to understand the impact of the probe on the test, which includes the meaning of:

1. The influence of the probe on the circuit under test;

2. Signal distortion caused by the probe. An ideal probe should have no effect on the circuit being tested and no distortion on the signal. Unfortunately, no real probe can meet both conditions at the same time, and usually some compromises need to be made between the two parameters.

For DC or general low-frequency signals, the oscilloscope probe is just a transmission cable formed by a specific impedance R. As the frequency of the signal to be measured increases and becomes irregular, the oscilloscope probe will introduce parasitic capacitance C and inductance L during the measurement process. The parasitic capacitance will attenuate the high-frequency components of the signal and slow down the rising edge of the signal. The parasitic inductance will form a resonant circuit together with the parasitic capacitance, causing the signal to resonate. All of these will bring challenges to the accuracy of our signal measurement.

Figure 1 Schematic diagram of the electrical characteristics of the probe

Oscilloscope probes can be divided into passive probes and active probes according to the power supply mode. Passive probes are divided into passive low-voltage, passive high-voltage and low-resistance transmission line probes, while active probes are divided into active single-ended, active differential, high-voltage differential probes, etc. In addition, in some special applications, current probes (AC, DC), near-field probes, logic probes, and various sensor (light, temperature, vibration) probes are also used.

Passive probes are the most commonly used type of voltage probes and are also the probes that come with the oscilloscope when you purchase it, as shown in Figure 2.

Figure 2 Passive probe schematic

Passive probes are generally connected to oscilloscopes using a universal BNC interface, so passive probes from most manufacturers can be used on oscilloscopes of different brands (except for probes with special interface standards from some manufacturers). However, since oscilloscopes generally cannot automatically identify probe types from other brands, you need to manually set the probe attenuation ratio on the oscilloscope to ensure that the oscilloscope correctly compensates for the signal attenuation caused by the probe during measurement.

Figure 3 shows the schematic diagram of the most common passive probe in daily life, which consists of input impedance Rprobe, parasitic capacitance Cprobe, transmission wire (usually about 1 to 1.5 meters), and adjustable compensation capacitor Ccomp. This type of passive probe generally has an input impedance of 10M? and an attenuation factor of 10:1.

Figure 3 Passive probe schematic

When using this type of probe, the input impedance of the oscilloscope will be automatically set to high impedance 1MΩ. At this time, the relationship between the voltage Vscope at the oscilloscope BNC channel input point and the voltage value Vprobe detected by the probe front end satisfies the following correspondence:

Vprobe/Vscope = （9M? + 1M?) / 1M? = 10 ： 1

From the relationship, we can see that the voltage obtained by the oscilloscope is one tenth of the voltage detected by the probe, which is also the origin of the 10:1 attenuation factor of the passive probe. The passive probe has a high impedance of 10MΩ, so it has a small load effect on the circuit to be tested (which will be described in detail in the second part), can cover the general low-frequency band (within 500MHz), has a strong voltage resistance (300V-400Vrms), is cheap, and has good versatility, so it is widely used.

When the attenuation factor of a passive probe is 100:1, 1000:1 or even higher, this type of probe is generally classified as a passive high-voltage probe. Because of its large attenuation ratio, it can measure high-voltage and ultra-high-voltage electrical signals.

Figure 4 R&S RT-ZH10 high voltage probe

There is another type of passive probe, whose attenuation ratio is 1:1. The signal is directly transmitted to the oscilloscope through the probe without attenuation. Its voltage resistance is not as good as other passive probes, but it has the advantage of testing small signals. Because the signal does not need to be amplified 10 times by the oscilloscope for display like the 10:1 attenuation ratio probe, the internal noise of the oscilloscope is not amplified, and the measurement noise is smaller. This type is more suitable for testing small signals or power supply ripple noise.

Figure 5 R&S HZ-154 1:1/10:1 adjustable attenuation ratio passive probe

Passive transmission line probes are another special type of passive probes, which are characterized by relatively low input impedance, generally several hundred ohms, and support for higher bandwidth, up to several GHz or more. Figure 6 is a schematic diagram of a 10:1 passive transmission line probe with an input impedance of 500Ω:

Figure 6 Schematic diagram of transmission line probe

Transmission line probes have the characteristics of low parasitic capacitance and low input impedance, and are generally used to measure high-frequency signals. When using a transmission line probe, you should pay attention to setting the oscilloscope input impedance to 50 Ω to match the 50 Ω impedance of the transmission line. The typical application of a transmission line probe is to measure electrical signals on a 50 Ω transmission line. Through different conversion connectors such as SMA-N, the transmission line probe can also be used on other test equipment such as spectrum analyzers.

Figure 7 Typical applications of transmission line probes

It should be noted that due to the low impedance of the transmission line probe, its loading effect will be more obvious. Therefore, this type of probe is only suitable for testing circuits with low output impedance (tens to 100 ohms). For circuits with higher output impedance, we can choose to use a high-impedance active probe, which will be described in detail later.

Figure 8 R&S RT-ZZ80 8.0GHz passive transmission line probe

After introducing passive probes, let's take a look at active probes. As the name suggests, the biggest difference between active probes and passive probes is that they are "active", that is, they need power to work. Nowadays, most active probes are equipped with special interfaces to obtain power from the oscilloscope by connecting to the oscilloscope, without the need for additional external power supply (except for some models). The figure below shows the schematic diagram of an active single-ended probe:

Figure 9 Schematic diagram of active single-ended probe

Active single-ended probes generally have high impedance (around 1MΩ) and low parasitic capacitance. There is a high-bandwidth amplifier at the front end, and the power supply of the active probe is mainly used for this amplifier. The amplifier drives the signal to reach the oscilloscope through a 50Ω transmission line, and the input impedance of the oscilloscope needs to be selected as 50Ω for matching. Due to its lower parasitic capacitance and 50 ohm transmission, active single-ended probes can provide higher bandwidth than passive probes, so they are mainly used in the field of high-frequency signal measurement.