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In essence, an oscilloscope probe establishes both a physical and an electrical connection between the test point or signal source and the oscilloscope. In practice, however, a probe is far more than a piece of wire that senses the test signal and delivers it to the oscilloscope input—it is a key component of the measurement system. A probe is therefore any device or network that connects a signal source to an oscilloscope input, and it must provide a sufficiently convenient, high-quality connection between the two. Three key questions define how adequate a probe connection is: the physical connection, the effect on circuit operation, and signal transmission.
Let's start with the types of oscilloscope probes. Probes come in many different models and types, each with its own purpose and characteristics; the market offers hundreds, even thousands, of different oscilloscope probes to meet the diverse needs of different measurement tasks. Frequency response is one probe specification, and classifying probes by frequency has its conveniences, but a probe's frequency coverage is limited, making it difficult to divide them into radio-frequency bands such as LF, HF, VHF, UHF, and RF. Oscilloscope probes are one category among all probes, and the most commonly used are voltage and current probes. Oscilloscope probes are generally classified by measurement object and fall mainly into the following types:
1. Passive probes Passive probes are made of wire and connectors, and where compensation or attenuation is needed, they also include resistors and capacitors. There are no active devices (transistors or amplifiers) in the probe, so no power supply is required. Passive probes are generally the most rugged and economical probes; they are simple and convenient to use and widely applied. 2. High-impedance passive voltage probes Judging by actual demand, voltage probes are the most widely used of all oscilloscope probe types, and high-impedance passive voltage probes account for the largest share. Passive voltage probes offer various attenuation factors—1×, 10×, and 100×—for different voltage ranges. Among these, the 10× passive voltage probe is the most commonly used. For applications with signal amplitudes of 1 V peak-to-peak or less, a 1× probe may be more suitable, or even essential. 3. Low-impedance passive voltage probes Most high-impedance passive probes have bandwidths ranging from under 100 MHz to 500 MHz or higher. Low-impedance passive voltage probes (also called 50-ohm probes, Zo probes, or divider probes) offer excellent frequency characteristics; using a matched coaxial cable, a probe can achieve bandwidths up to 10 GHz and rise times of 100 ps or faster. These probes are designed for 50-ohm environments, typically high-speed device characterization, microwave communications, and time-domain reflectometry (TDR). 4. Passive high-voltage probes "High voltage" is a relative concept. From the probe's perspective, we can define high voltage as any voltage exceeding what a typical general-purpose 10× passive probe can safely handle. High-voltage probes require good dielectric strength to ensure the safety of both the user and the oscilloscope. 5. Active probes Active probes contain or rely on active devices such as transistors. Most commonly, the active device is a field-effect transistor (FET), which provides very low input capacitance; low capacitance in turn yields high input impedance over a wider frequency range. 6. Active FET probes Active FET probes typically have specified bandwidths from 500 MHz to 4 GHz. Beyond higher bandwidth, the high input impedance of an active FET probe allows measurements at test points of unknown impedance with a much lower risk of loading effects. In addition, because low capacitance reduces ground-lead effects, longer ground leads can be used. Active FET probes do not have the voltage range of passive probes; their linear dynamic range is typically between ±0.6 V and ±10 V. 7. Active differential probes Differential signals are referenced to each other rather than to ground. An oscilloscope differential probe can measure signals on floating devices; in essence, it consists of two symmetrical voltage probes, each with good insulation from ground and high impedance. Differential probes provide a high common-mode rejection ratio (CMRR) over a wide frequency range. 9. Current probes In principle, current can easily be obtained by dividing the voltage measured with a voltage probe by the impedance under test. In practice, however, this method introduces large errors, so converting voltage to current is generally not used. A current probe measures the current waveform accurately by using a current transformer as its input: the magnetic flux from the signal current is converted to a voltage by the transformer and then amplified by an amplifier in the probe before being sent to the oscilloscope.
10. AC current probes In a transformer, as the AC current changes direction, it produces a changing electric field and induces a voltage. AC current probes are passive devices and require no external power supply. 11. DC current probes Traditional current probes can only measure AC signals, because a steady DC current cannot induce a current in a transformer. Using the Hall effect, however, a semiconductor device biased with a current produces a voltage corresponding to the DC electric field. A DC current probe is therefore an active device requiring an external power supply. Current probes thus fall basically into two categories: AC current probes and AC/DC current probes. AC current probes are usually passive, while AC/DC current probes are usually active. 12. Logic probes Logic probes are needed when using an oscilloscope to observe and analyze the analog characteristics of digital waveforms. To isolate the exact cause, digital designers often need to view specific data pulses occurring under specific logic conditions, which requires logic trigger capability. 13. Other oscilloscope probes Because oscilloscopes are used so widely, there are various specialized probes in addition to the types above. These specialized probes serve different functions depending on their front-end sensors; two of them are described below for reference. An optical probe is, in principle, a combination of an ordinary voltage probe and an optoelectronic converter, allowing direct measurement of optical signals from optical devices and fiber-optic transmission. A temperature probe is a combination of an ordinary voltage probe and a temperature sensor, allowing direct measurement of an object's temperature. Temperature probes are one type of sensor probe; various sensor probes, used with an oscilloscope, can measure many physical quantities. |
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