Ⅰ. Introduction to oscilloscope probe
The oscilloscope probe establishes a physical and electronic connection between the test point or signal source and the oscilloscope.In fact, an oscilloscope probe is a device or network that connects the source of the signal to the input of the oscilloscope.There are three key issues in connection adequacy: physical connection, impact on circuit operation, and signal transmission.
Ⅱ. Classification of oscilloscope probes
Hundreds, if not thousands, of different oscilloscope probes are available on the market.One of the technical indicators of oscilloscope probe is frequency characteristic. It is convenient to divide the types of probes according to frequency. However, the frequency coverage of oscilloscope probe is limited and it is difficult to divide it according to LF, HF, VHF, UHF, RF and other bands of radio frequency.Oscilloscope probe is one of all probes, the most commonly used probe is the voltage and current probe, and the probe is usually measured by the object classification.
Passive probes are made of wires and connectors, including resistors and capacitors when compensation or attenuation is required.There are no active components (transistors or amplifiers) in the probe, so there is no need to power the probe.Passive probes are generally the strongest and most economical probes. They are not only simple to use, but also widely used.
According to practical needs, voltage probe is the most used, among which high resistance passive voltage probe accounts for the most.Passive voltage probes provide various attenuation coefficients of 1×, 10×, and 100× for different voltage ranges.Among these passive probes, 10× passive voltage probe is the most commonly used probe.For applications where the signal amplitude is 1V peak-to-peak or lower, a 1× probe may be appropriate or even necessary. It is much easier to switch a 1×/10× probe in applications where the signal mixture is low amplitude and medium amplitude (tens of millivolts to tens of volts).However, the switchable 1×/10× probe is essentially two different probes in a product, with different attenuation coefficients, bandwidth, rise time, and impedance (R and C) characteristics.Therefore, these probes do not exactly match the input of the oscilloscope and do not provide the optimal performance achieved by the standard 10× probe.
Most high impedance passive probes have bandwidths ranging from less than 100MHz to 500MHz or higher.The low resistance passive voltage probe (also known as 50 ohm probe, Zo probe, voltage divider probe) has good frequency characteristics, with coaxial cable matching probe, bandwidth up to 10GHz and 100 picoseconds or faster rise time.The probe is designed for use in 50 ohm environments such as high speed equipment calibration, microwave communication and time domain reflectometer (TDR).
"High pressure" is a relative concept.From the probe perspective, we can define high voltage as any voltage that exceeds the voltage that a typical universal 10× passive probe can safely handle.The high voltage probe is required to have good insulation strength to ensure the safety of the user and oscilloscope.
Active probes contain or depend on active devices, such as transistors.Most commonly, an active device is a field effect transistor (FET) that provides a very low input capacitance, which leads to high input impedance over a wider range of frequencies.It can be seen from the following Xc formula:
The specified bandwidth of active FET probe is generally between 500MHz and 4GHz.In addition to the higher bandwidth, the high input impedance of the active FET probe allows measurements to be made at test points where the impedance is unknown, with a much lower risk of load effects.In addition, longer ground wires can be used because the low capacitance reduces the impact of ground wires.
Active FET probes have no voltage range for passive probes.The linear dynamic range of active probes is generally between ± 0.6v and ±10V.
Differential signals refer to each other, not to ground signals.The differential probe can measure the signal of the floating device. In essence, it is composed of two symmetrical voltage probes, which have good insulation and high impedance to the section respectively.The differential probe provides a high common mode rejection ratio (CMRR) over a wider frequency range.
In principle, the current value can be easily obtained by dividing the measured impedance value by the voltage value measured by the voltage probe.However, the error of this measurement is very large, so voltage conversion of current is not used.The current probe can accurately measure the current waveform by using the current transformer input, the signal current flux is converted into voltage through the mutual inductance transformer, and then amplified by the amplifier in the probe and sent to the oscilloscope.
3.1 AC current probe
The alternating current in a transformer changes with the direction of the current, generating electric field changes, and induced voltage.The ac current probe is a passive device without external power supply.
3.2 DC current probe
Conventional current probes can only measure ac signals because steady dc current cannot induce current in a transformer.However, using the hall effect, the current bias semiconductor device will generate a voltage corresponding to the direct current field.Therefore, the dc current probe is an active device that needs external power supply.
Therefore, current probes are basically divided into two categories: AC current probes and AC/DC current probes. AC current probes are usually passive probes and AC/DC current probes are usually active probes.
When using an oscilloscope to observe and analyze the analog characteristics of digital waveform, it is necessary to use a logic probe. In order to isolate the exact cause of formation, digital designers usually need to look at the specific data pulse under specific logical conditions, which requires the logical trigger function.Figure 3 is a schematic diagram of logical probe, which can be added to most oscilloscopes.
Because the scope of application of oscilloscope is very wide, so in addition to the above types of probes, there are a variety of special probes, these professional probes according to its different front-end sensors and have different functions, we introduce two of them below, only for the reader to understand.
Photoelectric probe is a combination of ordinary voltage probe and photoelectric conversion device in principle, which can directly measure the optical signal transmitted by optical device and optical fiber.
Temperature probe is a combination of ordinary voltage probe and temperature sensor, which can directly measure the temperature of objects.Temperature probe is a kind of sensor probe. Various sensor probes and oscilloscope can measure various physical quantities.
Ⅲ. Tips for use of oscilloscope probe
Connecting the signal to the oscilloscope is the first step of the test.