This section provides an overview for passive probe as well as their applications and principles. Also, please take a look at the list of 8 passive probe manufacturers and their company rankings.
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A passive probe is a device that is attached to the input terminal of an oscilloscope to facilitate measurement of electrical signals with the oscilloscope. It is also called a passive probe. This device is often included as an accessory when you purchase an oscilloscope. It is the most widely used and inexpensive of all probes. Many passive probe is set to an internal resistance that is nine times the oscilloscope's input resistance. When an input signal is received through a passive probe, the input voltage is reduced by a factor of 10, which also protects the circuitry in the oscilloscope.
A passive probe is used in conjunction with oscilloscopes to observe changes in electrical signals. By observing electrical signals, many oscilloscopes are available that can measure temperature, humidity, speed, and pressure in addition to voltage, and passive probe are used to improve the accuracy of these measurements and to protect the oscilloscope's circuitry. They are also used to check the cause of circuit failures and malfunctions, position magnetic heads, adjust signals, and detect noise by examining electrical signals.
When selecting a passive probe, it is recommended that you purchase a passive probe from the same manufacturer as the oscilloscope you are using, so they are compatible with each other.
Passive probes consist of a capacitor for adjusting the frequency response and an internal resistor connected in parallel. The internal resistor is often set to 9 MΩ, which together with the oscilloscope's input resistor, which is often set to 1 MΩ, makes the input resistance 10 MΩ.
The principle of operation of a passive probe is explained separately for the case where the electrical signal is high frequency and for the case where the electrical signal is low-frequency.
When the electrical signal is high frequency, the impedance of the capacitor used to adjust the frequency response is low, and the electrical signal passes through the capacitor and is observed by the capacitor inside the oscilloscope.
If the electrical signal is low frequency or DC current, the impedance of the capacitor becomes large and the electrical signal flows through the internal resistance to the oscilloscope's input resistance and is observed.
Passive probes are used for general measurements with oscilloscopes, and are selected from three types (1:1, 1:10, and 1:100) based on the characteristics of the circuit under test, depending on the built-in attenuator.
In the 1:1 type, the signal is directly applied to the oscilloscope's input terminal without an attenuator, so the oscilloscope's input impedance of 1 MΩ is connected to the circuit under test. If the circuit under test has a high impedance, it may affect the measurement circuit, so care must be taken. On the other hand, when observing minute signals, the input sensitivity of an oscilloscope can be used as it is, which is more advantageous than a probe with a built-in attenuator.
The 10:1 probe is the most general-purpose probe. Since the input impedance is 10 MΩ, it has little effect on the circuit under test and is easy to use.
The 100:1 probe has an attenuation factor of 1/100, so it is mainly used when the signal voltage exceeds 100V. It is also characterized by its extremely high input impedance of 100 MΩ, which has little effect on the circuit under test.
So-called high-voltage probes are also a type of passive probe, but they are large, special forms with completely different precautions.
Particular attention should be paid to the connection and handling of ground leads when using a passive probe. When observing multiple points simultaneously on a multi-channel oscilloscope, it is fundamental that the ground leads to each channel probe be connected to a single common point (preferably a single point ground). Connecting to different ground lines will create a ground loop on the oscilloscope's circuitry, which will adversely affect the measurement of small signals.
In addition, longer ground leads facilitate probing, but when observing high-frequency signals, faulty phenomena such as ringing and large fluctuations in signal amplitude can occur.
This is due to the resonance between the inductance component of the ground lead and the input terminal capacitance of the probe, resulting in extremely large amplitude near the resonance frequency. Therefore, when observing high-frequency signals with a frequency of 10 MHz or higher, consider using a ground spring instead of a ground lead.
The main measurement performance is determined by the frequency bandwidth and rise time of the measurement system that combines the oscilloscope and the probe. Therefore, the frequency bandwidth and rise time specifications are published for each oscilloscope body and a probe that is combined with the oscilloscope body.
The probe's input capacitance greatly affects the frequency response of waveform measurements. The higher the frequency, the smaller the capacitive reactance leads to a greater the load on the circuit under test. Furthermore, the frequency bandwidth of the probe itself is narrowed, the rise time is slowed down, and so on, adversely affecting the measurement performance.
What is important in the frequency response is that the amplitude of the waveform under test is 3 dB lower near the rated frequency. That is, at the upper end of the frequency band, the amplitude is 30% smaller.
Therefore, for accurate amplitude measurement, the required frequency bandwidth of the oscilloscope and probe should be about five times the frequency of the waveform to be measured. Following this guideline will ensure sufficient frequency bandwidth even for signals containing high-frequency components, such as square waves, and avoid distortion of the signal waveform.
The rise time of the measurement system should also be considered. To measure the rise and fall times of pulses with sufficient accuracy, the oscilloscope and probe should have a rise time less than 1/5 of the rise time of the pulse to be measured.
*Including some distributors, etc.
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