In the electronic manufacturing, we will inevitably use a variety of test equipment, oscilloscope is one of the most commonly used equipment. When using the oscilloscope, we use the probe to measure the physical factor such as time, frequency, and voltage. But have you ever wondered how the probe measures these physical factor?
To solve this question, we must tear down the oscilloscope probe and take a look inside. The section connected with oscilloscope is a BNC interface. If you use two wires to connect the oscilloscope directly instead of BNC connector, signal distortion will happen. A square wave input result in a sawtooth wave! How did this happen?
Oscilloscopes generally use a higher input impedance to reduce the impact on the test circuit. So you will see a 1M ohm resistor or a similar circuit behind the probe's BNC connector. A coin should has two side, the smaller external capacitance value will form a filter at input interface, which causes measured waveform distortion. How to solve this problem depends on the way the probe is processed.
In general, the oscilloscope's probe will use a parallel adjustable capacitor to offset the impact of this part of the cable. Some compensation capacitors allow us to adjust it by ourselves to reach an ideal effect. There will be a square wave source on the oscilloscope. We hook the probe on the signal source and adjust the capacitor so that the square wave displayed on the screen becomes the most standard "square wave". A large capacity allows the probe to form a low-pass filter and, conversely, will form a high-pass filter. So be careful to adjust the job.
Usually the probe will have an attenuator for attenuating the signal. Typically a multiple of 10 times. 1V signal goes in, showing 100mV. Some oscilloscopes can automatically recognize the status of the probe and display the correct values.
The probe utilizes high-impedance features to ensure that the circuit is not disturbed by the measurement section, but sometimes we need to measure some of the circuits with a low-impedance test. For example, 50 ohm impedance RF output circuit, this is only a click of the button for 50 ohm impedance measurement function of the machine; but for ordinary oscilloscope, the probe is not suitable for measurement. You need to use three-way BNC to match 50 ohm end resistance, and directly connected to the 50 ohm output at the other end.
For many enthusiasts, these questions are very simple but rarely think about. This is the same with the devices around us, we use them every day but we rarely pay attention to their principles? Need some thought!
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