Understand the working principle of oscilloscope probe in minutes

Oscilloscopes are the right-hand man of our electronic engineers. Playing with oscilloscopes well is one of our essential skills. As for probes, we often use them to measure physical quantities such as time, frequency and voltage. The ancients have long taught us not only to know what it is, but also why it is. Therefore, it is not enough to just play with it. It is even more important to understand the working principle of the probe in order to use the oscilloscope better. Next, prbtek will share with you:

To understand this problem, let's first look at the design principle of the oscilloscope:

The waveform first passes through the probe, is amplified by the front-end amplifier, then converted by the analog-to-digital conversion unit, stored in the acquisition memory, and then displayed on the monitor.

Then we disassemble the probe of the oscilloscope and take a look at the internal structure. There is a BNC interface at one end connected to the oscilloscope. If you do not use the BNC connector but directly use two wires to introduce the signal into the oscilloscope, you will see that the signal is distorted. A square wave goes in, but a sawtooth wave is displayed! What is the reason for this?

Oscilloscopes generally have high input impedance to reduce the impact on the circuit being measured. Therefore, you will see a 1M ohm resistor or a similar circuit behind the probe BNC interface. Such a small external capacitor value will also form a filter at the input, thereby distorting the measured waveform. How to solve this problem depends on the processing method of the probe!

Generally speaking, the probe of the oscilloscope will use a parallel adjustable capacitor to offset the influence of this part of the cable. Some compensation capacitors allow us to adjust them ourselves and choose the best effect. There is a square wave source on the oscilloscope. We hook the probe to the signal source and adjust the capacitor to make the square wave displayed on the screen the most standard "square wave". Too much capacitance will make the probe form a low-pass filter, and vice versa, it will become a high-pass filter. Therefore, it must be adjusted carefully.

The probe usually has an attenuator to attenuate the measured signal. The multiple is usually 10 times. A 1V signal will be displayed as 100mV. Some oscilloscopes can automatically identify the status of the probe and display the correct value.

The probe uses the high impedance characteristic to ensure that the circuit is not disturbed by the measured part, but sometimes we need to measure certain circuits in a low impedance test mode. For example, for a 50 ohm impedance RF output circuit, for a machine with a 50 ohm impedance measurement function, this is just a matter of pressing a button; but for an ordinary oscilloscope, the probe is not suitable for measurement at this time. You need to use a BNC tee and a 50 ohm terminal resistor to match, and connect the other end directly to the 50 ohm output terminal.

For electronics enthusiasts, although measuring instruments seem simple, we rarely pay attention to their principles in daily use. It seems that if we want to use them well, we need to think further.