The function and principle of oscilloscope probe compensation

Let me explain in detail why the probe needs to be compensated, and I hope everyone can understand it thoroughly.

All of this starts with understanding the compensation principle of the probe:

Oscilloscope input resistance

The oscilloscope probe cannot send the circuit signal into the oscilloscope. At first glance, it seems that it can be used by directly connecting it.

However, when we use a multimeter to measure the resistance at both ends of the oscilloscope probe, it is nearly 9M ohms, as shown in the following figure:

The multimeter measures the resistance at both ends of the probe at X10 level

If we look at an oscilloscope, careful friends will find that the 1MΩ input resistance parameter to ground is usually marked next to the BNC input interface of the oscilloscope. Many people may not understand what this means.

STO1104C oscilloscope BNC input interface

In fact, when using an oscilloscope probe to measure a circuit, since we do not want the connection of the oscilloscope probe to change the working state of the measured circuit itself, the oscilloscope probe must be high-impedance, that is, the input impedance is relatively large (megaohm level). The oscilloscope has a certain voltage input range, but different measurement occasions will have different voltages, so the oscilloscope probe will have different attenuation ratios (1X, 10X, 100X...). Then the simplest signal attenuation is achieved through resistor voltage division, as shown in the following figure:

In the figure, R1 is the resistance on the oscilloscope probe, and Rin is the input resistance of the oscilloscope. Generally, Rin = 1MΩ, R1 = 99MΩ at 100X, R1 = 9MΩ at 10X, and theoretically it should be 0Ω at 1X, but in fact R1 is about several hundred ohms, generally within 300 ohms.

The multimeter measures the resistance at both ends of the probe X1

Oscilloscope input capacitance

So can the oscilloscope be used according to the resistor voltage divider circuit introduced above? No.

As we all know, in reality, no circuit is an ideal circuit and has more or less parasitic parameters. The interface between the oscilloscope and the oscilloscope probe is no exception. Since the oscilloscope interface needs to connect the signal and GND to the oscilloscope probe at the same time (as shown in the figure below, the metal on the outer circle is generally GND, which can play a role of shielding with the outside, and the metal inside is the input signal), a capacitor is formed between the input signal and GND. No matter how the design of the oscilloscope interface is improved, the parasitic parameters of the oscilloscope input capacitor cannot be eliminated.

The typical values ​​of input capacitance of general oscilloscopes are 15pF, 14pF, and 12pF. The one shown in the figure is 14pF.

There are both R and C, isn’t this an RC low-pass filter?

Let's calculate the cutoff frequency of this RC circuit. Considering the 10X gear, R1 = 9MΩ, Rin = 1MΩ, Cin = 14pF, the cutoff frequency is

In this way, all signals above 12.64kHz are attenuated to the point where they are unreadable. So how do we solve this problem? Reduce Cin? Impossible, because physical limitations dictate that Cin must exist, and 14pF itself is already a very small capacitance value; reduce R1 and Rin? Too small a resistance will inevitably affect the measured circuit. It seems that there is no other way. But there are always smart people who can find a solution: compensation capacitor

Compensation capacitor

As long as it satisfies

This allows signals to be transmitted at the correct 10:1 attenuation ratio at relatively high frequencies without causing signal distortion.

But a new problem arises. Different oscilloscopes have different Cin. Even if the model is the same, different oscilloscopes may not meet the above ratio relationship when using the same probe due to inconsistent parameters. Doesn't that mean that each oscilloscope needs to have a specific probe installed? How to solve the universal problem? It's very simple, just add another variable capacitor. As shown in the figure Cp.

In this way, the problem can be solved by simply adjusting the size of the variable capacitor on the probe and observing the waveform distortion on the oscilloscope.

The oscilloscope generally outputs a 1KHz, 5V (or less) square wave signal, which is used for probe compensation calibration. This signal is often indicated by a square wave symbol plus a ground symbol. We can use this signal as a signal source. Start the oscilloscope, as shown in the figure below, connect the probe's BNC to channel 1, and the other end to the square wave signal output port.

When the following two situations occur, it means that the probe compensation is incorrect and you need to use the "adjustment stick" to adjust the compensation capacitor on the probe.

Overcompensated waveform

Under-compensating waveforms

Use the adjustment rod to turn the screw in the probe screw hole to adjust the compensation capacitor to obtain the correct waveform.

Some probes also set it at one end of the detection head, as shown below:

Therefore, when the oscilloscope is replaced with a new probe or the probe has not been used for a long time, we should perform compensation calibration on the probe.