How to calibrate the 10X file of the oscilloscope probe

Quasi-oscilloscope probes typically have a switch on the handle for 1X or 10X attenuation selection. When the switch is in the 10X position, a 9M ohm resistor (Ra) is actually placed in series with the oscilloscope's input. Since the input resistance (Rs) of the oscilloscope is generally 1M ohm, Ra and Rs actually constitute a 1/10 voltage divider, which means that it will attenuate the measured voltage to 1/10 of the original voltage and then send it to the oscilloscope.

Why is the probe's 10X file calibrated?

Since the input capacitance (Cs) is inevitable at the input of the oscilloscope, Ra and Cs form a low-pass filter. When the signal has high-frequency components, this low-pass filter can seriously affect the performance of the oscilloscope. This is because when the signal passes through this filter, the high-frequency component is attenuated, and the low-frequency component is reduced. The oscilloscope input does not reflect the original signal, but an already distorted signal.

Obviously, this problem is caused by Cs. In order to offset the influence of Cs, we can add a capacitor Ca in parallel with Ra to “compensate” the attenuation caused by Cs. With Ca, the higher frequency signal components can be attenuated by the Ca bypass, which improves the performance of the 10X file. Since the Cs of each oscilloscope is different, Ca must be adjusted to perform its function.

How to calibrate the probe's 10X file

Calibrating the probe 10X requires a square wave signal source with good signal quality. Generally, there is no strict requirement on its frequency and amplitude. The frequency ranges from a few hundred Hz to several hundred KHz, and the amplitude can range from a few volts to a few volts, but the most important. The edge must be steep and all corners should be right angles. When the signal source is ready, follow the steps below to make the calibration.

1. Connect the probe to the square wave signal source and switch the switch on the handle to the 10X position.

2. Adjust the oscilloscope parameters so that 1 - 2 cycles of the signal are displayed on the screen, and the amplitude is 2 - 3

3. Adjust Ca with a small screwdriver to make the shape of the square wave displayed on the oscilloscope the best, and the calibration is complete.

Why do you know that the best compensation is best when you get a good square wave waveform?

From the signal theory, a square wave actually contains a lot of high-frequency frequency components. When a square wave passes through the circuit, if some frequency components increase or decrease, the output waveform will be distorted and reflected from the shape. Conversely, if we see that there is no waveform distortion after the square wave signal passes through the circuit, we know that all the frequency components pass through the circuit in the same proportion.

Source: Jin Yue Electronics

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