How to choose an oscilloscope probe

 

    Oscilloscope bandwidth

 

    For electronic engineers who often deal with oscilloscopes, the bandwidth of the oscilloscope is undoubtedly one of the indicators they care about most. Bandwidth directly affects the fidelity of the signal and the accuracy of the measurement. The bandwidth we usually refer to is the -3dB bandwidth, as shown in Figure 1. When a 200M sine wave is input into an oscilloscope with a bandwidth of 200M, its amplitude will theoretically drop by 3dB. However, if you divide the measured signal amplitude by 0.707 to obtain the true signal amplitude, the result may not be correct. Because in fact, when the oscilloscope is designed, in order to ensure the bandwidth parameters and reduce signal distortion, the signal attenuation amplitude within the band is usually less than 3dB. Therefore, the 200M bandwidth is a design indicator to ensure that the attenuation of the sine wave amplitude within 200M does not exceed 3dB, so as to provide better measurement confidence.

 

    Figure 1 Bandwidth is defined as the frequency at which the amplitude of the response curve drops by 3dB

 

    Probe bandwidth

 

    An oscilloscope must use a probe to perform measurements. What is the impact of the bandwidth of an oscilloscope probe on the oscilloscope? Below we analyze the most widely used high-impedance passive probe. High-impedance passive probes have different attenuation ratios, such as 1×, 10×, 100×, etc. Even if it is the same probe, the bandwidth is different at different attenuation levels, so you should pay attention when using it.

 

    The principle of probe selection is that the bandwidth of the probe should not be lower than the bandwidth of the oscilloscope to ensure that the bandwidth of the oscilloscope can be fully utilized after being connected to the oscilloscope. When the bandwidth of the probe is lower than the bandwidth of the oscilloscope, the bandwidth of the system will be limited by the bandwidth of the probe, and additional waveform distortion may occur. This distortion is caused by the mismatch between the probe and the oscilloscope. Using the probe recommended by the oscilloscope manufacturer can avoid additional waveform distortion caused by the mismatch between the probe and the oscilloscope, and ensure that the oscilloscope bandwidth extends to the probe tip when the probe bandwidth is not lower than the oscilloscope bandwidth.

 

    Problem Review

 

    Let's go back to the previous question: Is it possible to use a 200M bandwidth oscilloscope with a 200M bandwidth passive probe? Will the bandwidth be reduced to 140M? From the above text, you should be able to easily get the answer: such a configuration is feasible, and the bandwidth will not be reduced to 140M, but in order to ensure the available bandwidth of 200M and avoid additional waveform distortion, you should use the probe recommended by the oscilloscope manufacturer as much as possible.

 

    Some readers may still have questions: When two 200M bandwidth systems are cascaded, why can the bandwidth still remain at 200M?

 

    In fact, the connection between the probe and the oscilloscope is not a simple cascade.

 

    Let's think about how the probe's 10x attenuation is achieved. It is achieved by dividing the voltage with the oscilloscope's input impedance. If the probe is not connected to the oscilloscope, it cannot achieve 10x attenuation by itself. Therefore, after the probe is connected to the oscilloscope, it should be viewed as a whole system, whether evaluating the system bandwidth or rise time. In addition, for an oscilloscope with a 200M bandwidth, the amplitude attenuation of a 200M sine wave is actually less than 3dB, and there is a certain margin for this. After the probe is connected, the amplitude attenuation of a sine wave within 200M still does not exceed 3dB. In order to obtain more accurate measurement results, an oscilloscope with a specified bandwidth 3 to 5 times higher than the highest frequency waveform of the measured signal should be selected. This is based on this consideration, that is, to reduce or avoid the impact of the frequency response characteristics of the measurement system on the test measurement results.