Common errors in oscilloscope probe selection (previously)2 of them)

In oscilloscope measurement, the choice of probe directly affects the accuracy of the results. The probe is the combination of the oscilloscope and the device under test（The performance of the "bridge" between DUT directly determines the accuracy, completeness, and reliability of oscilloscope measurement results. But many engineers tend to overlook its importance. The following provides a detailed analysis of the first two common errors and their avoidance methods:

1、 The1 error: Failure to understand the key technical specifications of oscilloscope probes

If the core technical specifications of the probe are not clear, it may not be able to match its own measurement needs, which may lead to selection deviation.

·The core impact of key indicators：
Among various indicators, the relationship between bandwidth and noise is particularly important. Noise is uniformly distributed at all frequencies,The higher the probe bandwidth, the more noise is introduced(Due to a wider frequency range covered).
For example, using high bandwidth probes that far exceed demand not only increases costs (high bandwidth probes are usually more expensive due to their higher technical complexity), but also introduces additional noise that interferes with measurement results; Meanwhile, the operational complexity of high bandwidth probes may be higher, increasing workload.

·Key principles for selection：
Different probes have their own advantages and disadvantages (such as passive probes with low cost but limited bandwidth, and active probes with high bandwidth but easily affected by load), which need to be selected based on specific testing scenarios. Instead of dwelling on a lengthy list of metrics in technical documents, it's better to focus on the matching degree between core metrics (such as bandwidth, load effect, noise level) and one's own needs ——Understanding the actual impact of indicators on measurement can greatly simplify the selection process.

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2、 The2 errors: The selected probe bandwidth is not appropriate

Bandwidth is the core indicator of a probe, and inappropriate bandwidth can lead to loss of signal details or introduction of unnecessary noise.

1. Definition of bandwidth:3dB point
The essence of probe bandwidth is The "3dB point" refers to the frequency at which the probe output decreases by 3dB relative to the nominal response.
Example: Using1: When measuring a low-frequency sine wave of 1Vpp with a probe, the oscilloscope displays a signal that is consistent with the actual value; As the frequency increases, the probe output gradually attenuates until it displays 0.7Vpp (approximately 70.7% of the original signal), at which point the frequency is the 3dB point of the probe (due to the 3dB attenuation of 0.7Vpp relative to 1Vpp).

2. Calculation of signal bandwidth
Before selecting the probe bandwidth, it is necessary to first determine the bandwidth of the measured signal. The calculation formula is as follows:

oIf the signal rise time is Measurement of "10%~90% threshold":Signal bandwidth（BW=rise time ÷ 0.35

oIf the signal rise time is Measurement of "20%~80% threshold":Signal bandwidth（BW=rise time ÷ 0.22

3. Experience in selecting probe bandwidth
After calculating the signal bandwidth, the probe bandwidth can be selected based on the following experience:

oSimulation application: The probe bandwidth should be higher than the fastest sine wave frequency in the signal3 times;

oDigital applications: Probe bandwidth should be higher than the maximum digital clock rate5 times.

4. The relationship between bandwidth and signal details
The higher the bandwidth, the more signal harmonics are captured, and the richer the waveform details:

oLow bandwidth probes may lose high-order harmonics, resulting in a slower rising edge and smoother corners of the waveform (such as when only capturing first-order harmonics);

oHigh bandwidth probes can capture more harmonics such as first, third, and fifth order, making the rising edge of the waveform steeper and the corners sharper, restoring the details of the top and bottom of the signal.

However, it should be noted that excessive bandwidth will introduce additional noise, and higher bandwidth is not necessarily better. It needs to be matched with the signal bandwidth.

In summary, the key to avoiding the first two errors is to clarify the key indicators of the probe (especially the relationship between bandwidth and noise), select the matching probe bandwidth by calculating the signal bandwidth, and balance signal details with noise interference.