Application of Tektronix Oscilloscope in Dynamic Parameter/Double Pulse Test of Power Devices

Oscilloscope issues have as much impact on probe selection as signal source issues. If the probe and oscilloscope do not match, the signal fidelity on the oscilloscope side of the probe will be compromised. So, how can you make your oscilloscope probe and oscilloscope more compatible? Antai Test recommends that you pay attention to the following indicators when selecting:

1. Bandwidth and rise time

It is important to realize that an oscilloscope and its probe work together as a measurement system. Therefore, the oscilloscope bandwidth and rise time specifications used should be greater than or equal to the probe specifications used to make it sufficient to examine the signal.

In general, the bandwidth and rise time interaction between probes and oscilloscopes is very complex. Because of this complexity, most oscilloscope manufacturers specify the oscilloscope bandwidth and rise time for the probe tip in a specific probe model for use with a specific oscilloscope. To ensure adequate oscilloscope system bandwidth and rise time for the signals you plan to examine, it is best to follow the oscilloscope manufacturer's probe recommendations.

2. Input resistance and capacitance

All oscilloscopes have input resistance and input capacitance. To maximize signal transmission, the oscilloscope's input resistance and capacitance must match the resistance and capacitance of the probe output, as shown below:

More specifically, a 50 ohm oscilloscope input requires a 50 ohm probe, and a 1 megohm oscilloscope input requires a 1 megohm probe. A 1 megohm oscilloscope can also be used with a 50 ohm probe when using the appropriate 50 ohm adapter.

The probe and oscilloscope capacitance must also be matched, which is achieved by selecting a probe designed for use with a specific oscilloscope model. In addition, many probes have compensation adjustments that allow precise matching by compensating for small capacitance changes. The first thing that should be done when the probe is connected to the oscilloscope is to adjust the probe's compensation. Failure to properly match the probe to the oscilloscope (both by selecting the appropriate probe and making the appropriate compensation adjustments) can result in significant measurement errors.

3. Sensitivity

The vertical sensitivity range of an oscilloscope determines the overall dynamic range of signal amplitude measurements. For example, an oscilloscope with a vertical display range of 10 divisions and a sensitivity range of 1 mV/division to 10 V/division has an actual vertical dynamic range of approximately 0.1 mV to 100 V. If you plan to measure a variety of signal amplitudes ranging from 0.05 mV to 150 V, then this oscilloscope's dynamic range will not meet your requirements at both the low and high ends.

However, this disadvantage can be compensated for by selecting the appropriate probe for the various signals to be processed.

For high amplitude signals, the dynamic range of the oscilloscope can be extended upward by using an attenuator probe. For example, a 10X probe effectively shifts the sensitivity range of the oscilloscope upward by ten, and the sensitivity range of the oscilloscope will be 1 mV/div-100 V/div. This not only provides enough range for a 150V signal, but also provides a top oscilloscope display range of 1000V. However, before connecting any probe to a signal, be sure that the signal will not exceed the maximum voltage capability of the probe.

Notice

Always observe the maximum specified voltage capabilities of the probe. Connecting the probe to a voltage that exceeds its capabilities may result in personal injury and equipment damage.

For low amplitude signals, the oscilloscope range can be extended to a lower sensitivity by using a probe amplifier system. This is usually a differential amplifier that can provide, for example, 10 mV/div sensitivity.

These probe amplifier systems are highly specialized and are designed to match specific oscilloscope models. As a result, when selecting an oscilloscope, be sure to check the manufacturer's recommended accessories for differential probe systems for small signal applications.

Notice

Differential probe systems often contain sensitive devices that can be damaged by overstress, including electrostatic discharge. To avoid damaging the probe system, always follow the manufacturer's recommendations and observe all precautions.

4. Reading function

Most modern oscilloscopes provide on-screen readouts of the vertical and horizontal sensitivity settings (volts/div and seconds/div). These oscilloscopes also typically provide probe sensing and readout processing so that the readouts correctly track the type of probe being used. For example, if a 10X probe is used, the oscilloscope should roughly reflect the sensitivity by adjusting the vertical readout by a factor of 10X. If a current probe is used, the vertical readout will change from volts/div to amps/div to reflect the correct units of measure.

To take advantage of these readout features, you should use probes that are compatible with the oscilloscope's readout system. You should also follow the manufacturer's recommendations for using probes with a specific oscilloscope. This is especially important with newer oscilloscopes, which may have advanced readout features that many general-purpose or commercial probes do not fully support.

How to match the corresponding probe?

There is really no “right” probe selection for any given application, but only “right” oscilloscope/probe combination options, which are determined first and foremost by the defined signal measurement requirements, including:

• Signal type (voltage, current, optical, etc.)

• Signal frequency content (bandwidth issue)

• Signal rise time

• Source impedance (resistance and capacitance)

• Signal amplitude (maximum, minimum)

• Test point shape (leaded device, surface mount package, etc.)

By considering the questions above and filling in specific application information, you can specify an oscilloscope and a variety of compatible probes to meet all of your application needs.