The impact of Tektronix oscilloscope probe selection on measurement

Common mistakes to avoid when probing with a Tektronix oscilloscope. Let’s take a look at them together!!!

Mistake 1 - Not calibrating the probe

The probes are calibrated before delivery, but they are not calibrated to the front end of the oscilloscope. If they are not calibrated at the oscilloscope input, then no measurement results can be obtained.

Oscilloscope Active Probes

If your active probe is not calibrated for your oscilloscope, you will see differences in vertical voltage measurements and rising edge timing (and possibly some distortion). Most oscilloscopes have a reference or auxiliary output capability and come with a guide to walk you through probe calibration.

Mistake 2 - Increasing Probe Loading Effects

Whenever you connect a probe to an oscilloscope and touch it to your device, the probe becomes part of the circuit. The resistive, capacitive, and inductive loading effects that the probe applies to your device affect the signal you see on the oscilloscope screen. These loading effects can change the operation of the circuit under test. Understanding these loading effects can help you avoid choosing the wrong probe for a particular circuit or system.

Mistake 3 - Not Getting the Most Out of Your Differential Probe

Many people think that differential probes should only be used when probing differential signals. Did you know that you can also use differential probes when probing single-ended signals? This will save you a lot of time and money, and improve the accuracy of your measurements. Get the best signal fidelity possible by getting the most out of your differential probes.

A differential probe can make the same measurements as a single-ended probe, and because the differential probe has common-mode rejection on both inputs, the differential measurement results are much less noisy. This allows you to see a better representation of the device under test signal without being misled by random noise added by probing.

Mistake 4 - Choosing the wrong current probe

High current and low current measurements require different details to be captured. You need to know which current probe is right for your application and what troubles you might run into if you use the wrong probe.

High current measurement:

If you are using a clamp probe to measure high current (10A - 3000A), your device must be small enough for the clamp probe to clamp onto it. If the device is too large for the clamp probe to clamp onto, then engineers may try to add extra wires to the probe jaws, but this will change the characteristics of the device under test. A better approach is to use the right tools.

The best solution is to use a high current probe with a flexible loop probe tip. You can wrap this flexible loop around any device. This type of probe is called a Rogowski coil. It allows you to probe the device without adding unknown characteristics, so the measurement results maintain a high signal integrity. They also allow you to measure high currents from mA levels to hundreds of kA. Note that they only measure AC current, so the DC component will be isolated. They are also less sensitive than some current probes. This is usually not a problem for high current measurements. But when measuring low currents, sensitivity and the ability to see the DC component become important. Remember that what works for one measurement may not work for another.

Mistake 5 - Incorrectly handling DC bias during ripple and noise measurements

Ripple and noise on a DC power supply are formed by a small AC signal on a larger DC signal. When the DC offset is large, you may need to use a larger voltage-per-division setting on your oscilloscope to display the signal on the screen. Doing so reduces the sensitivity of the measurement and increases the noise compared to the small AC signal. This means you are not getting an accurate representation of the AC portion of the signal.

If you use DC blocking capacitors to solve this problem, you will inevitably block some low-frequency AC content, making it impossible for you to observe the changes in the signal as it passes through the components on the device.

Using a power supply probe with a large offset capability allows you to center the waveform on the screen without removing the DC offset. This allows the entire waveform to be displayed on the screen while keeping the vertical scale small and zoomed in. With these settings, you can see details of transients, ripple, and noise.

Error 6 - Unknown bandwidth limitations

When making critical measurements, it is important to select a probe with adequate bandwidth. Insufficient bandwidth can distort your signals and make it difficult for you to make informed engineering test or design decisions.

Mistake 7 - Masking noise effects

Probe and oscilloscope noise can cause the device under test to appear noisier. Selecting a probe with the right attenuation ratio for your application will reduce the noise added by the probe and oscilloscope. As a result, you get a more accurate signal and a clearer view of the device under test.

Many probe manufacturers describe probe noise as equivalent input noise (EIN) and express it in units of Vrms. Higher attenuation ratios allow you to measure larger signals, but the drawback is that the oscilloscope will detect these ratios and amplify both the signal and its noise.

Mistake 1 - Not calibrating the probe

The probes are calibrated before delivery, but they are not calibrated to the front end of the oscilloscope. If they are not calibrated at the oscilloscope input, then no measurement results can be obtained.

Oscilloscope Active Probes

If your active probe is not calibrated for your oscilloscope, you will see differences in vertical voltage measurements and rising edge timing (and possibly some distortion). Most oscilloscopes have a reference or auxiliary output capability and come with a guide to walk you through probe calibration.

Mistake 2 - Increasing Probe Loading Effects

Whenever you connect a probe to an oscilloscope and touch it to your device, the probe becomes part of the circuit. The resistive, capacitive, and inductive loading effects that the probe applies to your device affect the signal you see on the oscilloscope screen. These loading effects can change the operation of the circuit under test. Understanding these loading effects can help you avoid choosing the wrong probe for a particular circuit or system.

Mistake 3 - Not Getting the Most Out of Your Differential Probe

Many people think that differential probes should only be used when probing differential signals. Did you know that you can also use differential probes when probing single-ended signals? This will save you a lot of time and money, and improve the accuracy of your measurements. Get the best signal fidelity possible by getting the most out of your differential probes.

A differential probe can make the same measurements as a single-ended probe, and because the differential probe has common-mode rejection on both inputs, the differential measurement results are much less noisy. This allows you to see a better representation of the device under test signal without being misled by random noise added by probing.

Mistake 4 - Choosing the wrong current probe

High current and low current measurements require different details to be captured. You need to know which current probe is right for your application and what troubles you might run into if you use the wrong probe.

High current measurement:

If you are using a clamp probe to measure high current (10A - 3000A), your device must be small enough for the clamp probe to clamp onto it. If the device is too large for the clamp probe to clamp onto, then engineers may try to add extra wires to the probe jaws, but this will change the characteristics of the device under test. A better approach is to use the right tools.

The best solution is to use a high current probe with a flexible loop probe tip. You can wrap this flexible loop around any device. This type of probe is called a Rogowski coil. It allows you to probe the device without adding unknown characteristics, so the measurement results maintain a high signal integrity. They also allow you to measure high currents from mA levels to hundreds of kA. Note that they only measure AC current, so the DC component will be isolated. They are also less sensitive than some current probes. This is usually not a problem for high current measurements. But when measuring low currents, sensitivity and the ability to see the DC component become important. Remember that what works for one measurement may not work for another.

Mistake 5 - Incorrectly handling DC bias during ripple and noise measurements

Ripple and noise on a DC power supply are formed by a small AC signal on a larger DC signal. When the DC offset is large, you may need to use a larger voltage-per-division setting on your oscilloscope to display the signal on the screen. Doing so reduces the sensitivity of the measurement and increases the noise compared to the small AC signal. This means you are not getting an accurate representation of the AC portion of the signal.

If you use DC blocking capacitors to solve this problem, you will inevitably block some low-frequency AC content, making it impossible for you to observe the changes in the signal as it passes through the components on the device.

Using a power supply probe with a large offset capability allows you to center the waveform on the screen without removing the DC offset. This allows the entire waveform to be displayed on the screen while keeping the vertical scale small and zoomed in. With these settings, you can see details of transients, ripple, and noise.

Error 6 - Unknown bandwidth limitations

When making critical measurements, it is important to select a probe with adequate bandwidth. Insufficient bandwidth can distort your signals and make it difficult for you to make informed engineering test or design decisions.

Mistake 7 - Masking noise effects

Probe and oscilloscope noise can cause the device under test to appear noisier. Selecting a probe with the right attenuation ratio for your application will reduce the noise added by the probe and oscilloscope. As a result, you get a more accurate signal and a clearer view of the device under test.