Understand oscilloscope probes. Not all probes are suitable for all these indicators.

This article lists the specifications in alphabetical order; not all of them apply to all probes. For example, the insertion impedance specification only applies to current probes; other specifications, such as bandwidth, are universal and apply to all probes. I hope this article can help you better understand oscilloscope probes.

1. Distortion (general indicator)

Aberration is any deviation in amplitude from the expected or ideal response of an input signal. In practice, aberration often occurs immediately between fast waveform transitions, where it manifests as so-called "ringing".

The distortion is measured or specified as a percentage of the final impulse response level. This specification may also include the time window of the distortion, for example:

In the first 30ns, aberrations should not exceed ±3% or 5% peak-to-peak. When you see excessive aberrations on a pulse measurement, be sure to consider all possible sources before assuming that the aberrations are the source of probe failure. For example, is the aberration actually part of the signal source? Or is it caused by the probe grounding technique?

One of the most common sources of observed distortion is neglecting to check and properly adjust the compensation of the voltage probe. A severely overcompensated probe can cause noticeable peaks immediately after the pulse edge.

2. Accuracy (general indicator)

For voltage sensing probes, accuracy is generally referred to as the probe's attenuation of a DC signal. Calculations and measurements of probe accuracy should generally include the oscilloscope's input resistance. Therefore, the probe accuracy specification is correct or applicable only when the probe is used with an oscilloscope that has an assumed input resistance. Examples of accuracy specifications are as follows:

10X over 3% range (for 1M ohm ±2% oscilloscope input) For current sensing probes, accuracy specifications refer to the accuracy of the current to voltage conversion. This depends on the value and accuracy of the current transformer winding ratio and the terminating resistors. For current probes using dedicated amplifiers where the output is calibrated directly in amps/div, accuracy specifications are specified as attenuator accuracy as a percentage of the current/div setting.

3. Ampere-second product (current probe)

For current probes, the ampere-second product specifies the energy handling capability of the current transformer core. If the product of the average current and the pulse width exceeds the rated ampere-second product, the core will saturate. This core saturation causes the portion of the waveform that occurs during the saturation process to be clipped or suppressed. If the ampere-second product is not exceeded, the signal voltage output of the probe will be linear and measurement accuracy is maintained.

4. Attenuation coefficient (general indicator)

All probes have an attenuation factor, and some probes may have an attenuation factor that can be selected. Typical attenuation factors are 1X, 10X, and 100X. The attenuation factor is how much the probe reduces the amplitude of the signal. A 1X probe will not reduce or attenuate the signal, while a 10X probe will reduce the signal to 1/10 of the amplitude at the probe tip. The probe attenuation factor allows the oscilloscope's measurement range to be extended. For example, a 100X probe allows the measurement of signals that are 100 times higher in amplitude.

1X, 10X, 100X These names come from the days when oscilloscopes did not automatically sense probe attenuation and adjust the scale factor accordingly. For example, the 10X name reminds you that all amplitude measurements need to be multiplied by 10. The readout system on current oscilloscopes automatically senses the probe attenuation factor and adjusts the scale factor readout accordingly. Voltage probe attenuation factors are implemented using a resistive voltage shunt technique. As a result, probes with higher attenuation factors generally have higher input resistance. Additionally, the shunt effect divides the probe capacitance, and higher attenuation factors effectively represent lower probe tip capacitance.

5. Bandwidth (general indicator)

All probes have a bandwidth. A 10MHz probe has a bandwidth of 10MHz, and a 100MHz probe has a bandwidth of 100MHz. The bandwidth of a probe is the frequency at which the probe response causes the output amplitude to drop to 70.7% (-3dB).

It should also be noted that some probes also have low-frequency bandwidth limitations. This applies to AC current probes, for example. Due to their design, AC current probes cannot pass DC or low-frequency signals, so their bandwidth must be specified using two values, one for low frequencies and one for high frequencies.

For oscilloscope measurements, the real concern is the combined total bandwidth of the oscilloscope and probe. This system performance ultimately determines the measurement capability. Unfortunately, connecting a probe to an oscilloscope results in some degradation in bandwidth performance. For example, using a 100MHz general-purpose probe with a 100MHz oscilloscope results in a measurement system with bandwidth performance slightly less than 100MHz. To avoid overall system bandwidth performance uncertainty, Tektronix specifies passive voltage probes to provide a specified measurement system bandwidth at the probe tip when used with a specified oscilloscope model.

When selecting an oscilloscope and oscilloscope probes, realize that bandwidth affects measurement accuracy in many ways.

In amplitude measurements, the amplitude of a sine wave becomes increasingly attenuated as its frequency approaches the bandwidth limit. At the bandwidth limit, the amplitude of a sine wave is measured as 70.7% of its actual amplitude. Therefore, to achieve maximum amplitude measurement accuracy, you must select an oscilloscope and probe with a bandwidth several times higher than the highest frequency waveform you plan to measure.

The same applies to measuring waveform rise and fall times. Waveform transitions, such as pulses and square wave edges, are made up of high-frequency components. Bandwidth limitations attenuate these high-frequency components, causing transitions to appear slower than they actually are. To accurately measure rise and fall times, the measurement system used must have sufficient bandwidth to preserve the high frequencies that make up the rise and fall times of the waveform. Most commonly, this is dictated by the rise time of the measurement system, which should generally be 4-5 times faster than the rise time to be measured.

6. Capacitance (general indicator)

Generally, probe capacitance specifications refer to the capacitance at the probe tip. This is the capacitance of the probe at the circuit test point or device under test. Probe tip capacitance is important because it affects the way pulses are measured. Low tip capacitance minimizes errors in making rise time measurements. In addition, if the duration of the pulse is less than five times the probe RC time constant, it will affect the amplitude of the pulse.

The probe also presents a capacitance to the oscilloscope input, and this probe capacitance should match the oscilloscope capacitance. For 10X and 100X probes, this capacitance is called the compensation range, which is different from the tip capacitance. For probe matching, the oscilloscope's input capacitance should be within the probe's compensation range.

7. CMRR (differential probe)

Common-mode rejection ratio (CMRR) is the ability of a differential probe to reject any signal common to both test points in a differential measurement. This is a key specification for differential probes and amplifiers and is given by:

CMRR = |Ad/Ac|

Where: Ad = voltage gain of the differential signal, Ac = voltage gain of the common-mode signal.

Ideally, Ad would be large and Ac would be equal to 0, so the CMRR is infinite. In practice, a CMRR of 10,000:1 is considered very good. This means that a 5V common-mode input signal will be suppressed so that it appears as 0.5 millivolts at the output. This suppression is very important for measuring differential signals in the presence of noise.

Since CMRR decreases as frequency increases, the frequency at which CMRR is specified is as important as the CMRR value. A differential probe with high CMRR at high frequencies is better than a differential probe with the same CMRR at low frequencies.

8. Decay time constant (current probe)

The decay time constant specification indicates the ability of a current probe to sustain a pulse. This time constant is the secondary inductance (probe coil) divided by the termination resistance. The decay time constant is sometimes called the probe L/R ratio. The larger the L/R ratio, the longer the current pulse can be represented without a noticeable decay or drop in amplitude. The smaller the L/R ratio, the longer it will take for the pulse to decay to zero before it actually completes.

9. DC (current probe)

DC reduces the permeability of the current probe coil core. The reduced permeability results in reduced coil inductance and L/R time constant, which in turn reduces low-frequency coupling performance and causes loss of low-frequency current measurement response. Some AC current probes offer current compensation options that can null out the effects of DC.

10. Frequency current rating reduction (current probe)

Current probe specifications should include an amplitude vs. frequency derating curve that relates core saturation to increasing frequency. The effect of increasing frequency on core saturation is that the peak amplitude of a waveform with an average current of zero amperes is clipped as the waveform frequency or amplitude is increased.

11. Insertion impedance (current probe)

Insertion impedance is the impedance transformed from the coil of the current probe (the secondary) to the conductor carrying the current being measured (the primary). Generally speaking, the impedance reflected by a current probe can be in the range of a few milliohms, which has little effect on circuits with impedances of 25 ohms and above.