Probe Specifications Explained

1.Distortion (general specification)

Distortion is any amplitude deviation of the input signal from the expected response or ideal response. In practice, distortion often occurs immediately between fast waveform transitions, manifesting as so-called“ringing”。

Distortion as the final pulse-response level ± measured or specified as a percentage（See Figure1). This specification may also include a time window for distortion, for example: Within the first 30 ns , distortion shall not exceed that of the peak-to-peak value ±3% or 5%。

Figure 1  Relative to 100% Example of distortion in pulse height measurement

When excessive distortion is observed in pulse measurements, and when the distortion is considered to originate from a probe fault, all possible sources must be considered. For example, is the distortion 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 failure to check and properly adjust the compensation function of the voltage probe. A severely overcompensated probe causes a pronounced peak immediately after the pulse edge (see Figure 2).

Figure 2  Distortion caused by an overcompensated probe

2. Accuracy (general specification)

For voltage-sensing probes, accuracy generally refers to the probe's DC signal attenuation. Calculation and measurement of probe accuracy generally should include the oscilloscope input resistance. Therefore, the probe accuracy specification is correct or applicable only when the probe is used with an oscilloscope having the assumed input resistance. Examples of accuracy specifications are: in 3% the range 10X ( for 1 megohm ±2% oscilloscope input )

For current-sensing probes, the accuracy specification refers to the accuracy of current-to-voltage conversion. This depends on the current transformer turns ratio and the value and accuracy of the terminating resistor. For current probes using a dedicated amplifier, the output is calibrated directly in amperes / per division; the accuracy specification is specified by the attenuator accuracy as a percentage of the current / per division 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 average current and pulse width exceeds the rated ampere-second product, the core will saturate. This core saturation causes the portion of the waveform occurring during saturation to be clipped or suppressed. If the ampere-second product is not exceeded, the probe's signal voltage output will be linear and measurement accuracy is ensured.

4. Attenuation factor (general specification)

All probes have an attenuation factor; some probes may have selectable attenuation factors. Typical attenuation factors are 1X、10X and100X。

The attenuation factor is the degree to which the probe reduces the signal amplitude.1X probe does not reduce or attenuate the signal, whereas 10X probe reduces the signal to the amplitude at the probe tip of 1/10. The probe attenuation factor allows the measurement range of the oscilloscope to be extended. For example,100X probe allows measurement of amplitude higher by 100 times the signal.

1X、10X、100X These names originate from the fact that older oscilloscopes did not automatically sense probe attenuation and adjust the scale factor accordingly. For example,10XThe name reminds you that all amplitude measurement results must be multiplied by 10. The readout system on the current oscilloscope automatically senses the probe attenuation factor and adjusts the scale-factor reading accordingly.

Voltage probe attenuation factors are implemented using resistive voltage-divider technology. As a result, the higher the probe attenuation factor, the higher the input resistance generally is. In addition, the divider effect divides the probe capacitance; the higher the attenuation factor, the lower the effective probe tip capacitance.

5. Bandwidth (general specifications)

All probes have bandwidth.10 MHz The probe has 10 MHz bandwidth,100 MHz The probe has 100 MHz bandwidth. Probe bandwidth refers to the frequency at which the probe response causes the output amplitude to fall to 70.7% (-3 dB) as shown in Figure3.

Figure 3 Bandwidth is the frequency at which the sine-wave amplitude falls to 70.7% (-3 dB) in the response curve

It should also be noted that some probes have a low-frequency bandwidth limit. For example, this applies to AC current probes. Owing to their design,AC current probes cannot pass DC or low-frequency signals; therefore, their bandwidth must be specified using two values, one for low frequency and one for high frequency.

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 causes some degradation in bandwidth performance. For example, using 100 MHz general-purpose probe and 100 MHz oscilloscope results in the bandwidth performance of the measurement system being slightly lower than 100 MHz. To avoid uncertainty in overall system bandwidth performance, Tektronix specifies passive voltage probes to provide the specified measurement system bandwidth at the probe tip when used with the specified oscilloscope model.

When selecting an oscilloscope and oscilloscope probe, it is important to recognize that bandwidth affects measurement accuracy in many ways.

In amplitude measurements, as the sine-wave frequency approaches the bandwidth limit, the sine-wave amplitude becomes increasingly attenuated. At the bandwidth limit, the sine-wave amplitude is measured relative to the actual amplitude. 70.7% Therefore, to achieve the maximum amplitude measurement accuracy, it is necessary to select an oscilloscope and probe with a bandwidth several times higher than the highest-frequency waveform planned for measurement.

The same applies to measuring waveform rise time and fall time. Waveform transitions ( such as pulse and square-wave edges ) are composed of high-frequency components. The bandwidth limit attenuates these high-frequency components, causing the displayed transition to be slower than the actual transition speed. To accurately measure rise time and fall time, the measurement system used must have sufficient bandwidth to preserve the high frequencies that make up the waveform rise time and fall time. In the most common case, this is specified using the rise time of the measurement system, rise timeGenerallyshould be faster than the rise time to be measured 4-5 times.

6. Capacitance (general specification)

In general, the probe capacitance specification refers to the capacitance at the probe tip. This is the capacitance of the probe at the test point in the circuit under test or on the device under test. Probe tip capacitance is very important because it affects the way pulses are measured. Low head capacitance minimizes errors in rise-time measurements. In addition, if the pulse duration is less than the probe RC time constant multiplied by five, it will affect the pulse amplitude.

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

7.CMRR(differential probe)

common-mode rejection ratio (CMRR) refers to the ability of a differential probe to reject any signal common to the two test points in a differential measurement. This is a key specification for differential probes and amplifiers, and its formula is:

CMRR = |Ad/Ac|

where:

Ad =Voltage gain of the differential signal.

Ac =Voltage gain of the common-mode signal.

Under ideal conditions,Ad should be very large, while Ac should be equal to 0, therefore CMRR is infinite. In practice,10,000:1 ofCMRR is already considered very good. This means that it will suppress a 5 V common-mode input signal, causing it to appear at the output as 0.5 millivolts. This rejection is very important when measuring differential signals in the presence of noise.

Because CMRR decreases as frequency increases; therefore, the frequency at which CMRR is specified is as important as the CMRR value. At high frequencies, a differential probe with high CMRR is better than one with the same CMRR at low frequencies.

VIII. Decay Time Constant (Current Probe)

The decay time constant specification indicates the current probe's pulse capability. This time constant is the secondary inductance (probe coil)divided by the termination resistance. The decay time constant is sometimes called the probe's L/R ratio.L/R ratio; the larger it is, the longer the current pulse that can be represented without significant decay or droop in amplitude.L/R ratio; the smaller it is, the longer the pulse will be seen to decay to zero before the pulse actually completes.

IX. DC (Current Probe)

DC reduces the magnetic permeability of the current-probe coil core. The decrease in permeability causes the coil inductance and L/R The time constant decreases, which in turn degrades low-frequency coupling performance and leads to loss of low-frequency current measurement response. Some AC current probes provide current compensation options that can cancel DC effect.

10. Current rating derating with frequency (current probe)

Current probe specifications should include an amplitude and frequency rating derating curve that relates magnetic core saturation to increasing frequency. The effect of increasing frequency on magnetic core saturation is that, as the waveform frequency or amplitude increases, the amplitude peaks of a waveform whose average current is zero amperes are clipped.

11. Insertion impedance (current probe)

Insertion impedance is the impedance from the current probe coil ( secondary ) referred to the current-carrying conductor under test (the primary) impedance. Generally, the impedance reflected by a current probe can be in the range of a few milliohms, with little effect on circuits with impedance of 25 ohms and above.

12. Input capacitance (general specification)

The probe capacitance measured at the probe tip.

13. Input resistance (general specification)

The input resistance of the probe is at zero hertz (DC) the impedance when the probe is placed on the test point.

14. Maximum rated input current (current probe)

The maximum rated input current is the total current that the probe can accept while still achieving specified performance (DC plus peak AC). In AC current measurement, the peak-to-peak rating must be derated with frequency to calculate the maximum total input current.

15. Maximum rated peak pulse current (current probe)

This rating must not be exceeded; it accounts for magnetic-core saturation and secondary voltage buildup that may damage the equipment. The maximum rated peak pulse current is usually specified as an ampere-second product.

16.Maximum rated voltage (general specification)

Voltages approaching the probe maximum rating should be avoided. The maximum rated voltage depends on the rated breakdown voltage of the probe body or of the probe device at the measurement point.

17. Propagation delay (general specification)

Each probe introduces a small amount of time delay or phase shift that varies with signal frequency. Propagation delay is a function of the probe devices and of the time required for the signal to travel through these devices from the probe tip to the oscilloscope connector.

Under normal conditions, the most significant shift is caused by the probe cable. For example,42 the inch-long dedicated probe cable section has 5 ns of signal delay. For 1 MHz signals,5 ns Delay causes a phase shift of two degrees. The longer the cable, the longer the corresponding signal delay.

Propagation delay is usually a problem only when making comparative measurements between two or more waveforms. For example, when measuring the time difference between two waveforms, matched probes should be used to acquire the waveforms so that each signal experiences the same propagation delay through its probe.

Another example is the combined use of a voltage probe and a current probe for power measurement. Because voltage probes and current probes have significantly different constructions, their propagation delays also differ. Whether these delays affect the power measurement depends on the frequency of the waveform under test. For Hz signals and kHz signals, the delay difference is generally not significant. However, for MHz signals, the delay difference can have a significant effect.

18. Rise Time (General Specification)

The probe's response to a step function 10 - 90% indicates the fast measurement transition that the probe can transmit from the tip to the oscilloscope input. To accurately measure rise time and fall time on a pulse, the rise time of the measurement system ( the oscilloscope and probe combined ) should be faster than the fast test transition to be measured by 3-5 times.

19. Tangential Noise (Active Probe)

Tangential noise is a method used for active probes to specify the noise generated by the probe. The tangential noise coefficient is approximately RMS Twice the noise.

20. Temperature range (general specifications)

Due to heating caused by energy induced in the coil's magnetic shield layer, a current probe has a maximum operating temperature. Increased temperature leads to increased losses. Therefore, a current probe has a maximum amplitude versus frequency derating curve.

Attenuator voltage probe ( i.e., 10X, 100X etc. ) May cause accuracy changes due to temperature variation.

21. Threshold voltage (logic probe)

Logic probes measure and analyze signals differently from other oscilloscope probes. A logic probe does not measure analog detail; it detects logic threshold levels. When you use a logic probe to connect a mixed-signal oscilloscope to a digital circuit, you care only about the logic state of the signal. In this case, there are only two logic levels of interest. When the input exceeds the threshold voltage (Vth) the level is called“high”or“1”; conversely, below Vth the level is“low”or“0”. When sampling the input, a mixed-signal oscilloscope stores a“1”or a“0”，depending on the signal level relative to the voltage threshold.

A logic probe can capture a large number of signals at once, which is what distinguishes it from other oscilloscope probes. These digital acquisition probes connect to the device under test; at the probe's internal comparator, the input voltage and threshold voltage (Vth) are compared to determine the logic state of the signal (1 or 0). The threshold value is set by the user, from TTL levels to CMOS、ECL and user-defined thresholds.

Reprinted from Tektronix