“Electronic products are increasingly complex, and the market places higher demands on the bandwidth and accuracy of oscilloscopes. This is not a problem that can be solved by purchasing a high-end oscilloscope. It also requires the right probe and the right test method. This article starts from the principle of the probe, and describes how to select and use the probe correctly.
electronic products are increasingly complex, and the market places higher demands on the bandwidth and accuracy of oscilloscopes. This is not a problem that can be solved by purchasing a high-end oscilloscope. It also requires the right probe and the right test method. This article starts from the principle of the probe, and describes how to select and use the probe correctly.
Getting to know oscilloscope probes
The signal under test cannot be directly connected to the oscilloscope, which requires a device to establish an electrical connection between the test point and the oscilloscope. Depending on the needs, this device can be a wire or a more complex circuit. The device responsible for connecting the test point to the oscilloscope is the oscilloscope probe. So the oscilloscope probe is crucial, without the probe the oscilloscope will not be able to make measurements.
The above figure is a schematic diagram of the oscilloscope probe measurement. From the above figure, it can be seen that the oscilloscope generally has three typical parts, the probe head, the probe cable and the probe compensation device. Among them, the function of the probe head is to directly contact the test point, so as to generate an electrical connection with the system under test, and finally obtain the signal to be measured. The function of the probe cable is to prevent the oscilloscope and the probe head from interfering with each other. The probe head can be moved at will without moving the oscilloscope, so that it can easily contact the test point. The final probe compensation equipment is mainly to eliminate the negative influence of the probe cable as much as possible, and to maintain the measurement accuracy of the probe to a certain extent.
It can be seen from the basic structure of the probe that the probe cannot be regarded as a transparent device, and there must be many performance limitations. For example, the probe cable and compensation equipment determine the bandwidth of the probe, and the size of the device in the probe also determines. The input voltage of the probe. So the probe will have some basic parameters. Here is a summary:
1. Attenuation coefficient
Attenuation coefficient, a parameter that all probes have, refers to the degree to which the probe reduces the signal amplitude. Some probes may have selectable attenuation factors. Typical attenuation coefficients are 1×, 10× and 100×. A 1× probe means that the signal will not be attenuated. 10× means that the signal will be attenuated by 10 times and then input to the oscilloscope. The origin of the names 1× and 10× is that the previous oscilloscopes did not have the ability to automatically identify the probe attenuation coefficient and adjust automatically, so the names of 1× and 10× are needed to remind the tester to remember to multiply the measured results by corresponding multiples.
Bandwidth is also a necessary parameter for the probe, which refers to the frequency point at which the probe causes signal attenuation by -3dB. As shown below:
For example, a 100MHz probe has a 100MHz bandwidth, and a 500MHz probe has a 500MHz bandwidth. Some probes will also have a low frequency bandwidth frequency, such as some AC probes, which cannot transmit DC signals, it will have a bandwidth parameter in the low frequency band. It is worth mentioning that the bandwidth refers to the -3dB spectrum. At this time, the measured amplitude of the signal is only 70.7% of the real signal, so the measurer needs to consider whether the result is acceptable, otherwise a higher bandwidth needs to be used. probe.
3. Rise time
Bandwidth refers to the measurement of a single sine wave. If the measurement is a square wave, the rise time of the probe needs to be considered. required time. This parameter is actually used to evaluate the margin of error. For example, the rise time of the rising edge of the square wave signal under test is 10ns, after a probe with a rise time of 3.5ns, the final output rise time is roughly:
Rise time degrades by 5.9%.
If a 0.7ns probe is used instead, the rise time of the output is:
The rise time is only degraded by 0.24%. Therefore, when measuring, it is necessary to try to choose a probe whose rise time is much smaller than the rise time of the measured signal, generally 3 to 5 times.
4. Maximum input voltage
The maximum input voltage is the maximum rated voltage that the probe can input. The maximum input voltage depends on the breakdown voltage rating of the probe body and the probe’s internal components. Generally, this item will be given by some safety regulations instead of a single voltage. For example, the maximum input voltage of a passive probe of 10× is 300VRMS CAT II. Among them, CAT II refers to a type of test scenario, and 300VRMS CAT II refers to the maximum voltage that can be measured in this type of test scenario. And this voltage is not a constant value. Rather, it varies with frequency. Generally, the probe will give its own voltage rating curve, as shown in the following figure:
5. Input capacitance
Input capacitance is the capacitance measured from the probe tip end of the probe. For active probes, this capacitance includes the parasitic capacitance of the probe tip and capacitance in the probe’s internal circuitry. For some passive probes, also include the parasitic capacitance of the probe cable and the capacitance of the oscilloscope itself. The smaller the capacitance value, the higher the frequency that the probe can measure.
6. Input resistance
The input resistance of the probe is the resistance measured at the probe head end of the probe, which is measured at DC. For passive probes, the larger the attenuation ratio, the higher the input resistance of the probe.
7. Oscilloscope compensation range
Most passive probes are a general-purpose device that can vary from oscilloscope to oscilloscope and even from channel to channel on the same oscilloscope. In order to be compatible with these differences, the probe comes with a compensation network to compensate for the differences between different oscilloscopes. Under-compensation or over-compensation can lead to erroneous measurement results. And this compensation network must have a range that can be adjusted, and this range is the compensation range of the oscilloscope. The oscilloscope compensation range of a general passive probe is 10~35pF.
8. Cable length
Each probe must have a length of probe cable for easier measurement. And this cable will cause a certain signal propagation delay. For example, a probe cable of about 1m will have a delay of about 5ns. For a 10MHz signal, this results in a delay of about 20°. The longer the cable, the longer the phase signal delay will result. In general, this delay will not affect the measurement, because within a certain bandwidth range, this delay will not change with the signal frequency, so it will not cause distortion of the group delay. Propagation delay has an effect only when more than two channels are measured together, especially when voltage probes are used for power measurement together with current probes, the delay between different probes can have a large impact. Therefore, it is necessary to calculate the approximate delay according to the length of the cable before measurement. If the delay is too large, you need to use the delay calibration function in the oscilloscope.
Common probe types
Common probe classifications are shown in the following figure:
Figure 4 Classification of oscilloscope probes
Among them, passive probes commonly have three specifications: 1×, 10×, and 100×. In general, 1× probes are mostly low-bandwidth probes, which are suitable for measuring low-frequency and low-voltage signals. The withstand voltage value of 100× probes is generally higher. , which is suitable for some high-voltage measurement scenarios, while the bandwidth of the 10× probe is generally relatively high, which is suitable for the measurement of higher-speed signals.
Among active probes, high-speed differential probes are suitable for high-speed signal measurement, with high bandwidth and low probe load benefit, but are generally expensive. High-voltage differential probes are generally suitable for testing high-voltage occasions. Compared with passive probes, not only the input voltage is higher, generally above 1000V, but also because the impedance of the two measurement lines to ground is very high, it can be directly tested. For non-grounded measurement, for example, when measuring the mains, the ground wire of the passive probe must be connected to the ground wire of the mains, and only the voltage between L or N and the ground wire can be measured, while the high-voltage differential probe can perform any two measurements. measurement between lines. Current probes are used to measure current. Some current probes can only measure AC, and some can also measure DC.
Precautions for the use of probes
In the use of probes, there are also some issues to consider:
When measuring with a probe, the most important thing is safety. For example, when using a passive probe, the ground wire of the probe and the ground of the oscilloscope are connected together. When the oscilloscope is safely grounded, the probe is safe. But when the oscilloscope is not grounded safely, the ground wire of the probe will have a certain voltage, which will bring danger to the user. The specific situation is shown in Figure 5:
Figure 6 Oscilloscope grounding
Due to the Y capacitor, the original grounding point voltage is not 0V, but half the voltage between L and N, which is 110V, which will harm the human body. Therefore, during the measurement process of the oscilloscope, it is necessary to ensure that the grounding is good, or use an isolation transformer for complete safety isolation, otherwise it may cause electric shock to the user.
And at this time, if the ground wire of the probe is connected to a relatively high voltage, such as the live wire of the mains, it will cause the entire oscilloscope shell to have a high voltage of 220V, which will happen when people touch the oscilloscope again. Direct contact with electricity is very dangerous, which is equivalent to directly inserting your hand into the mains power outlet.
2. Connection sequence
The probe is indirectly grounded through the ground wire of the power cord of the oscilloscope, and the system under test may be a suspended system. To avoid danger, the ground wire must be connected to the ground first, and the oscilloscope and the system under test can be grounded together, and then the probe tip can be connected to the ground. at the test point. When disconnecting the probe, also disconnect the probe first, and then disconnect the ground wire.
When using a probe, the matching problem with the oscilloscope needs to be considered. Common passive probes generally require the impedance range of the oscilloscope to be 1MΩ impedance. Some active probes require 50Ω impedance. Before using the probe, read the description of the corresponding impedance in the manual to select the corresponding oscilloscope gear to match it.
When a probe is used for measurement, the bandwidth of the entire system is composed of the probe bandwidth and the oscilloscope bandwidth. If any one of the bandwidths is insufficient, the final measurement result will not meet the requirements. For example, if a 100M probe is selected to match a 500M oscilloscope, the bandwidth is only about 100M at most, so the bandwidth advantage of the 500M oscilloscope cannot be used. Therefore, a reasonable choice of oscilloscope and probe can make the test bandwidth meet the requirements.
When choosing a probe, you also need to consider the voltage of the test signal. On the one hand, you must ensure that the test voltage is not higher than the maximum input voltage of the probe, and on the other hand, you must also ensure that the signal output by the probe is within the measurable range of the oscilloscope. This requires the tester to select the probe according to the voltage of the test signal. For example, if you want to measure a voltage signal of 600Vpp, you need to choose a probe with a withstand voltage value of more than 600Vpp. Generally, the maximum measurement voltage of an oscilloscope is 80V, so you need to choose a probe with an attenuation ratio greater than 8. However, if the signal to be measured is a voltage signal of 10mVpp, a non-attenuated probe needs to be used for measurement.
6. The influence of the ground wire
Traditionally, the grounding method of the oscilloscope is the long grounding clip. This grounding method is indeed a simple and convenient grounding method, but it is not a rigorous and accurate grounding method.
Figure 8 Schematic diagram of grounding clamp
Since the ground clip line is relatively long, it will form a parasitic inductance Lgnd. As the clip line grows, this inductance will also increase, and this loop inductance will resonate with the input capacitance Cin of the oscilloscope probe. This causes the amplitude-frequency characteristics of the oscilloscope to become uneven, resulting in inaccurate measurements. The figure below shows the equivalent circuit when ground clips are used.
Figure 9 Equivalent circuit diagram of ground clip
The following figure shows the spectral characteristic curve simulated by this equivalent circuit:
Figure 10 Spectral characteristic curve
It can be seen that at frequencies above 60MHz, the amplitude has produced an overshoot of more than 3dB, and when it reaches about 100M, the overshoot reaches the maximum amplitude. Therefore, if the ground clip is used, the measurement of signals exceeding 60MHz will produce relatively large distortion. The correct way should be to use a ground spring. The ground spring has very little inductance and can greatly increase the bandwidth of the probe.
Figure 11 Schematic diagram of grounding spring
Moreover, the use of springs will also reduce the ground loop area and greatly reduce the radiation interference of space noise.
7. Adjustment of compensation capacitor
For a 10× passive probe, the equivalent model is shown in the figure below.
Figure 12 Probe Equivalent Circuit
Among them, C1 and Cline1 are parasitic capacitances of the oscilloscope and communication cables, which cannot be removed. Because of R2, a low-pass filter is bound to occur, which makes the probe unable to pass high-frequency signals. In order to increase the probe bandwidth, C2 will be added to the probe to compensate.
Calculate the zeros of the system:
The system poles are:
Only by making these two points coincide can the probe bandwidth be flattened. The input capacitance C1 of the oscilloscope and the capacitance Cline1 of the communication cable have a certain range of variation, so an adjustable capacitance Cadj must be added to match. In actual use, the probe bandwidth needs to be adjusted to be flat through this adjustable capacitor. The specific method is to link the probe to the calibration sheet of the oscilloscope, and then adjust the square wave to be flat by adjusting the adjuster on the probe compensation device. The figure below is an example of adjustment. When the waveform on the screen becomes a square wave, it means that the probe adjustment is successful.
Figure 13 Schematic diagram of probe compensation
8. Loading effect of the probe
As mentioned above, the probes all have input capacitance and input resistance, which makes them not regarded as a device with infinite impedance. The equivalent model of the measurement is shown in the figure below.
According to the circuit knowledge, for the DC signal, Vout will decrease compared with before the probe is not connected, because the current load is no longer RL, but the resistance of RL and Rin in parallel, which is bound to be smaller than the original Resistor RL. For the AC signal, the impedance generated by a capacitor Cin needs to be connected in parallel. And this impedance will gradually decrease as the frequency increases. This is the reason why the circuit cannot work normally when the probe is on in some cases. When measuring with a probe, it is necessary to evaluate the effect of the loading effect of the probe on the system under test. For example, for a general 1× probe, the input capacitance is as high as about 150pF. If it is connected to a 50Ω system, a low-pass filter of about 40M will be generated. If the measured signal is close to or higher than this frequency, it will lead to the measured signal. The system is working abnormally. When making high frequency measurements, the smaller the input capacitance of the probe, the less impact it will have on the system.
Probes are critical in an oscilloscope measurement system. The ZDS4000 oscilloscope has excellent performance and is equipped with a passive probe with excellent performance as standard. However, there are many matters needing attention in the use of the probe. That is, safe and accurate measurement results are obtained.