The influence of 50Ω on oscilloscope measurement

Transmission line

It's like talking about history and having to insert a military theory class. If you want to understand what we have said, then you have to talk about this transmission line. As we all know, electrical signals actually propagate in the transmission line in the form of electromagnetic waves. When the size of the transmission line is no longer much smaller than the wavelength of the electromagnetic wave, the characteristics of this "wave" have to be considered.

Light will be reflected when the transmission medium changes, and the same is true for electrical signals. What will the reflection bring? Your signal may be like this.



Is the whole person bad? In order to prevent reflections, uniform transmission lines have appeared, such as PCB microstrip lines and coaxial lines. They have uniform media and the same cross-sectional geometry at any point, so that electrical signals will not be reflected in the transmission line. But the problem is coming again. To send you a thousand miles away, you have to say goodbye. Sooner or later, the transmission line still has to hand over the signal to the signal load. Once the signal reaches the end of the transmission line, does it still have to be reflected? Fortunately, our electrical signals are not as hypocritical as light. As long as her instantaneous impedance remains the same, she can also not reflect back at once.

The instantaneous impedance is the impedance of an electrical signal at a certain point on the transmission line. After research, it is found that the instantaneous impedance of a uniform transmission line is purely resistive, independent of frequency, like a resistor, and the instantaneous impedance is only related to the geometry and filling of the transmission line. The material is related, so it is also called characteristic impedance. Since the instantaneous impedance is like a resistor, then we connect a resistor in parallel to the load to make the total resistance equal to the characteristic impedance, so that the signal will not be too disgusting, and will be condescendingly transmitted to the load without being reflected back. , Your circuit will be clean. This method is called terminal matching. Another method is source end matching, that is, a resistor is connected in series at the source end to make the output resistance of the source equal to the characteristic impedance of the transmission line, so that the load of the reflected wave can be equal to the impedance of the transmission line, thereby absorbing the reflected wave. Don't let it bump into the transmission line. In many cases, these two types of matching are used at the same time.

50Ω

The characteristic impedance will affect the signal transmission power, transmission loss, crosstalk and other electrical properties, and its plate and geometric structure will affect the manufacturing cost. In this case, only a compromise can be found. And 50Ω is a balance point of the transmission power, transmission loss and manufacturing cost of the coaxial line. Therefore, most high-speed signals will use a 50Ω characteristic impedance system, which has become a standard and has been used to this day as an impedance standard. For example, the common PCIE, its single-ended impedance is required to be 50Ω.

This is the origin of this 50Ω, but it has not been explained why there is a 50Ω on the oscilloscope. Is it to prevent signal reflection? Yes, this is indeed a reason, but besides this, it has other meanings.

Oscilloscope loading effect

I believe everyone has this experience. I debug a circuit with a problem and want to see the waveform. As a result, the probe circuit is normal when the probe circuit is connected, and the problem occurs again when the probe circuit is removed. This is caused by the load effect. The equivalent model of the oscilloscope in the 1MΩ impedance mode is more complicated, and it can be roughly equivalent to a parallel connection of 1MΩ and a capacitance of more than ten pF.



This 1MΩ is the specification of the oscilloscope. The capacitance is a parasitic parameter that we don't want but unavoidable. At DC and lower frequencies, 1MΩ plays a dominant role. When the frequency exceeds 10M, the capacitor will become the main load. Due to the introduction of these two parameters, the signal during measurement will be different from the original signal, which will cause errors in the measurement results. So how big the difference is, it also depends on the output resistance and load of your circuit under test. Take the example in the figure above. According to, it can be changed to:



It can be seen that the original signal is; the difference of the low frequency signal is mainly determined by the partial pressure of Thevenin output resistance Re and 1MΩ, while at high frequency, the partial pressure of Re and 16pF capacitive reactance needs to be added.

The calculation shows that if the value of Re is 10Ω, but 200M, the load effect of the oscilloscope will cause a deviation of about -0.2db. And if the Re of your system is 25Ω, then the deviation will reach -1db. If it is 50Ω or 100Ω, the error will undoubtedly become larger and larger.

In order to make the measurement more accurate, the oscilloscope must add some internal compensation measures to compensate for these deviations (of course, this compensation is only relative to the measurement result and the original signal. The internal compensation cannot reduce the signal and the original signal during the measurement. difference between). It should be compensated according to the specific situation. As we have already known before, 50Ω system is widely used in high-speed signals, so we choose 50Ω system to compensate under the condition of Re=25Ω. Oscilloscope manufacturers will compensate the signal in this case. So if you are a 50Ω system, the effect of the result is close to the original signal. If your equivalent output resistance is much different from 25Ω and the frequency to be measured is high, you need to evaluate whether the measurement error is within your allowable range. It is recommended to use a 10:1 probe for measurement, because its parasitic capacitance is lower than that of an oscilloscope, and the load capacitance of a 1:1 probe is basically about 50pF, and its load effect is much more serious than the oscilloscope itself. If the 10:1 probe still does not meet your needs, you must choose an active probe with a smaller parasitic capacitance for measurement.

Imagine, if you use an oscilloscope to directly connect to the high-frequency signal to measure the high-frequency signal output by the signal, and the output resistance of the high-frequency signal generator is all 50Ω, what will happen. It can be seen from the above that the corresponding load will seriously affect the measurement results. Combined with the transmission line theory, it can be known that there will be a reflected wave reflected back to the signal source, which may be fatal for some sophisticated instruments. So you need to add a 50Ω termination adapter or use the internal 50Ω gear at this time. In this way, the reflection of the signal is greatly reduced, and the measured signal can be affected by the load effect. This is what the 50Ω impedance of the oscilloscope does.

Oscilloscope measurement related to 50Ω

Combining transmission line theory and oscilloscope load effect, I will talk about some points for attention related to 50Ω in oscilloscope measurement. Of course, you do not need to consider these when measuring lower frequency signals.

1. When the oscilloscope uses 50Ω termination or internal 50Ω gear, only 50Ω coaxial cable or some active probes that require 50Ω matching can be used. When directly measuring onboard signals, only active probes can be used. Coaxial cables are only suitable for measuring unloaded signals (such as signal generators).

2. When measuring high frequency signals, pay attention to the load effect of the oscilloscope. The measurement system is a 50Ω system, and considering the transmission line theory, the probe is used to directly measure the load end instead of the middle PCB.

3. When measuring the output waveform of the signal generator, a 50Ω termination is required. Otherwise, the measurement result will be seriously affected by the oscilloscope load effect.

Extra Story

One brother of the Ming Dynasty, Yang Minggong, taught us: "When you are aware of action, you will know when you are careful, and when you know the truth, you will do." True knowledge is obtained in the continuous process of unity of knowledge and action. In this process, you must need some partners who can help you discover the truth. The newly launched ZDS2024 Plus will be your good choice. It supports a large storage of 250M points, and you can see much more details than other oscilloscopes under a larger time base. It supports more than 20 kinds of protocol triggering and decoding, and has a variety of calculation functions such as FFT, which will make your "action" and "knowledge" more quickly "unity".