What are the common response characteristics of oscilloscopes?

The oscilloscope is a commonly used electronic measuring instrument with many advantages such as high measurement accuracy, good stability, strong durability, and long service life. When using an oscilloscope, do users have a specific understanding of the common response characteristics of the oscilloscope? Today, the editor will introduce to you the common response characteristics of the oscilloscope, hoping to help you.

The response characteristics of the oscilloscope will affect the waveform of the signal and change the calculation of the signal rise time. When Pentium4 entered the gigahertz era, high-speed interfaces or buses such as SerialATA and PCIExpress also surpassed Gbps. Choosing the right probe is of course an important thing, but choosing the right oscilloscope is also an indispensable task.

The measured waveform is sampled and processed from the input connector and displayed on the screen, while the data is saved. Once an inappropriate oscilloscope is selected, the waveform may be deformed. Especially when measuring waveforms of high-speed serial interfaces such as PCI Express, it is necessary not only to measure the sampling frequency and bandwidth, but also to have some understanding of the response characteristics of the oscilloscope. For example, when measuring very steep signal changes, there will be differences due to differences in the response characteristics of the oscilloscope.

Reaction systems are divided into two categories

The response characteristics of an oscilloscope generally refer to the "transfer characteristics" of the entire measurement system from the input connector to the screen display. It can usually be divided into two categories: Gaussian Response and Brick-wall Response. Brick-wall Response is also called Flat Response.

The easiest way to distinguish or compare the differences between these two types of systems is to look at the two basic parameters: "-3dB frequency characteristics" and "step waveform response".

Commonly used analog oscilloscopes are Gaussian response systems, and their frequency characteristics will slowly decline at the right shoulder end. Even if the step waveform input is steep, it is not easy to produce waveform distortion, that is, there will be no instantaneous preshoot of the step waveform, overshoot after the waveform, or ringing of the waveform shaking up and down. This is an ideal characteristic when measuring digital circuit signals with short transition times.

Analog oscilloscopes must convert the tiny voltage signal of a few mV at the input end into a voltage of hundreds of mV through several stages of amplifier circuits to ensure that it is sufficient to drive the CRT display. The frequency response characteristics of these amplifier circuits are Gaussian.

When measuring the waveform of a high-speed serial interface, a broadband digital oscilloscope with real-time sampling is generally used. This type of oscilloscope often uses a brick-wall response type response system.

The brick wall response is also called the "highest flat response". The frequency response is extremely flat within the frequency band, and the signal is quite steep when it rolls off outside the frequency band. With such an ideal frequency characteristic, the signal amplitude within the frequency band will not be attenuated. Beyond the frequency band, the signal amplitude becomes zero.

Compared with Gaussian response oscilloscopes, brick wall response oscilloscopes still have several disadvantages:

In response to the input step waveform, pre-shoot or over-shoot waveforms are likely to occur

The oscilloscope has a longer rise time, in other words, it reacts more slowly

The oscilloscope rise time mentioned here refers to the rise time from the step input to the output waveform. The shorter this time is, the more faithfully the oscilloscope can show the waveform measured from the input connector. Therefore, the oscilloscope rise time is synonymous with its high-frequency characteristics. At the same time, the rise time of a digital signal generally refers to the time it takes to migrate from a low level to a high level. It usually refers to the rise migration time of 10% to 90% of the signal level, and for high-speed digital communications, it mostly refers to the time migration of 20% to 80%.

The following two mathematical formulas can be used to estimate the rise time of brick-wall and Gaussian oscilloscopes:

Brick wall response oscilloscope rise time (ns) = 0.45/bandwidth (GHz)

The rise time (ns) of a Gaussian response oscilloscope is 0.35/bandwidth (GHz), ideally it should be 0.338/bandwidth (GHz)

Although the rise time of the brick-wall response oscilloscope is slightly inferior, the real-time sampling broadband digital oscilloscope models mainly adopt the brick-wall response characteristics. A closer look reveals two main reasons. First, it is to avoid the error of the input signal and output signal voltage amplitude, because the amplitude error of the Gaussian response oscilloscope within the frequency band is too large. The frequency response diagrams of the two oscilloscopes shown in Figure 2 show their advantages and disadvantages in this regard. Assuming that the input signal bandwidth is 1GHz and the sampling frequency is 4GHz, it can be seen from Figure 2 that the frequency characteristics of the Gaussian response oscilloscope slowly decline at the right shoulder, especially in the frequency band area exceeding 1/3 of the bandwidth, the waveform is obviously attenuated, that is, the signal error is large.