The latest high-speed signal virtual detection and equalization technology

1. Introduction
In high-speed serdes systems, factors such as the impedance discontinuity of long-distance transmission lines, connectors, and vias on PCBs cause great attenuation of signals. For example, the eye diagram of a 6.25Gb/s signal cannot be opened after traveling 25 inches or more on FR4. In order to solve this attenuation problem, there are usually two methods, one is to use pre-emphasis technology at the transmitter, and the other is to use equalization technology at the receiver.
Since an equalizer is integrated inside the SERDES receiver chip to compensate for excessive signal attenuation, it is basically meaningless to directly measure the signal at the receiver during testing, because this cannot represent the true performance of the received signal at all. Integrating filters inside the oscilloscope and allowing customers to customize filtering functions to simulate the equalization characteristics of the receiving chip is one of the current solutions for high-speed testing.
As the company's high-speed test platform, ZTE's EDA high-speed laboratory has solved many problems such as E1 service boards, Ethernet ports, Gigabit optical ports, and high-speed interconnection systems, and has also studied cutting-edge technologies such as equalization testing. This laboratory will take LeCroy's SDA6000A (bandwidth 6GHz, 20GS/s) as an example to introduce the latest high-speed serial signal virtual detection and equalization technology. The virtual detection and equalization analysis tool of SDA6000A is called Eye Doctor, which is a powerful software package integrated into the LeCroy oscilloscope.
2. Use Eye Doctor for high-speed signal testing
I. Introduction to Eye Doctor functions
Virtual detection:
allows testing at any convenient location in the system, and then reflects the waveforms at other locations. These locations include locations that cannot be detected in the test system, such as inside the chip.
It can improve test accuracy: remove the influence of instruments, probes, and fixtures; test at the place with the strongest signal (usually the transmitting end), and then deduce the waveform at the place with weaker signals. Testing at the transmitting end has a higher signal-to-noise ratio, which is better than testing directly at the receiving end.
Use universal S parameters as the language of the simulator (S parameters can be extracted using a vector network analyzer or simulation modeling software).
Receiver equalizer simulation:
High-speed serial chips often embed receiver equalizers. Reproducing signals through the oscilloscope's equalization simulation technology can reflect the true performance of the system.
2. Principle
1. Virtual detection

Figure 1 Virtual detection principle
Figure 1 shows the virtual detection principle of Eye Doctor, which consists of a virtual detection device, equivalent to a black box, and its performance is related to the input S parameters, which represent the network parameters of the transmission system. The virtual detector inputs the waveform obtained by the actual test on the left (such as the waveform at the transmitter end), and the output on the right is the virtual detection waveform (such as the waveform at the receiver end or a point that is difficult to detect). It can support a system with up to 8 ports of input and output. The description language is similar to Spice, and it receives S parameters in Touchstone format.
2. Receive equalizer
In high-speed data transmission, there are usually two methods to prevent signal distortion: one is to use pre-emphasis at the transmitter end, and the other is to use equalization technology at the receiver end. Its structure is shown in Figure 2.

Figure 2 High-speed data transmission system
Pre-emphasis: To offset the attenuation of the transmission channel, pre-emphasis pre-distorts the transmitted signal by emphasizing the first data symbol after any signal change, thereby eliminating the leading edge overshoot and trailing edge tailing of the pulse response in the channel.
Receive equalization: The receive equalizer compensates for transmission losses by amplifying the high-frequency components in the signal at the receiving end, while the low-frequency components remain unchanged, making the signal eye diagram open as wide as possible. It is an alternative or complementary solution to pre-emphasis.
Decision feedback equalization (DFE): Changes the amplitude of the signal according to the decoded data. The structure of a classic decision feedback equalizer is shown in Figure 3. [page]

Figure 3 Structure of decision feedback equalizer
The function of feedforward equalization (FFE) is to remove signal intermodulation interference and "bleach" the noise from the received data stream. DFE monitors the output of the decision slicer, extracts the noise related to the slicer decision, feeds it back to the input of the slicer, and subtracts it from the next signal. In the FFE-DFE system, the number of taps of FFE and DFE has a great influence on the entire equalizer.
The function of Eye Doctor is shown in Figure 4:

Figure 4 Application example of Eye Doctor
III. Application example:
The following is an actual application case of ZTE EDA high-speed laboratory to illustrate the application of Eye Doctor. A high-speed backplane interconnection system shown in Figure 5 was tested. The signal passed through the connector and the backplane, and the routing length was about 40In.

Figure 5 High-speed interconnection system
1. Transmitter signal waveform and eye diagram
Signal waveform (Figure 6):

Figure 6 Transmitter signal waveform
The eye diagram quality is very good, as shown in Figure 7:

Figure 7 Eye diagram at the transmitter [page]
2. Signal waveform and eye diagram at the receiver
The signal passes through the backplane and connector and reaches the receiver, where the signal deteriorates and the eye diagram becomes smaller, as shown in Figures 8 and 9. If equalization technology is not used, the system is prone to bit errors, so an equalizer is usually embedded in the high-speed receiver chip.

Figure 8 Receiver signal waveform

Figure 9 Receiver eye diagram
3. Receiver waveform obtained by virtual detection
Next, let's take a look at the receiver waveform obtained by virtual detection using eye doctor. During the detection process, the S parameters of the transmission network need to be input into the virtual detection device (Figure 10). The S parameters are obtained by the EDA high-speed laboratory using a network analyzer plus an auxiliary daughter card. If the S parameters are not easy to obtain in actual application, they can also be extracted through some simulation software. EDA high-speed laboratories usually use SIwave and HFSS to extract S parameters.

Figure 10 Using eye doctor for virtual detection [page]
The virtual detection waveform and the actual test waveform are compared as shown below. It can be seen that the agreement is quite good (Figure 11). Virtual detection can be used as an important auxiliary means of testing.

Figure 11 Virtual detection vs. measured waveform: yellow is the detection waveform, cyan is the measured waveform
4. Equalization simulator
Add the attenuated signal passing through the backplane to the equalization simulator of the oscilloscope to view the eye diagram of the equalized signal. According to the actual situation of the receiving chip, we set the number of taps of FFE and DFE to 3 and 1 respectively, click Train, and train the equalizer (Figure 12), and the equalized signal is obtained, and its eye diagram is shown in Figure 13.

Figure 12 Equalizer settings

Figure 13 Equalized receiver eye diagram [page]
Compared with Figure 9, the eye diagram quality is greatly improved, which represents the real performance of the system. Embedding equalization technology in the test instrument has become one of the solutions for high-speed testing.
Note: Explanation for the waveform cut-off phenomenon when the eye diagram is displayed:
The equalization algorithm will increase the amplitude of the sampling point to minimize the difference between the equalized waveform and the ideal waveform (the decoded waveform in the figure below). For high-frequency codes, in order to compensate for its attenuation, the waveform after equalization may have a relatively high overshoot-like waveform on the rising or falling edge, and the cut-off waveform may be seen when the eye diagram is displayed.

3. Conclusion
In this case, ZTE EDA High-Speed ​​Laboratory solved the problem of testing high-speed serial signals by using virtual detection and equalization technology. In high-speed data transmission, equalization processing technology is often used to extend the transmission distance of the signal. It is basically meaningless to directly measure the signal at the receiving end because it cannot represent the true performance of the received signal. Embedding equalization technology in the test instrument to reproduce the true performance of the system has become an important solution for high-speed testing.
Eye Doctor is a software package for LeCroy high-end oscilloscopes. It contains two functions: virtual detection and equalization simulation. This is a very important function in high-speed serial data testing and is sometimes necessary.
Virtual detection is a new method in high-speed testing. It can be done quite accurately, but it requires the S parameters of the system, which is not easy to obtain and can be extracted through some simulation software.
References
1. (US) LeCroy Corporation, Eye Doctor Operator\'\'\'\'s Manual, January 2007
2. (US) Leo Wong, Rambus, How to solve the signal integrity problem of high-speed backplane, 2006