Introduction to three common signal integrity test methods

Signal integrity testing methods can be divided into three main categories. Some of these methods are described below.

Jitter testing is gaining more and more attention now, as dedicated jitter testing instruments, such as the TIA (Time Interval Analyzer), SIA3000, are very expensive and use less. The most used is the oscilloscope plus software processing, such as TEK's TDSJIT3 software. Through software processing, separate components such as RJ and DJ, and individual components in the DJ are separated. For this type of test, the selected oscilloscope, long memory and high speed sampling are necessary, such as memory above 2M, sampling rate of 20GSa/s. However, at present, the jitter test, the results of the solutions of various companies are quite different, and no one is an authority or industry standard.

The first is to require the bandwidth of the host and probe together to be sufficient. Basically, the bandwidth of the test system is more than three times the bandwidth of the test signal. In actual use, some engineers just look for some probes to test, even the probe of company A is inserted into the oscilloscope of company B. This kind of test is difficult to get accurate results.

Waveform test is the most commonly used method in signal integrity test. It is usually performed by oscilloscope. It mainly tests waveform amplitude, edge and glitch. By testing the parameters of the waveform, it can be seen whether the amplitude, edge time, etc. meet the interface level of the device. Request, is there any signal glitch? Since the oscilloscope is a very versatile instrument, it will be used by almost all hardware engineers, but it does not mean that everyone is using it well. Waveform testing also has to follow some requirements in order to get an accurate signal.

Second, pay attention to the details. For example, the test points are usually placed on the pins of the receiving device. If the conditions are not restricted, for example, the BGA packaged devices can be placed on the PCB traces or vias closest to the pins. It is too far away from the receiving device pin. Because the signal is reflected, the difference between the test result and the actual signal may be large. The ground wire of the probe should be selected as short as possible.

Finally, you need to pay attention to the match. This is mainly for the case of testing with coaxial cable. The coaxial is directly connected to the oscilloscope. The load is usually 50 ohms and is DC-coupled. For some circuits, DC bias is required to directly connect the test system. Will affect the working state of the circuit, so that the normal waveform is not tested.

Eye diagram testing is a common test method, especially for interfaces with specifications, such as E1/T1, USB, 10/100BASE-T, and optical interfaces. Eye diagram testing of these standard interface signals, mainly using oscilloscopes with MASK (template), including general purpose oscilloscopes, sampling oscilloscopes or signal analyzers. These oscilloscopes have built-in clock extraction function to display eye diagrams. For oscilloscopes without MASK, You can use an external clock for triggering. When using the eye diagram test function, you need to pay attention to the number of test waveforms. Especially when judging whether the interface eye diagram meets the specifications, the number is too small, the waveform jitter is relatively small, and there may be some violations, such as a waveform entering a part of MASK. It may not be collected, and the misjudgment is passed. The number is too large, which will lead to the long test time and the efficiency is not high. Under normal circumstances, the number of test waveforms is not less than 2000, and it is suitable at around 3000.

At present, there are some instruments that can use the analysis software to view the details of the violations in the eye diagram. For example, some sampling points are dropped in the MASK. In the past, it was not known which cases fell in because all the sampling points were accumulated. Going in, the overall effect looks like a long afterglow display. The new instrument takes advantage of its long storage and collects the waveform for processing and display. Therefore, every detail of the waveform can be retained, so it can view the violation of the waveform, such as the waveform is 000010 or 101010. It can help hardware engineers find the root cause of the problem.

Ethernet Cable Wiring

Ethernet cables connect devices such as PCs, routers, and switches within a local area network.Most technicians refer to these standards as CAT5 and CAT6, respectively. Also CAT3 available. Because of this, many online stores that sell network cables use this abbreviated language as well.

The connector can by shield or non-shield type, raw cable can be UTP, STP, FTP type. Also the molded shape can be custom mould by straight, right-angle, 105 degree, etc.

These physical cables are limited by length and durability. If a network cable is too long or of poor quality, it won't carry a good network signal. These limits are one reason there are different types of Ethernet cables that are optimized to perform certain tasks in specific situations.

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