PCB Tech - Fast positioning of signal integrity in high-speed PCB design

In high-speed PCB design, the traditional method of locating signal integrity problems is to use hardware triggers to isolate events, and/or to use deep acquisition storage techniques to capture events before finding the problem.

With the increase in the speed and complexity of high-performance circuit systems, the limitations of oscilloscopes in locating signal integrity problems have gradually emerged. With the emergence of new event location technology, this situation will be greatly improved.

Ultimately, this powerful event location system will effectively help PCB design engineers quickly and easily identify signal integrity issues.

Traditional location method for signal integrity problems

The traditional hardware trigger/deep acquisition storage method has two advantages in locating signal integrity problems. First, when hardware triggers are used to lock related events, there is no dead time. The hardware trigger system makes the oscilloscope acquisition system run continuously until the target event is found. After the target event is locked, the hardware trigger circuit will trigger to complete the data acquisition of the oscilloscope and display the event in the center of the screen.

This method is really convenient. Secondly, using deep acquisition and storage technology, the user does not need to know the type of signal integrity problems faced by the target system, just set the oscilloscope to the maximum storage mode, and set the trigger mode to edge trigger or even automatic trigger, and then let the oscilloscope start run. The oscilloscope will capture a relatively long screenshot of the target system execution, and then the user can analyze the data at any time to determine whether there is a problematic event.

This technique is also known as the "extensive swallow and drowning" technique. These methods of using oscilloscopes to verify designs are very effective and have taken root in the electronic design engineer community.

But compared with emerging technologies in the test/measurement industry, this approach has many limitations.

A new method for locating signal integrity problems The new method for locating signal integrity problems is event recognition software. Event recognition software is essentially a kind of intelligent software that can scan the waveform captured by the oscilloscope to identify various signal integrity problems or events with signal problems. This method does not have the "no dead time" function of the hardware trigger method, because there is already a "dead time" when processing the previously captured data, and it does not have the in-depth "extensive inspection" function acquisition and storage technology can provide.

However, event recognition software has the following unique advantages, attracting more and more oscilloscope users. 1. Simultaneous monitoring of multiple events: The hardware trigger method only identifies problematic events, and the hardware trigger circuit is set to trigger when a specific event occurs, which fundamentally eliminates the possibility of monitoring multiple events at the same time. The event recognition software is not affected by this limitation, and the software can be programmed to scan 5 events on any channel or multiple channels at the same time.

This greatly reduces the time required to gradually reduce the range of potential causes of signal integrity problems and isolate complex related events. 2. Understand how the same event occurs multiple times: The hardware trigger circuit only recognizes the occurrence of one event per capture. In fact, the event is repeated many times before or after the event is isolated by the hardware, but the hardware trigger method cannot detect these repeated events. This can be done with event recognition software, which can find all events captured by the waveform memory.

Therefore, PCB design engineers can not only find the first fault, but also the second and third faults. 3. Event navigation: Once the user captures long waveforms through deep storage, the next step is very boring and error-prone manual work, which is to replay these waveforms, check every part of the waveform, and identify potential signal integrity issues. Deep acquisition storage technology can capture information from 10,000 screens. It is impractical to view all this information manually. It is also impractical and time-consuming to upload these oscilloscope data to a single controller and write custom software to analyze the data. Once the event recognition software recognizes all occurrences of the target event, it can switch back and forth between multiple occurrences, using the same intuitive playback control keys as the DVD player. 4. Identify multiple events: A typical hardware trigger system can isolate about 10 different types of events or trigger modes. However, for oscilloscope manufacturers, the development of new hardware trigger modes is very troublesome, requiring a lot of development resources and expensive IC production costs. The cost of developing event recognition software is much lower. Current event recognition software can isolate any event that can be measured by waveform measurement (modern oscilloscopes can perform more than 30 waveform measurements), and can also detect problematic events, such as non-monotonic edges caused by incorrect signal terminals.

It is almost impossible to trigger wavelet phenomena, such as non-monotonic edges, using hardware trigger circuits. 5. The speed of identifying events: The speed of the hardware trigger circuit is mainly affected by the speed of its transistors, and analog technology is used. The most high-end hardware trigger circuit can now achieve as low as 300ps pulse width (or pulse interference) trigger, and 3.25Gbps sequence trigger (serial trigger). Although these indicators are very good, the speed of the hardware trigger circuit still cannot keep up with the speed of today's top systems exceeding 8.5Gbps. The event recognition software is only limited by the sampling rate of the oscilloscope and basically uses digital technology. The industry-leading oscilloscope has a sampling rate of up to 40GSps, and the software event recognition system recognizes events faster than the hardware trigger mode. The new technology can observe events with a pulse width of 70ps, and its sequence search speed can be as high as 8.5Gbps

The PCB software combination system can generate a trigger classifier (trigger sequencer), or it can use hardware to limit the waveforms that the software wants to check, thereby improving efficiency. Event recognition software is an effective supplement to traditional hardware triggering or deep acquisition and storage methods, and is used to identify signal integrity problems. When the oscilloscope has no "dead time" problem, that is to say, the frequency of occurrence of events is higher than once per second (one second is a considerable period of time for high-speed circuits), the new technology of event recognition software will become the positioning electronics One of the most effective and flexible tools for signal integrity issues in design.