Signal Integrity in High-Speed Rigid PCB Designs

We use the term signal integrity a lot, what actually is it? Is it something related to signal parameters or system parameters? In easy words when a signal travels through a piece of wire or transmission line some parameter gets changed from where it is transmitted and where it received. In case of high speed signals the signal loss is even more, which rises to the problem of data losing and signal corrupting. So what type of signal is getting disturbed and how is it getting changed? We have discussed the 4 main phenomena why the signal changes its properties during transmission.

And coming to the second question, what type of signal: basically if high frequency there should be zeroes and ones which are switching at a higher rate. If any 0 becomes 1 and 1 becomes 0 there will be data loss. Yeah there are some code correction techniques also but that is the topic for another day. To solve this problem we have to consider factors like stackup design and impedance control in mind. Design teams can improve the integrity by pushing for ever-smaller form factors devices. Which also reduces the parasitic effects in the signal. Here in this article we will see the  fundamentals of signal integrity, layout strategies and ways to fully solve or eliminate this.

1.    Signal Integrity Basics:

Traces act as straightforward connections at low frequencies. These same traces, however, function as transmission lines at high frequencies, causing ringing, reflections, crosstalk, and other undesirable consequences. Achieving dependable communication between high-speed ICs requires maintaining appropriate signal integrity.

Several factors that fall under several categories might cause a signal's quality to deteriorate on a circuit board. All of the aforementioned criteria are covered in other articles here as well. You should be mindful of these four primary areas of poor signal integrity:

1) Electromagnetic Interference (EMI):

EMI is a type of interference brought on by undesired electrical impulses, according to the fundamentals of PCB design.If high-speed transmissions are not adequately controlled, they can result in electromagnetic interference (EMI) and signal loss. In essence, it is a form of antenna effect, whereby the electromagnetic interference of one chip interferes with that of the second chip and continues in all designs. Missing return pathways are the cause of this kind of issue. See our in-depth EMI article to learn more.

2) Unintentional Electromagnetic Coupling (Crosstalk):

Unintentional interactions between signals on tightly routed wires can result in crosstalk, which could cause one signal to interfere with another. Consider two discussions taking place next to one other. Speakers may become distracted if they are too near to one another and overhear one another. Similar to this, interference can occur when traces on a circuit board are too close together because one signal may inadvertently "hear" another.

3) Simultaneous Switching Noise (Ground Bounce):

When a circuit board has a lot of components that alternate between high and low states, the voltage level could not go back to the ground potential as it should when it goes low. The signal's low state might be mistakenly seen as a high state if the low state's voltage level bounces too high. When many of these occur at the same time, the circuit may malfunction and cause erroneous or duplicate switching.

4) Impedance Mismatch:

According to the fundamentals of signal integrity, an impedance mismatch happens when there are variations in the electrical resistance (impedance) along a trace. This is especially important for high-speed signals that enter or exit an integrated circuit. Signal distortion results from reflections of the signal caused by this discrepancy. Visit our in-depth post here to learn more about impedance mismatch.

2.    When to Worry About Signal Integrity?

Technically, any design will have some issues with signal integrity, but unless you are working with high speed digital signals, these issues usually don't affect a product's operation or produce excessive noise. Not every PCB needs to be designed using high-speed methods. To find out if your design fits within this category, follow these steps:

⦁ Maximum frequency content (Fm) exceeds 50 MHz

⦁ Fastest rise/fall time (Tr) is less than 10ns

⦁ Data transfer rate is greater than 20 Mbps

⦁ Using the approximation: Fm ≈ 0.5/Tr

3.    How to test Signal Integrity:

S-parameter measurements using a vector network analyser (VNA) and eye diagram testing using a standard test bitstream are two of the most crucial tests for digital systems, while there are other tests that may be carried out to assess signal integrity. While an oscilloscope is often used for bit error rate calculations and eye diagrams, certain VNAs are capable of producing eye diagrams.

For assessing digital channels, eye diagram measurements and extracted bit error rates are essential. They offer a summative assessment that makes it possible to quantify losses, ISI brought on by signal reflections, jitter, and the requirement for equalisation adjustment.

4.    Eye Diagram Analysis:

Yes it is the method to see signal integrity in real systems. What it does is, it takes the transmitter signal as reference and compares the signal with received one. Match the two and plot the output with the help of an eye. What type of EYE is it? How we can measure and calculate the signal integrity from here. All these questions are covered in a recent blog on eye diagrams.

Here we can only say, if the signal has more distortion then the shape of the eye is more closed. If the signal is the same as the input one, we get a perfect open eye. For the reference I have a diagram above, where you can see both the phenomena.

5.    How to Solve Signal Integrity Issues:

Clearly defining ground and maintaining ground close to crucial lines during routing are key components of maintaining signal integrity. The majority of EMI and signal integrity issues may be resolved with a well-designed stackup, power and ground plane selection, and signal layer identification. Properly constructed stackups also have a significant positive impact on power integrity.

A common configuration including ground, power, and alternating signal layers. In addition to preventing reflections, a low-impedance return path with a well defined trace impedance and ground near signals also lowers EMI emission and reception and offers shielding from signals on various levels. Here are a few quick remarks and a guidance to solving the problem:

⦁ Use Short, Direct Routes for high-speed signals.

⦁ Avoid Acute Angles in trace routing to minimize impedance variations.

⦁ Use Solid Ground Planes under signal layers for stable return paths.

⦁ Length Match Differential Pairs to minimize skew.

⦁ Minimize Layer Transitions use microvias where possible.

Conclusion:

In conclusion, signal integrity will remain a crucial component of rigid PCB performance as electronic systems develop, particularly for high-speed designs. Engineers may guarantee reliable, fast connectivity in their designs by carefully regulating impedance, lowering reflections, limiting crosstalk, and selecting the appropriate materials. Modern electrical devices perform better, are more reliable, and reach the market faster when SI analysis is incorporated early in the PCB design process. Data corruption, elevated bit error rates (BER), or noncompliance with electromagnetic compatibility (EMC) standards are all signs of poor SI.