Aug 15, 2020|General

With the introduction of high speed PCB design, the circuit building industry is changing for designers, engineers, and PCB manufacturing. If you need a refresher on PCB technology, need to know how to design a PCB, or are a beginner in circuitry, our comprehensive guide is here to help. 

What is High Speed PCB Design?

High speed PCB design is engineering circuit board layouts for high speed circuitry. These high speed circuits are designs in which a circuit board’s physical components affect the signal integrity. Such characteristics of high speed circuits include the layout, the layer stack-up, and interconnections, among others. 

With high speed PCB design, you have to spend more time than usual on the exact placement of traces, their width, their proximity to additional signals, and the type of components the traces connect to.

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Signals and Signal Integrity in a Nutshell

No matter if you’re dealing with ordinary or high speed PCB design, your circuit board will send a signal through your traces to reach an endpoint. So, what are high speed signals? There are two types: analog and digital. 

Digital Signals

Digital signals are simpler than analog ones, as they are a series of low points and high points. You can also think of them as 0 and 1, or off and on.

Analog Signals

Analog signals vary much more than digital ones and may contain an array of positive and negative signals. The signals fluctuate depending on the intensity and frequency of the signal. 

High Speed PCB Design

When it comes time to design your circuit, keep these common issues and solutions in mind to enhance the performance of your board.

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Issues

High speed PCB designs are sensitive, so you are likely to encounter some problems as you work through your project. Here are the three most common issues to monitor.  

Timing

If the timing of your signals is off, you will probably receive corrupted data. With a timing issue, you want to make sure each signal in your layout arrives at the correct time in relation to all of the other signals, along with the signal from the clock. 

Integrity

Signal integrity means that your signals arrive at their destination in the correct form. If your signals do not look the way they should when they reach their endpoint, they most likely encountered interference along the way.

Noise

Every PCB releases noise, but if there’s too much, your data may be damaged. Noise occurs when there is an unexpected oscillation from one of the other signals on the board that affects the signals around it.

Solutions

Luckily, there are fixes for all of these problems, and they are part of what is crucial for high speed design.

Impedance

Impedance is a fundamental solution to several of the most common issues affecting PCB design. When you have the correct impedance between your transmitter and receiver, it will directly influence your signals’ quality, integrity, and sensitivity.

Matching

Matching helps with the proper timing of your signals. If you match the lengths of two coupled traces, it will ensure that they meet their destination simultaneously and synchronize with their clock.

Spacing

Leaving adequate space between your tracers helps protect them from noise and other kinds of signal interference. When you avoid placing your tracers too close, you reduce the chance of interference.

High Speed PCB Layout

When we talk about what is high speed design in PCB, we have to remember an overwhelming number of guidelines. Planning your layout ahead of time will help maintain your project’s timeline and prevent mistakes.

The Schematic

The first step is to map out your circuit with a schematic. When drawing your schematic, think about signal flow and try to capture a natural flow from left to right while including as much helpful information as possible.

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Requirements

Be specific with your PCB layout instructions. Make sure to include a description of the circuit’s purpose, a sketch of the circuit, the board stack up, the components’ location, and the proximity of the traces and circuits.

You may also need to think about the types of signals on each layer. For example, if you’re working on radio frequencies, you will need to consider what is RF PCB design. These are high-frequency signals that have special requirements. In your instructions, include anything that seems vital to the proper functioning of the board.

Location

The location of components on your circuit board is among the most crucial elements of your PCB layout. You also must consider the circuit’s place on the board and what other circuits are around it.

Power-Supply Bypassing

To decrease noise in high speed circuitry, bypassing the power supply at the amplifier’s supply terminals is essential.

If you’re dealing with high speed op-amps, there are two techniques used for bypassing. They are rails to ground, which works in most cases, and rail to rail, which is useful in unique situations. 

Parasitics

Parasitics are stray capacitors and inductors that sneak into high-speed layouts and cause problems in your circuit. Because it doesn’t take much to disrupt them, high speed PCB circuits are very susceptible to parasitics. 

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Ground Plane

A ground plane serves as a reference voltage, offers shielding capabilities, allows dissipation of heat, and minimizes stray inductance. You have to be careful, though, because a ground plane may also result in more parasitic capacitance.

In most cases, you will want to dedicate an entire layer of your circuit as the ground plane and keep it unbroken. 

Packaging

When dealing with op-amps, they come in many different packages. The one you choose will impact the high-frequency performance of the amplifier. Mostly, packaging affects parasitics and signal routing.

Routing and Shielding

Routing and shielding reduce interference between signals. Several methods of routing and shielding exist in PCB design. Ground planes work well for shielding. You can also route traces orthogonally to reduce capacitive coupling and minimize the proximity of traces.

How to Tell If Your Project is High Speed

While there is no absolute definition of high speed PCB design, there are a few ways to figure out if your project is high speed. 

Problems with the integrity of signals are an accurate indication. If you’re working on a cell phone project or a motherboard, it’s definitely a high-speed design. Using specific technologies may also be a helpful clue.

If your project is a distributed system where you have a bunch of subcircuits that work independently within a more extensive system, you are also dealing with high speed PCB.

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