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High-Speed PCBs

Key Takeaways

  • When the digital signal delay on PCB traces is greater than 20% of the rising edge time, the circuit can be regarded as one requiring high-speed PCB design considerations.

  • High-speed PCBs are preferred when the speed of the signal is high enough to alter the circuit impedance and the behavior of circuit components, digital radiations are present in the circuit board, or ionization loss or the skin effect is present in the board.

  • If the length of the interconnection is greater than or equal to λm/12, then the PCB must be designed as a high-speed PCB.

High-Speed PCB

High-speed PCB design guidelines must be strictly followed when signal frequencies are above 50 MHz

The transition from analog circuits to digital circuits was a turning point in the history of the PCB manufacturing industry. In the sixties, most electronic products were analog circuit-based and dealt with low signal speeds. Since the signal speeds were low, PCBs didn’t face problems such as signal integrity, power integrity, EMI/EMC issues, etc. However, the digital revolution in the electronics industry led to a drastic rise in signal speeds, causing traditional PCB design to fail. To address this, PCB engineers optimized circuit design and PCB material and developed high-speed PCBs.

Let’s look at analog and digital signals and learn how to identify when high-speed design needs to be considered.

Analog and Digital Signals 

Generally, signals can be classified into analog or digital signals.

Signal Classifications

Analog signals

Signals that contain positive and negative values. These vary much more than digital signals.

Digital signals

Digital signals are waveforms with high and low points. They can also be considered 0s and 1s or on-and-off states.

In both analog and digital signals, the frequency of the signal variation can be high. If this is not addressed in the circuit board, physical components disturb the signal integrity and are capable of causing glitches, errors, or unexpected outputs.

Usually, signals with frequencies ranging from 50 MHz to 3 GHz are vulnerable to signal integrity, power integrity, and EMI problems. In this context, high-speed PCB design guidelines must be strictly followed when the signal frequencies are above 50 MHz. Such circuit boards can be regarded as high-speed PCBs.

High-Speed PCBs

Modern electronic products rely on high-speed signals. PCBs are made to handle very high clock frequencies (greater than 50 MHz) and very short signal rise times. If high clock frequencies and extremely short rise times are not taken into consideration during PCB design, signal integrity issues may start to appear. High-speed PCBs are required whenever the circuit handles signals of very high speeds. 

High-Speed PCBs vs. High-Frequency PCBs

High-Speed Circuits

High-Frequency Circuits

The circuit voltage rises and falls in a short time period.

The circuit cycle is short.

The higher the signal frequency, the shorter the cycle. In a cycle, the signal rise time and fall time are kept short to maintain signal stability. This causes the signal speed to increase. These high-frequency signals subsequently become high-speed signals.

Identifying Whether High-Speed PCB Design Is Required

To successfully design a PCB, it is necessary to identify the design type. Designing an original high-speed circuit as an ordinary or traditional PCB will cost money and cause signal integrity issues.

When to Use High-Speed PCBs

  • The speed of the signal is high enough to alter the circuit impedance and the behavior of circuit components.

  • When the digital signal delay on PCB traces is greater than 20% of the rising edge time.

  • Digital radiations are present in the circuit board.

  • There is the presence of ionization loss, skin effect, etc. in the board.

  • If the length of the interconnection is greater than or equal to λm/12. 

There is a two-step process that designers practice to determine whether a design needs to be high-speed or not. 

Step 1:

State the following values:

  1. The maximum frequency content in the high-speed signals (Fm)
  2. If it's a digital signal, the fastest rise and fall time (tr)
  3. The maximum data transfer rate (DTR)

The following equation is a shortcut to get a sense of the other two parameter values by measuring one:

Torque formula

If Fm turns out to be greater than 50 MHz, design the board as a high-speed PCB.

Step 2: 

Determine the wavelength (λm) of the signals of a given frequency (Fm)   on the PCB. V is the signal speed on the PCB. The V value of the signals through a microstrip of the same dimensions in different PCB materials varies with the dielectric constant of the PCB material. For a microstrip line, the signal speed in vacuum or air is 11.8 in /ns, and with other PCB materials of an effective dielectric constant greater than unity (dielectric constant of air), the speed of the signal decreases.

equation for wavelength

Cadence Software Supports High-Speed PCB Design

Cadence’s OrCAD software can assist design engineers in designing high-speed PCBs with minima signal integrity issues and EMC. The PCB design and analysis tools from OrCAD can help you in precisely designing high-speed mixed signal PCB boards that adhere to design rules governing track impedance control, line terminations, routing to minimize noise coupling, etc. Simulation tools can model the layout and help in improving the performance by analyzing it for design iterations.

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