With 5G’s arrival imminent, electrical engineers must reconsider (and sometimes redesign) their PCBs and other infrastructure in order to support the high frequencies of the new frequency spectrum. Signal integrity will become a paramount concern in the design of 5G boards.
5G’s place on the electromagnetic spectrum. Image used courtesy of the BBC
For this article, let’s take a look at some of the implications of higher frequencies on PCB signal integrity and ways to mitigate these concerns.
Why 5G Frequencies are Bad For Signal Integrity
Increasing frequencies in board design comes with many undesirable effects on signal integrity—specifically, increasing noise and the effects of attenuation.
With respect to noise, the first thing to consider is that signal reflections become more relevant as system frequencies increase. Based on the transmission line theory, reflections are directly related to the ratio of the transmission line length to the signal wavelength.
We also know that signal wavelength decreases as frequency increases (λ = v/f). Thus, as 5G introduces higher frequencies, designers must also consider the effects of signal reflections, such as ringing or other distortions, which cause more noise in the system and effectively decrease SNR.
Capacitive and Inductive Coupling
Furthermore, the effects of capacitive coupling and inductive coupling become more relevant because capacitance and inductance are related to the rate of change in voltage and current, respectively. This too will create noise and distortion, decreasing SNR.
Attenuation and the “Skin Effect”
With respect to attenuation, one important consideration is what is known as the skin effect. The skin effect essentially states that as signals increase in frequency, the depth that they penetrate within the conductor decreases.
Depiction of the skin effect.
The important implication of the skin effect is that as higher frequencies travel through smaller areas, they experience more resistance and induce greater IR losses. This loss will also decrease SNR.
Ways to Improve SNR in 5G Designs
There are many factors working against signal integrity in high-speed designs. So, what can designers of 5G boards do?
Control Board Impedance
An important step to mitigate signal reflections and attenuation is to control the board impedance. Having appropriately terminated lines and well-designed impedance matching networks will be crucial to both prevent signal reflections and deliver maximum power to circuit blocks.
Focus on Impedance in Manufacturing: mSAP
Traditional PCB manufacturing processes, such as the subtractive process, have the disadvantage of creating tracks with a trapezoidal cross-section. These cross-sections modify the impedance of the tracks themselves, placing serious limits on 5G applications.
One solution is to use the mSAP (semi-additive fabrication process) technique, which allows a manufacturer to create traces with greater precision. Controlling the geometry of your lines can also help to mitigate the skin effect and losses in signal power because of it.
Subtractive vs. mSAP process. Image used courtesy of Proto-Electronics
Place Components and Traces
When alleviating effects such as coupling, the most important thing one can do is to judiciously place components and traces relative to each other and to ground. For example, multilayer PCBs with buried ground and power supply planes can be a useful solution.
Placing sensitive lines next to the ground plane forces capacitive coupling with ground (as opposed to with other lines) and offers low inductance return paths for the high-speed signals.
Many More Considerations for 5G Design
While this article did not address the exhaustive list of problems or solutions, we reviewed some of the high-level issues of 5G frequency on signal integrity and the possible design solutions to address them.
It is clear that 5G will introduce signal integrity challenges for PCB engineers since both the frequency-dependent effects of noise and attenuation will work to lower SNR. Some considerations not considered in this article, such as dielectric and substrate material selection, are equally as important to successful 5G designs.