Design considerations for RF PCBs can vary greatly. Many of the newer RF PCB designs are intended to accommodate millimeter-wave (mmWave) technology and as mmWave chipsets continue to diversify, there are some things that will remain the same for these applications. Understanding the basic needs of circuit design, which are good for mmWave performance when using PCB technology, can be advantageous to the circuit designer.
PCB design attributes greatly influence RF performance, particularly in relation to frequency. Low- and high-frequency applications require different PCB design disciplines. As a general statement related to this article, the comments about lower frequency are typically at 10 GHz or less. Millimeter-wave frequencies are defined to be approximately 30 GHz to 300 GHz and this article will focus more attention on mmWave applications from about 40 GHz to 90 GHz.
Considering Wavelength
Wavelength is a critical consideration for mmWave design. For those who are less familiar with RF technology, wavelength can be confusing, but there are some general definitions that can clarify the topic. It is easier to think about wavelength in physical terms, instead of the more detailed electromagnetic definitions. Wavelength is as the name implies: the physical length of a wave. To give some examples with comparisons for a PCB may be helpful.
As an example, using a simple double-sided circuit that is microstrip (signal conductor on top, ground plane on bottom), with a 5-mil thick dielectric material that has a Dk of 3, the length of the wave on that circuit when operating at 2 GHz is approximately 3.9 inches (99.1 mm). If the frequency doubles to 4 GHz, the length of the wave will be twice as small and would be ~ 1.95" (49.5 mm). A higher frequency will give a wave of shorter length. When considering 60 GHz for this example, the wavelength is about 0.13" (3.3 mm). Here is another way to think about wavelength: If the circuit has a special conductor feature that is 0.26" (6.6 mm) in length, then at 60 GHz that circuit feature will have two full waves propagating on it and it can be said the circuit length is two wavelengths. If the circuit feature is changed to be much shorter and is 0.065" (1.65 mm) in length, that conductor can be defined in terms of wavelength as a half-wavelength conductor when operating at 60 GHz.
As a quick reminder, the waves that propagate on the PCB can be thought of as sine waves; a sine wave is made up of 360 degrees. If the propagating wave on a circuit encounters an anomaly, like a circuit etching defect, some portion of the wave will be affected by that anomaly. If the anomaly is small and it only affects 10 degrees of the propagating wave, then the wave will not be distorted much, and the circuit will perform as expected. If the anomaly is much larger and can affect 90 degrees of the propagating wave, the wave will have some distortion and the circuit will probably not perform as expected.
The idea of the physical length of the conductor, as it relates to the physical length of the wave (wavelength), is critical for understanding why mmWave circuits need to be manufactured with much more precision than circuits operating at lower frequency. Again, looking at some of the example comparisons, if a circuit is operating at 2 GHz with a wavelength of about 3.9" (99.1 mm) and the circuit has an etched anomaly about 0.033" (0.84 mm) in length, that will equate to about 3 degrees of the wave being affected and that will not cause a difference in the overall wave performance.
However, that same anomaly for a circuit operating at 60 GHz will be equivalent to about 90 degrees of the propagating wave, which is a significant portion of the wave. The wave properties will be distorted, and the circuit will not perform as expected.
The examples given here are relatively simple and meant to help the thought process. However, there are a lot of special conductor geometries which the RF engineer will purposely design for the circuit performance they want. The propagating wave will be much more sensitive to some of these special conductor geometries and very small differences in the geometry can make big differences in circuit performance. For radar circuits operating at 77 GHz, there have been notable RF performance differences due to certain circuit geometry varying as little as just a few mils.
The PCB designer working with applications at different frequencies should be mindful of circuit feature sizes as it relates to the wave size (wavelength) and consider the possible variations of circuit geometry due to the PCB fabrication process.
This column originally appeared in the June 2023 issue of Design007 Magazine.
