Substrate Integrated Waveguide Design for RF PCBsBy ZM Peterson • Nov 19, 2019
Unless you’re a pro RF designer, you are probably only passively familiar with some alternative routing structures for RF PCBs. Some well-known structures like grounded coplanar waveguides and their variants are known for providing low loss single-ended and differential mode routing.
You can thank applications like car/UAV radar, upcoming 5G rollouts, and current research on 6G for making RF interconnect structures more prevalent. Substrate integrated waveguides are another tool for RF designers to create low-loss interconnect architectures for use at mmWave frequencies. Grounded coplanar waveguides provide low insertion loss beyond ~40 GHz, which is where microstrip and stripline routing configurations start to see a large drop in transmission at successively higher frequencies. Recent research into substrate integrated waveguide design has shown this new structure offers low insertion loss up to ~80 GHz frequencies, making this structure ideal for new 5G, radar, and other systems that run deep into mmWave bands.
Substrate Integrated Waveguide Design
Any substrate integrated waveguide design has a similar structure. If you think about the way a waveguide like an optical fiber works, it is easy to draw an analogy to substrate integrated waveguides. In a PCB, a substrate integrated waveguide is a structure with two parallel copper planes, forming the top and bottom of the waveguide. The walls of the waveguide are formed by plated through-hole vias that are drilled through the two layers (see Fig. 1). This type of structure is quite easy to manufacture using standard PCB etching and drilling processes. Note that the through-hole vias should be backdrilled to remove any remaining stub from the substrate. The alternative in an HDI structure is to use blind vias.
In this structure, the waveguide is filled with the dielectric, and the metal boundaries provide confinement of the electromagnetic wave in the structure. The level of confinement depends primarily on the via diameter (d) and the via spacing (s). As this structure is somewhat open, you need to properly design the geometry in order to provide confinement and prevent conducted and radiated emissions in other areas of the system. In essence, the via wall acts like a typical via fence in an RF PCB layout, which confines electromagnetic radiation within the parallel arrangement of vias.
Substrate integrated waveguide design and geometry.
Just like a standard rectangular or cylindrical waveguide, this structure has an infinite set of modes that correspond to the eigenfunctions of this structure. These modes are indexed using two integers n and m (or whatever other symbols you like). As this structure is forced with a specific frequency, the propagation constant along the z direction is defined in terms of the source frequency and the integers n and m. The spatiotemporal distribution of the electromagnetic field TE modes and the dispersion relation defining the (n, m) wavenumber are shown in Eq. (1).
Eq. (1): Electric and magnetic field function in a substrate integrated waveguide.
The total electromagnetic field is just the sum of fields from each mode. Note that the sine functions satisfy closed boundary conditions in the structure at the edges. As we are only dealing with TE modes, we can choose to excite a specific set of modes in the structure simply choosing the particular frequency. Note that, if you want to excite a very specific mode, you would need to excite the structure with the exact spatial distribution for mode (n, m). In practical terms, this is not possible.
One should immediately notice that the wavenumber in Eq. (1) will be complex for certain modes when the frequency is below the value for the n and m terms. This means that these modes will decay. You could excite modes (1, 1) and (1, 0), depending on the geometry, if the frequency were high enough, while all higher order modes with (n > 1, m > 1) would decay to zero. This allows the designer to engineer the electromagnetic field structure they need in their PCB.
Normally, most designers only care about the TE10 mode, unless they are working with multiplexing. The cutoff for the TE10 mode is rather simple and is shown in Eq. (2):
Eq. (2): Cutoff frequency for the TE10 mode.
If the source wavenumber is less than this value, then all modes will decay. This shows how the structure provides significant isolation against lower frequency signals in your board, allowing it to be easily used alongside other circuits that may run at lower frequency.
The waveguide structure can only provide the necessary level of confinement when the following conditions are met:
- The via spacing s must be less than double the via diameter d.
- The signal wavelength in the substrate (calculated using the signal frequency and the effective refractive index) must be greater than 5 times the via diameter d.
Similar conditions can be derived when multiple modes are excited with a single frequency. This allows a designer to easily engineer the field distribution required for a number of different passive RF devices.
What You Can Do With a Substrate Integrated Waveguide
Substrate integrated waveguide design allows a designer to engineer the electromagnetic field they need when working with other passive RF components. Some examples of these other components include:
- Directional/multi-port couplers. These structures offer an easy way to split a coherent wave into multiple directions with minimal losses.
- Antennas. All antennas must support a coherent electromagnetic wave, which then transmits into the surrounding environment. A substrate integrated waveguide can be used to create a slotted antenna by cutting a hole directly in the end of the waveguide. The ability to engineer the electromagnetic field distribution allows the directionality of the antenna to be easily engineered without using components like antenna switches or arrays.
- Amplifiers. RF amplifiers are designed to increase the field strength through interference in a resonant structure. The field can then be coupled into another component, such as a multi-port coupler or antenna.
All these applications rely on constructive interference involving one or more electromagnetic waves in a well-defined structure. A substrate integrated waveguide design and other structures are extremely useful in the V band/M band and higher, where active RF components are still lacking in terms of performance.
Perhaps the greatest advantage for use in mmWave circuits is the reduced losses in the V band/M band. Similarly, the structure has significant isolation, allowing it to be easily used alongside other circuits on standard PCB materials. The mode structure can be further engineered by simply choosing the appropriate laminate with the desired dielectric constant. Designers can also easily implement these structures with standard PCB design and simulation software.
PCBs designed to operate at mmWave frequencies take the right design tools and analysis experience. Substrate integrated waveguide design and fabrication planning is much easier when you work with a knowledgeable research and design firm. If you’re looking for a knowledgable firm that offers cutting-edge PCB design services and technology research services for innovative electronics companies, contact NWES for a consultation.