Millimeter-wave frequencies (about 30 to 300 GHz) were once associated with at least two things: circuits for these frequencies are extremely difficult to fabricate, and they will probably be used for some military-electronics application. However, the United States’ Federal Communications Commission (FCC), among other organizations around the world, is doing its part to free wide portions of bandwidth for unlicensed radio use at millimeter-wave frequencies.
The FCC is treating wide millimeter-wave bandwidths, such as the 7 GHz span centered at 60 GHz (57 to 64 GHz) as Industrial-Scientific-Medical (ISM) band frequencies so that they can be used for commercial and other unlicensed applications by the general public. Because these frequencies are available for use without licenses, a growing number of circuit designers are considering different applications at these higher frequencies and, of course, choosing the right printed-circuit-board (PCB) material is an important part of any practical efforts to realize millimeter-wave circuits.
Millimeter-Wave ISM Bands
Organizations such as the FCC have set aside a number of different millimeter-wave bands for unlicensed use in addition to 60 GHz, such as 94, 140, and 220 GHz. Receivers and transmitters at these frequencies are currently being produced in the form of integrated circuits (ICs) based on gallium arsenide (GaAs) and even silicon semiconductor processes, such as silicon CMOS and silicon germanium (SiGe) technologies. As the speeds of computers increases, and the demand for faster Internet access grows, the fast (better than 1-Gb/s) data rates available in these unlicensed millimeter-wave ISM bands makes the use of millimeter-wave links attractive for a variety of short-range communications links.
For example, fixed-frequency millimeter-wave wireless links offer the bandwidth possible with fiber-optic links, but with a fraction of the time and cost required to install a high-speed fiber-optic communications link. For this reason, millimeter-wave links are popular solutions for providing radio backhaul for cellular-communications base stations.
Of course, while government agencies around the world may be freeing millimeter-wave frequency bands for unlicensed use, the task of designing and fabricating circuitry at these elevated frequencies has not become any easier. This blog series has already taken a look at some of the circuit-material characteristics that can impact the performance of millimeter-wave circuits. For years, military-electronic systems have employed phased-array radar systems at millimeter-wave frequencies. And millimeter-wave frequencies have been used extensively in high-end automotive electronic systems, including for long-range adaptive cruise control at 77 GHz and anti-collision systems at 79 GHz. At higher frequencies, millimeter-wave circuits have been part of airport security and imaging systems at 94 GHz. But with increasing opportunities for millimeter-wave circuit applications, especially for communications at ISM bands, it may help to review some of the key circuit material considerations when working at millimeter-wave frequencies.
Circuit Material Considerations at Millimeter-Wave Frequencies
Wavelengths for signals from 30 to 300 GHz are extremely small, from about 1 cm to 1 mm. Although this translates into reduced circuit dimensions, it also makes possible the use of modest-sized antennas with focused beamwidths. As an example of the reduction in size that is possible at these higher frequencies, an antenna with 1-deg. beamwidth for a line-of-sight communications link at 3.5 GHz has a nominal diameter of 12 ft. But for a line-of-sight link at 60 GHz, an antenna with a 1-deg. beamwidth is a mere 8 in. in diameter.
In terms of millimeter-wave circuitry, it is important to remember the impact of various circuit parameters on performance at millimeter-wave frequencies. Circuit designers typically work with a material that is familiar to them based on such characteristics as dielectric constant and dissipation factor, using those parameters where possible in a computer simulation program to project the performance of a particular circuit configuration. Because the physical size of a high-frequency circuit transmission line is dependent on the dielectric constant of the PCB material, the value of the circuit material’s dielectric constant is particularly critical at millimeter-wave frequencies, where circuit dimensions can be so small.
For this reason, PCB materials with the lowest possible dielectric-constant values are to be preferred for millimeter-wave circuit applications. Circuit dimensions shrink with higher values of dielectric constant, but reducing the size of necessarily small circuit dimensions can make those circuits difficult to fabricate with consistency.
The consistency of the dielectric constant across a circuit board can also be an important concern at millimeter-wave frequencies since, at those frequencies, variations in the dielectric constant can introduce variations in the signal phase. For a given consistency of dielectric constant, the phase variations will increase with increasing frequency, and will hinder the performance of circuits that depend on reliable phase behavior, such as in phase-modulated communications systems and phased-array radar systems.
Millimeter-wave phase performance can also be affected by the composition of the PCB material. For example, circuit materials that are reinforced using a glass weave can exhibit phase-based problems when the glass weave is not consistent throughout the material. In a manner somewhat akin to an inconsistent dielectric constant, this can lead to perturbations in a circuit’s signal propagation velocity, which cause signal integrity issues, including uneven phase performance. The unwanted results can be distortions in phase modulation and errors between phase-matched channels in radar systems.
Minimizing Atmospheric Losses
Millimeter-wave communications links typically support high data rates at line-of-sight distances to about 1 km, but they are subject to atmospheric losses even for such short links. To minimize losses at those higher frequencies, PCB materials with the lowest possible dissipation factors should be used for millimeter-wave circuits. The quality of a PCB’s conductor surface can also play a role in loss performance at millimeter-wave frequencies. A rough copper surface will yield higher conductor losses at higher frequencies, so that selecting a PCB material with smooth copper conductor surface can help minimize loss at millimeter-wave frequencies.
The thickness of a PCB’s conductor layer can also be a concern at millimeter-wave frequencies because of a parameter known as skin depth. Skin depth refers to the thickness into the conductor material at which a propagating electric field has decreased by about 37%. Skin depth decreases rapidly with increasing frequency, at about 6.6 μm at 100 MHz, about 0.66 μm at 10 GHz, and about 0.2 μm at 100 GHz. With the small skin depth at millimeter-wave frequencies, it is easy to see the impact that copper conductor roughness can have on loss performance.
The thickness of the PCB material is also a consideration at millimeter-wave frequencies, since moding effects and unwanted resonances can result from the use of a thick circuit material with such small wavelengths. Circuit materials used for millimeter-wave circuits are typically in the thickness range of 2 to 10 mils. As they do with increasing dielectric constants, circuit line widths also shrink with thinner PCB materials. Since circuit loss decreases with the increasing thickness of circuit dielectric materials, selecting a PCB material for millimeter-wave applications is something of a tradeoff between creating stable, practical, and producible circuits and achieving low loss for those circuits.
Given this challenging set of requirements, what types of real-world materials are suitable for millimeter-wave circuits? Two examples are RT/duroid® 5880 and RT/duroid 5870 laminates from Rogers Corp. Both are PTFE-based composite materials with low dielectric constants, good consistency of dielectric constant, and low loss. RT/duroid 5880 laminate has a dielectric constant of 2.20 in the z direction at 10 GHz with dissipation factor of a low 0.0009 at 10 GHz. RT/duroid 5870 laminate has a dielectric constant of 2.33 in the z direction at 10 GHz with dissipation factor of 0.0012 at 10 GHz. These materials can be supplied in sheets as thin as 3.5 mils for excellent performance at millimeter-wave frequencies.
In summary, when working at millimeter-wave frequencies, circuit materials should ideally be electrically homogeneous, as thin as possible, with low dielectric constant, low dissipation factor, and with a smooth conductor surface.
Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.