Much of the buzz on the show floor at the 2017 IMS in Honolulu was about millimeter-wave devices and circuits. At one time, frequencies above 30 GHz were considered “exotic” and only for military or scientific applications. But times have changed, and available spectrum is scarce. Millimeter-wave frequencies are now used in commercial vehicular radars, and big plans are being made for these small wavelengths in Fifth Generation (5G) wireless communications networks, in support of moving massive amounts of data quickly. More and more design engineers are faced with developing practical millimeter-wave circuits to 77 GHz and beyond. But first, they must decide upon the best transmission-line technology for those high frequencies as well as the circuit material that can support those circuits with quality, low-loss signal propagation. Drivers and cell-phone users everywhere will be counting on them!
At microwave frequencies, microstrip is by far the most popular transmission-line technology, compared to stripline and coplanar waveguide (CPW). It has a signal plane on the top copper layer and bottom ground plane. It is relatively simple and cost-effective, and allows surface mounting of components for ease of construction.
Unfortunately, as signal frequencies move into the millimeter-wave range, microstrip circuits can behave like antennas, radiating electromagnetic (EM) energy away from a desired signal propagation path and resulting in much higher radiation losses than at lower frequencies. Microstrip radiation losses are also dependent upon the thickness and dielectric constant (Dk) of the circuit substrate material. Thinner substrates suffer less radiation loss than thicker substrates. Also, circuit materials with higher Dk values have less radiation loss than circuit materials with lower Dk values.
In microstrip, the effective Dk is a combination of the Dk of the substrate material and air, since EM waves in a microstrip transmission line propagate in part through the dielectric and in part through the air above it. In contrast to microstrip, stripline is like a flattened coaxial cable. It consists of a conductor surrounded by top and bottom dielectric layers which in turn are covered by ground planes. The Dk of stripline is the same as that of the dielectric material, since air is not involved in the propagation process.
CPW circuits are fabricated with a number of variations, including as standard, grounded coplanar waveguide (GCPW), and conductor-backed coplanar waveguide. Standard CPW metallizes parallel conductors (in the form of a flat waveguide) on the top of a dielectric layer, with ground metal areas just beyond the conductors. GCPW adds a bottom ground-plane layer but requires plated-through-hole (PTH) viaholes through the dielectric substrate material to connect the top and bottom ground planes. The extra ground planes on the top copper layer helps GCPW achieve high isolation between signal lines and can be designed to minimize spurious wave propagating modes. Placement of the PTH viaholes is critical, and can impact transmission-line impedance and loss.
Like microstrip, GCPW has an effective dielectric constant as the result of EM waves propagating through the dielectric material as well as through the air around the conductors. GCPW, like microstrip, also allows surface-mounting of components for ease of fabrication, in contrast to stripline where PTH vias need to connect the components on the outer circuit layers to the inner signal layer. In terms of millimeter-wave frequencies, GCPW has lower dispersion than microstrip, with less radiation loss, and is capable of supporting higher-frequency propagation than microstrip circuits. GCPW also achieves more effective suppression of spurious propagation than microstrip, and is more amenable to practical signal-launch configurations (such as from waveguide, cables, and connectors) at millimeter-wave frequencies than microstrip.
Finding the Right Circuit Material
If GCPW is the optimum transmission line for millimeter-wave circuits, it should then be fabricated on a circuit material with optimum characteristics for millimeter-wave frequencies. Since signal power tends to decrease with increasing frequency, an optimum circuit material for millimeter-wave circuits should have low loss at those high frequencies. The insertion loss of millimeter-wave transmission lines is due mainly to the aforementioned radiation losses, conductor losses, and dielectric losses. Radiation losses tend to be design-specific, whereas conductor and dielectric losses will depend upon the choice of circuit material.
Dielectric losses are a function of the type of dielectric material, and usually well defined by a material’s dissipation factor (Df), with lower values indicating lower dielectric losses. A circuit material capable of consistent performance at millimeter-wave frequencies will also exhibit minimal variations in Dk, so that dielectric losses do not change dramatically with frequency.
In considering circuit materials for millimeter-wave circuit applications, the thermal coefficient of dielectric constant (TCDk) parameter provides reliable insight into the stability of a material’s Dk with temperature. The TCDk parameter provides an understanding of what to expect of a particular circuit material’s performance at millimeter-wave frequencies, with lower TCDk values indicating less change of Dk with temperature and less variations in frequency phase response resulting from variations in Dk with temperature.
Conductor losses can be traced to a number of variables at millimeter-wave frequencies, including the surface roughness of the copper conductors and the choice of plated finish for the conductors. Copper is an excellent conductor, but increasing surface roughness results in increasing conductor loss and greater propagation phase delays. The main area of concern for copper surface roughness is at the copper-substrate interface, with conductor loss due to the copper surface roughness increasing as a function of increasing frequency. The small wavelengths of millimeter-wave signals result in less skin depth in the circuit material as part of EM propagation, and circuit materials with greater copper surface roughness will more severely impact the insertion loss and phase response at millimeter-wave frequencies. The effect of copper surface roughness on insertion loss is also dependent upon the thickness of the circuit material, with thinner circuits affected more by copper surface roughness than thicker circuits.
At millimeter-wave frequencies, circuit materials with excessive copper surface roughness will have more impact on the conductor loss of microstrip circuits than on the conductor loss of GCPW circuits. Switching to a circuit material with smoother copper finish will bring less of an improvement in conductor-loss performance for a GCPW circuit than for a microstrip circuit, especially at millimeter-wave frequencies. In particular, tightly coupled GCPW circuits, which feature closely spaced conductors and ground areas, are less subject to the effects of copper surface roughness than loosely coupled GCPW circuits (with greater spacing between conductors and ground).
An optimum circuit material for millimeter-wave circuits should cause minimal phase angle variations, since such behavior can be critical to many millimeter-wave applications, such as 77-GHz vehicular radar systems. By minimizing variations in certain material-based attributes, such as copper thickness, Dk, conductor width, and substrate thickness, variations in phase angles can be minimized at millimeter-wave frequencies.
Additional details on finding the right combination of circuit material and transmission-line technology are available as part of the Microwave Journal webinar, “Design Considerations and Tradeoffs for Microstrip, Coplanar and Stripline Structures at Millimeter-wave Frequencies,” presented by John Coonrod of Rogers Corp.
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