When Digital Signals Reach Microwave Frequencies
Feb 23, 2011
John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University. www.rogerscorp.com/acm
Digital circuit design once had less demands. When clock speeds were 100 MHz or less, signal loss wasn’t an issue. Digital circuits, in fact, have long been designed to be more tolerant of signal level variations than analog circuits. But with digital circuits continuing to increase in speed, they are assuming more of the characteristics of analog microwave signals, and requiring more attention to design detail and even choice of PCB material as in the case of high-frequency analog circuits.
When 100 MHz was considered fast, the choice of a low-cost laminate like FR-4 was a sound decision. If anything, the performance provided by an epoxy-based circuit-board material like FR-4 was often better than necessary for most digital circuit designs. In early digital circuits operating at 100 MHz or less, it was important for a PCB material to provide a consistent relative dielectric constant across the material in order to maintain circuit traces with consistent impedance and maintain even timing of digital signals across the circuit. In an analog/microwave circuit, consistent impedance is also important, since impedance mismatches can cause unwanted signal reflections and shifts in signal phase and frequency.
But times have changed and now digital signals travel at microwave speeds. The same design guidelines and choices in PCB materials used for lower-speed digital circuits can’t be applied at higher operating speeds without penalties, such as timing problems, signal crosstalk, and even electromagnetic-interference (EMI) problems. High-speed signals more and more resemble the fast pulses of microwave radar systems, and designers of high-speed digital circuits must think more like microwave circuit engineers to create successful designs.
Digital circuit designers have come to learn what microwave circuit designers have known for years: the choice of PCB material can have a great impact on the performance of high-speed digital circuits as well as analog microwave circuits. Digital circuit designers once paid little attention to the loss of a PCB, since that material characteristic had little effect on digital functionality at lower speeds. But analog microwave systems, such as receivers, depend on signal strength, and any loss in a system or its components, including the PCB material, must be minimized. For that reason, microwave circuit designers have paid attention to a laminate’s dissipation factor, choosing materials with low values corresponding to low loss. As digital circuits move to multi-GHz speeds, loss can also be a concern, especially for its effect on digital signal integrity.
Losses in a high-speed or high-frequency PCB can stem from a number of sources, including dielectric losses (especially above about 3 GHz), conductor losses, ground-plane losses, and even losses attributed to the surface roughness of a PCB’s conductive metal used to form signal and power traces. The performance of a high-speed digital PCB can also be affected by crosstalk between signal traces on one layer of the PCB or between layers of a multilayer PCB, by noise from the power and ground planes due to inadequate decoupling, and by external radiated and conducted noise sources. High-speed digital circuit designers must also control levels of EMI to meet global regulatory requirements.
Microwave circuits may never match the complexity of high-speed digital circuits in terms of number of layers in a multilayer PCB assembly, but there are a number of parallels that can be drawn between the two circuit technologies. In fact, it is not unusual to find both types of circuit on one multilayer- circuit construction. Both benefit from PCB materials with typical low values of relative dielectric constant that are also consistent across a substrate and with frequency. Both circuit approaches can benefit from materials with low dissipation factor for low loss, which translates into good signal integrity in a high-speed digital circuit and low insertion loss in an analog microwave circuit.
Relative dielectric constant is only one of many different PCB material parameters, but it can be used as a starting point when considering laminate choices for a high-speed digital circuit design. Ideally, a PCB material for high-speed digital circuits would have isotropic dielectric constant characteristics—that is, with dielectric constant values that are about the same in the x, y, and z directions of the material. Another benefit would be to have the dielectric constant remain within a narrow range of values across a wide frequency range. Not only would this support broadband microwave use, but would ensure excellent signal integrity in high-speed digital circuits.
Unfortunately, most PCB materials are anisotropic in their relative dielectric constant values, with different values in each direction. And most exhibit dielectric constants that decrease with increasing frequency. For high-performance digital as well as microwave circuits, it can be helpful to select PCB materials that are as closely matched in relatively dielectric constant in all three axes, and with dielectric constant that is fairly consistent with frequency. For an increase in dielectric constant, the impedance of a circuit trace or transmission line will decrease and the speed of a wave through the trace or line will slow. In a digital circuit, a material with dielectric constant that significantly decreases with frequency can cause digital signal edges to reflect more at higher frequencies than at lower frequencies, causing timing problems. The problem can be minimized by selecting a PCB material with relative dielectric constant that is as flat as possible with frequency (low dispersion). The chart in figure 1 displays dispersion when measured using a stripline transmission line. Typically stripline transmission lines are considered a non-dispersive medium so what effect is shown with dielectric constant in relation to frequency is likely material dispersion.
One other material parameter that is worth noting in selecting substrates for high-speed digital circuits is the coefficient of thermal expansion (CTE) in the z-axis (through the thickness) of a laminate. This parameter is one indicator of the expected reliability of plated through holes (PTHs), which are plated vias used to make signal, power, and ground connections between the different layers in a multilayer PCB.
A low-dielectric-constant PCB material can support good performance in high-speed digital circuits, but it must also be a material that is compatible with the processing steps used to fabricate the multilayer circuits typical of digital designs. As an example, Theta™ circuit materials from Rogers Corporation are low-loss, low-dielectric-constant materials with excellent thermal and mechanical properties for high-layer-count circuit boards. They maintain dielectric constant within a fairly narrow window with frequency, with z-axis values of 3.90 at 1 GHz and 4.01 at 10 GHz. Theta materials also have a z-axis CTE (about 50 ppm/°C) that is approximately 30% lower than that of FR-4, and more closely matched to that of copper, for highly reliable PTHs in multilayer circuits. These materials are halogen free and compatible with lead-free soldering processes. They have the mechanical traits needed for multilayer digital designs, but also the excellent electrical characteristics suitable for both microwave and high-speed digital circuits or, in fact, both types of circuits within the same multilayer PCB.