Except for the most basic designs, system clock rates are inexorably creeping upward across a wide range of applications, from 5G telephony to automotive advanced driver assistance systems (ADAS), medical, industrial, and mil/aero systems. As frequencies rise into the GHz range, the need for impedance control to minimize signal attenuation and protect signal integrity gets more critical and complex.
Controlling dielectric losses is challenging and gets more challenging as the frequency increases. At a low frequency, conductor losses dominate. As the frequency increases, dielectric losses increase and become the primary source of signal attenuation. Both conductor losses and dielectric losses increase at higher frequencies (Figure 1). The increases are the result of dielectric absorption and the skin effect:
- Dielectric absorption results from energy absorbed by the printed circuit board (PCB) insulator and increases signal attenuation.
- Skin effect results from high-frequency signals being mainly confined to the surface (the skin) of a conductor, reducing the effective conductive area, increasing resistance, and signal attenuation.
In addition to attenuation, high-frequency operation results in distortion. The losses that cause signal attenuation unequally impact various signal harmonics. As successively higher harmonics are increasingly attenuated, the signal is distorted as well as weakened. Attenuation causes slower signal rise times, rounding the front edge of the signal and increasing the likelihood of data errors (Figure 2). Dealing with attenuation-related distortion can be more challenging than dealing with the attenuation itself.
In addition to the basics of dielectric absorption and the skin effect, there are numerous ‘second order’ factors that impact signal attenuation on PCBs:
- Noise causes signal interference that increases attenuation; reducing noise can help reduce attenuation.
- Longer PCB trace lengths increase attenuation. Shorter traces contribute to lower signal attenuation.
- Wider PCB traces attenuate signals less than narrow traces, but there can be a tradeoff between wide traces and crosstalk.
- Crosstalk in nearby traces increases attenuation. The use of wider PCB traces can result in smaller spacings between traces and increased crosstalk.
- Signal interfaces and connectors. Moving a signal through different conductor materials and connector interfaces increases attenuation.
- Transmission frequency needs to be optimized. Higher frequency signals suffer from greater attenuation.
- Ground loop resistance also increases with frequency; the ground loop narrows, using less copper area, and increases resistance and attenuation.
- Copper surface roughness can be a hidden source of attenuation. Losses are large if the surface roughness is greater than the skin depth. The roughness must be less than the skin depth for minimum attenuation (Figure 3).
Laying the foundation
PCB dielectric materials provide the non-conductive foundation layer supporting the conducting copper layers in a circuit board. PCB dielectrics have many important properties related to electrical, thermal, mechanical, and chemical performance. Choosing the appropriate PCB dielectric material is important and can be challenging when a circuit is required to handle high power, high frequency, and elevated temperatures. This FAQ is narrowly focused on signal attenuation in PCBs.
The dielectric constant, or, more correctly, the relative permittivity, is the measure of the insulation provided by a material. It also indicates the degree to which an electromagnetic wave slows down in the material. The relative permittivity of a PCB material can depend on the frequency of the embedded system. A stable relative permittivity over a wide range of frequencies is preferable to ensure safe and stable operation.
FR-4 is a group of materials that meets the National Electrical Manufacturers Association’s (NEMA) standards. FR-4 is a woven glass fiber epoxy compound with fire retardant (FR) properties and self-extinguishing flammability characteristics. The relative permittivity, or dielectric constant, of FR4, ranges from 3.8 to 4.8, depending on factors like the glass weave, thickness, resin content, and copper foil roughness. Collectively, FR-4 materials represent the most used PCB dielectric.
Composite epoxy materials (CEM) are a group of composites made from woven glass fabric surfaces and a non-woven glass core combined with epoxy synthetic resin. Like FR-4, CEM PCB materials are listed by NEMA. CEM-1 and CEM-3 are the most used. CEM-1 materials combine paper, woven glass epoxy, and phenol compounds and are often used in single-sided PCBs. The dielectric properties of CEM-1 are like FR-4, with a dielectric constant of about 4.2. CEM-1 can provide a lower-cost alternative to FR-4. CEM-3 materials are more suitable for double-sided PCBs with plated holes. CEM-3 has a higher dielectric constant of 5.0.
Polytetrafluoroethylene (PTFE) is a synthetic polymer of carbon and fluorine suited for high-speed, high-frequency, microwave, and RF power PCBs. Depending on the formulation and construction PTFE PCBs have a dielectric constant of 2.5 to 2.8.
Polyimide is used for flex circuits and has a dielectric constant of 3.8. It provides high mechanical strength, chemical resistance, and high flexibility over a wide operating temperature range. In addition to its flexibility, polyimide’s properties make it suitable for high-power, high-frequency microwave PCBs.
Dealing with signal attenuation
After selecting the optimal PCB material for a project and considering the first-order and second-order attenuation sources, designers can still use more tools to deal with signal attenuation.
Use a repeater and/or an amplifier to strengthen an attenuated signal. A repeater regenerates the original signal by reducing attenuation. It also enhances the range of the signal so that it can be transmitted over longer distances without failure. Because repeaters alter the signal, they are effective at reducing noise. An amplifier can also be used to strengthen an attenuated signal. Amplifiers operate on both the signal and noise components of the transmission. An amplified signal can be noisier than a regenerated signal (Figure 4).
Use pre-emphasis to boost a signal’s high-frequency components while leaving the low-frequency components unchanged. Pre-emphasis works by boosting the high-frequency energy every time a transition in the data occurs. The data edges contain the signal’s high-frequency content. The signal edges deteriorate with attenuation and the loss of the high-frequency signal components. A two-tap finite impulse response (FIR) filter can provide a pre-emphasis for a signal. The FIR compares the previously transmitted bit with the current bit. The current bit is transmitted when the two bits are at the same level. If the bits are not the same, the current bit is transmitted at the higher magnitude. Pre-emphasis techniques can also help overcome the effects of pattern-dependent jitter, including rounded signal edges, loss of amplitude, and displacement in time.
Use receiver equalization to compensate for signal attenuation. One of the sources of attenuation is that data channels often have transfer functions with much lower bandwidths than the data frequency. Equalization circuitry processes the data through a transfer function inverse of the data channel transfer function. It attenuates the signal’s low-frequency components as they arrive at the receiver to compensate for line losses.
Use a programmable differential output voltage (VOD) setting to match the signal drive strength with the line impedance and the PCB trace length. Increasing the VOD of the driver will enhance the signal of the receiver. A lower VOD swing can reduce power consumption, while a higher VOD swing improves voltage margins at the receiver.
To deal effectively with signal attenuation, designers must select the best PCB substrate material with optimal electrical and mechanical properties and a suitable dielectric constant and consider both first-order and second-order causes of attenuation. In addition, designers have various circuit design tools available that can further optimize attenuation-related performance in high-speed, high-frequency, and high-power RF and microwave applications.
Basic Principles of Signal Integrity, Altera/Intel
Common PCB Dielectric Materials and Their Properties, Cadence
Difference Between Repeaters and Amplifiers, JavaTpoint
High-frequency compatible copper foil, Furukawa Electric
PCB traces and dielectric attenuation at the end of the trace, iPCB Circuits
What is insertion loss, Polar Instruments