Understanding Signal Integrity
When a signal is transmitted, the received signal will always be distorted as a consequence of natural impedance and other effects. This is why designers work toward minimizing anything that may affect signal integrity. They try to limit the extent to which electronic noise, inductive coupling, capacitance and line resistance change the shape and amplitude of the signal. As signal frequency increases, these effects are magnified, and special care is needed to control their undesirable effects in electronic circuitry.
Many PCBs now operate at digital signal frequencies of 10 GHz and higher, and this means that appropriate measures are needed to prevent unacceptable signal degradation and their consequential errors.
Key Factors That Cause Signal Degradation
Several factors contribute toward signal degradation. These include the characteristics of the signal, system impedance, propagation delays, attenuation, crosstalk, voltage fluctuations and electromagnetic interference.
Signal characteristics: A digital signal is, in theory, a square wave, but in practice it takes a finite time for the signal to switch from one state to another, so there’s always a degree of signal distortion present that is related to the frequency response of the circuit. The rise time of the signal determines the maximum data transmission rate that is possible and is often measured by calculating the signal knee frequency. The goal of circuit design is to manufacture a circuit with a flat response at all frequencies up to the knee frequency.
Impedance: Changes in the impedance that a signal sees causes reflections, ringing and distortion. The degree of interference is exacerbated by the higher frequencies associated with digital circuits. PCB trace branches, line stubs, connector pins and vias all create impedance discontinuities.
Propagation delays: Signals that travel different distances or through different mediums do not arrive at their destination at the same time. These discrepancies, called signal skew, cause signal sampling errors, particularly at high clock frequencies.
Attenuation: The amplitude of a signal is attenuated by the resistance of PCB traces and the board’s dielectric dissipation factor. The effect is more noticeable at high frequency due to a tendency for high frequency signals to travel along the surface of traces. Attenuation leads to slow signal rise times and increases the possibility of data errors.
Crosstalk: Rapid voltage and current transitions induce voltages in adjacent traces due to inductive and capacitive coupling. These voltage spikes are known as crosstalk and may cause data errors.
Supply voltage fluctuation: As devices switch, the current that flows generates a voltage drop in supply and ground rails. This in turn leads to fluctuations in the supply voltage at each device, and the cumulative impact of this effect creates noise and may result in high bit error rates.
EMI: Every switching operation creates a certain amount of noise, and its intensity is magnified because devices switch at the clock frequency. This noise can be radiated by traces acting as antenna. The strength of the radiated signal is proportional to switching frequency and may cause unwanted interference.
Steps to Mitigate Signal Degradation
It’s essential that signal degradation is minimized by addressing the factors that contribute toward signal interference because their cumulative effect significantly decreases the stability and reliability of high speed digital circuitry. Here are some guidelines to reducing signal degradation.
Impedance matching: At digital circuit frequencies, signal lines act like transmission lines, and designers should match the impedance of the source, receiver and signal traces to minimize reflections. Terminating resistors may be used, the length of stub traces reduced and devices daisy chained. Keep signal lines short and use wide return paths.
Minimize effects of propagation delay: Signal skew can be minimized by matching the length of signal traces.
Reduce signal attenuation: The use of low-loss dielectric materials and low resistance traces will minimize signal attenuation.
Reduce crosstalk: Maximize the distance between signal traces and use wide return paths and, if possible, uniform planes as the ground plane. Don’t split the return paths, and use low dielectric material in the PCB construction. Consider using differential signaling, which is less sensitive to the effects of crosstalk.
Minimize supply voltage fluctuations: The use of power and ground planes mounted on the outer PCB layers and covering as much surface area as possible will reduce volt drops. It also helps to keep all leads short and to use multiple decoupling capacitors placed close to the device power pins.
Reduce EMI: Use a ground plane, or alternatively, route the signal return line under the signal line, to minimize creating a loop that may radiate radio frequencies. Keep trace inductances low to limit radio frequency radiation.
Pulling It Together
Good PCB layout and design is a complex task with many challenges. Apart from the need to route traces to where they are needed, the capacitive and inductive effects of these decisions should be evaluated. Special attention must be paid to ensuring line impedances are carefully matched and discontinuities avoided. Low-loss PCB dielectric materials should be chosen to limit stray capacitances. The use of ground and power planes go a long way toward minimizing noise by limiting power supply fluctuations, reducing inductive coupling and avoiding EMI.