Signal Distortion in Your PCB: Sources and Solutions
Length matching for high-speed signals is all about synchronization...
Signal distortion often gets a passing mention in many discussions on signal integrity and circuit analysis. As more networking products start running at higher speeds and use complicated modulation schemes, you’ll find that signal distortion becomes a serious problem that contributes to bit error rates. Distortion sources are cited as one of the primary bottlenecks preventing faster data rates in electrical interconnects.
The same problems can be seen in analog signals, particularly those that run at 10’s of GHz frequencies. More designers in the RF/wireless domain will need to understand these distortion sources during design, test, and measurement.
Linear vs. Nonlinear Signal Distortion
All sources of signal distortion can be classified as linear or nonlinear. They differ in terms of harmonic generation. Sources of nonlinear distortion generate harmonics as a signal propagates through the source, while linear signal distortion sources do not generate harmonics. Both sources of distortion can alter the magnitude and phase of the frequency components that make up a signal.
The different sources of signal distortion will affect different types of signals (analog or digital) in different ways, depending on the bandwidth of the distortion source and the frequency content in the particular signal. Different sources of distortion also have different effects on modulated signals, depending on the type of modulation.
Obviously, the breadth of different signal distortion sources is broad and we cannot cover every source in detail. However, we can summarize a few important sources of linear and nonlinear signal distortion in your PCB traces and components.
Linear Distortion Sources
Frequency response and phase distortion. If you’re familiar with frequency sweep simulations in linear circuits, then you know that a transfer function defines the change in phase and amplitude of a signal in a linear circuit. The transfer function of a circuit, specific component, or interconnect will apply a phase shift and will adjust the magnitude of the signal. These changes in the phase and magnitude are functions of frequency and are visualized in a Bode plot. This means different frequency components are delayed by different amounts, and these different frequency components are amplified or attenuated by different amounts.
Discontinuities. This broad class of distortion sources includes impedance discontinuities along an interconnect (e.g., vias and trace geometry) and discontinuities in material properties (e.g., from the fiber weave effect).
Dispersion distortion. This arises due to dispersion in a PCB substrate, conductors, and any other material in your board. This source of distortion is unavoidable, although it can be small enough that it is unnoticeable when interconnect lengths are short. Dispersion in the substrate causes different frequency components in a digital signal to travel along a trace at different velocities. Dispersion also affects the loss tangent seen by a signal on a trace, which contributes to signal distortion. This causes a pulse to stretch (i.e., the group velocity becomes frequency dependent), similar to what happens in ultrafast lasers without dispersion compensation.
One solution for compensating dispersion in a PCB interconnect is to use a DSP algorithm, or to use a layered substrate weave with alternating positive and negative group velocity dispersion such that the net dispersion is zero in the relevant frequency range. This particular topic is broad enough that it deserves its own article. Take a look at this excellent article in Signal Integrity Journal for a complete discussion of dispersion in PCB traces.
Dispersion is the same effect that causes a prism to split light
Nonlinear Distortion Sources
Nonlinear frequency response and phase distortion. In the same way as in the linear case, nonlinear circuits can distort frequency components in a signal by different amounts, depending on frequency and input signal level. This occurs in amplifiers, ferritic components, and other transistor-based devices once they reach saturation.
Intermodulation distortion. This type of amplitude distortion (both the active and passive variety) occurs when two frequency components are input into a nonlinear circuit. This occurs in 5G-capable devices as the two signals used for carrier aggregation interfere with each other (passive intermodulation). It also occurs in any nonlinear component that is used to manipulate a modulated signal, such as in power amplifiers in an RF signal chain.
Harmonic distortion. This is the second type of amplitude distortion. This occurs when a signal is input to a component or circuit that saturates. In effect, this causes the amplitude of a signal to level off (called clipping) once the input exceeds a certain level.
Harmonic signals are effectively immune to linear frequency response and phase distortion. As an example, a filter or passive amplifier circuit (such as an LC oscillator) will induce a phase shift and change in the amplitude of the input signal, but no additional harmonics will be generated. The same applies to dispersion distortion as the signal contains only a single frequency component. Discontinuities can distort the signal as it travels along an interconnect, effectively creating lower amplitude copies of the signal that are superimposed on the original.
All nonlinear distortion sources cause harmonic generation in analog signals. The only way to solve these problems is to work in the linear range for all components and enforce impedance matching. Component manufacturing imperfections, and roughness on microstrip and stripline traces, are also responsible for nonlinear distortion at mmWave frequencies.
Because digital signals are composed of multiple frequency components, they are particularly sensitive to frequency response and phase distortion. In the linear case, this causes different frequency components to be delayed and attenuated by different amounts. The result is a change in the shape of the component. If discontinuities and dispersion are added into the mix, portions of the signal can be delayed, effectively stretching the signal. In the case of signal reflections at impedance discontinuities, this can lead to ghosting when the distance between two discontinuities is longer than the spatial span of the signal. This can also produce the well-known stair-step response in digital signals seen on transmission lines.
Signal reflections from impedance discontinuities can produce ghosting. Image source: wirelesswaffle.com
Nonlinear distortion sources also cause harmonic generation in digital signals, creating unique changes in the signal spectrum and in the time domain. When a signal input to an amplifier switches faster than an amplifier can respond, intermodulation distortion will be seen in the output from the amplifier. This particular type of distortion is called slew-induced distortion as it is related to the slew rate of the input signal.
The powerful post-layout analysis tools and schematic simulation tools in Altium Designer® are ideal for creating and analyzing complicated PCB layouts. You can examine the effects of different signal distortion sources alongside your standard PCB design tools. You’ll also have a complete set of tools for documenting all aspects of your project, managing your supply chain, and preparing deliverables for your manufacturer.