Breaking Down High Speed Design: Switching Speed, Rise Time, and EMI Coupling
Kids these days have no idea why electromagnetic coupling between nearby circuits is called “crosstalk.” Back when phones plugged into the wall and they didn’t include a touch screen or internet access, it was possible to hear faint whispers of other conversations bleed through into your phone line.
Understanding crosstalk and EMI in general, whether within a PCB or from some external source, requires understanding how digital signalling defines the transition to high speed behavior. In a real PCB, high speed signalling can influence EMI in your board, and addressing these problems takes some creative design and routing strategies.
High Frequency Analog vs. High Speed Digital Signalling
Compared to digital signals, analog signals are simple. The main factor to concern yourself with is the signal frequency and propagation delay due to the finite speed of electromagnetic waves. A comparison between the oscillation period (i.e., inverse of the signal frequency) and the propagation delay in a given interconnect determines whether you need to worry about transmission line behavior and whether trace termination becomes critical.
Many engineers I have taught in the past do not think of digital signals as waves, rather they look at digital signals as being either on or off, where the electric field exists everywhere throughout the interconnect that carries the digital signal. At very low interconnect lengths, this is technically correct, as the constant electric field along the wire or PCB trace does not create any EMI problems elsewhere in the PCB via induction.
If you are using a modulation technique with analog signals for an antenna or optoelectronic component, you’ll need to be mindful of the electromagnetic damping over time due to the equivalent RC constant in the circuit. This transient behavior determines the bandwidth and power spectrum for your analog signal. This same issue applies in digital signalling, although digital signals have a much broader power spectrum.
Compared to analog signals, digital signals are really a combination of a large number of analog waves, all with different frequencies. The lowest frequency in the power spectrum corresponds to the digital switching speed, and the frequency content extends out to infinity. The other important frequency in a digital power spectrum is the knee frequency, which is equal to approximately one-third of the inverse of the signal rise/fall time (from 10% to 90%).
So we have two important parameters that determine how digital signals can cause EMI in other areas of a PCB: the switching speed and the rise time. Approximately 70% of the signal power is concentrated between the switching frequency and the knee frequency, and any element with an inductive resonance frequency between these two frequencies is highly susceptible to EMI. Some examples include vias on signal lines and on power/ground busses.
Waveform on a light blue background
Breaking Down the Term “High Speed”
Given what we know about the effects of these two critical frequencies, the transition to what PCB designers like to call “high speed” behavior in a PCB really depends on the signal rise/fall time rather than just the switching speed. A faster signal rise/fall time has the potential to create EMI problems at higher frequencies in various portions of a PCB, even if the switching frequency is quite low in comparison.
A faster switching speed just means that EMI will occur more often and can even be more severe as more power is concentrated at higher frequencies. If you remember your physics 101 classes, then you may remember that induced currents are proportional to the oscillation frequency of the magnetic field (assuming harmonic time dependence). Therefore, concentrating the power spectrum for digital signals in a higher range of frequencies causes more severe EMI throughout a PCB.
There is No Magic Bullet
While EMI in high speed digital signalling cannot be removed completely, it can be suppressed using a number of methods. The simplest method is to route all high speed signal traces as differential pairs directly over the reference ground plane, as this will prevent self-EMI between signal nets in your PCB. The counter-propagating pulses in differential pairs will generate magnetic fields that are out of phase by half a period, thus they will nearly cancel each other.
Routing differential pairs directly over a ground plane also provides some benefits against self-EMI and EMI from some external radiation source. Each trace in a differential pair must share symmetry and be length-matched to other traces in a net in order to prevent skew accumulation.
If differential signalling is not used, and the culprit and victim traces on your PCB can be identified, the so-called poor man’s shield can actually provide about a 20 dB reduction in crosstalk. This inexpensive solution requires placing a ground trace between the culprit and victim traces. Note that this ground trace must be the ground reference for both the culprit and victim traces.
Blue PCB with dense traces
High speed signals should usually be routed either in microstrip or stripline configuration with controlled impedance. This also applies to differential pairs, which should be routed as differential microstrips or differential striplines.
PCB design for EMI suppression, utilizing the power supply you need, managing capacitance and characteristic impedance as well as all other sorts of circuit performance factors like components, power integrity and signal integrity issues, and noise can be made easier with the right CAD tools. Don’t let your circuit board fall victim to its components in layout, make sure your design software can manage any number of PCB layers.
Trying to suppress EMI in high speed circuits presents a number of issues to consider during design. Fortunately, routing becomes faster and easier when you use a great PCB design package like Altium Designer®. You’ll have access to the best CAD tools, automated routing tools, and signal integrity analysis tools. Altium Designer uses a rules-driven design engine that helps ensure that your board is designed with EMI immunity in mind.