Reduce EMI to Improve Radio Range
Chances are, your radio range is limited by interference. Improve range by minimizing Electromagnetic Interference (EMI). Radio range often limits the usefulness of Internet of Things devices and untethered Robots.
Some sources of interference are local and intermittent. WiFi protocols have automatic retries. Eventually, most interference sources will stop, and communication problems will go away by themselves. Other sources of interference are continuous, and if they don’t block communications entirely, they will reduce the range of a product’s on-board radio. Learn how to reduce these continuous interference sources with good mechanical and printed circuit board design.
Transient sources of interference such as motors, cell phones, and microwave ovens can be annoying and cause performance problems. The WiFi protocol fixes many of these problems by rebroadcasting the data. This fix comes at the expense of communication speed and latency.
In addition to local and transient interference sources, there are more sources inside your product. Go beyond the FCC requirements to improve radio range. Follow the tips below to minimize EMI that comes from inside your product.
Cable Shields
Fixes made earlier in the design process are easier and lower cost than late fixes.
Hand-built fixes made of copper tape are expensive. Fabricating and installing this sort of cable is labor-intensive and time-consuming. A typical fix is to wrap a cable in copper tape, and ground both ends. What is this fixing? Does the tape shield the electric field, the magnetic field, or does the cable need a lower resistance ground connection? Experiment with different solutions to find the root cause of the problem. Sometimes more ground connections in the cable will serve the same purpose.
Snap-on ferrite cores are often a good fix for magnetic fields. In most cases, this same functionality can be provided by magnetic components loaded on a circuit board. These components are lower cost and more convenient.
Shielding of Printed Circuit Boards
Metal shields provide isolation between circuits. They contain interference and prevent it from reaching the antenna. Sensitive circuits can also be protected by a shield, but the antenna needs to be outside of the shield in order to receive signals. This means it is better to shield the sources of interference rather than the receiver.
The best shield is a Faraday cage. This is a seamless conductive box around shielded circuitry. The challenge is to create metal shields that have low contact resistance. Aluminum is a good shield material, but a non-conductive layer tends to form on its surface. Clear Chromate is a good finish to provide conductivity while resisting corrosion. This finish does have some resistance, but it is able to maintain this resistance over time, rather than degrading to a much higher resistance, as bare aluminum does.
To achieve low-contact resistance, use an overlapping structure. This provides increased area, which decreases contact resistance. The increased area also increases the capacitance between the shield components, and this helps at high frequencies.
Gaskets further reduce contact resistance. Gaskets require compression force, typically provided by screws.
Place the screws outside of the gasket for more reliable contact force.
High-performance PC board shielding can use clamshell aluminum shields with a gasket. A shield goes on the top and bottom of the PC board. Add many vias to provide a connection from an inner-layer ground plane to the top and bottom of the circuit board. Signals can be sent inside the shield on a inner layer, between the ground vias. Place at least two vias between each signal being routed this way.
Notice that the screw holes in this shield design are inside the gasket, and the drawing above says that they would be more effective placed outside of the gasket! In either case, the gasket needs to form an unbroken ring around the circuitry that is being shielded.
Choose Good Clock Frequencies
Some circuits never turn off, and any interference from them will decrease the sensitivity of a receiver. For example, harmonics of clock oscillator signals in a controller board are difficult to control. For example, the Raspberry Pi design has two clocks: 19.2 MHz and 25 MHz. The harmonics of the clock oscillators can cause interference. Where do these harmonics fall?
The 25MHz clock harmonics fall between the popular non-overlapping WiFi frequencies. This does not completely fix the interference issue, but it helps. The 19.2MHz clock frequency doesn’t take advantage of this. These fall inside the WiFi frequency channels.
Use Fast Power Diodes
An often-overlooked source of interference is power diodes. These components generate high-frequency energy far beyond their switching frequency. When diodes switch from their ‘On’ state to their ‘Off’ state, the stored charge in the diode causes the diodes to conduct in the reverse direction for a short amount of time, typically in the nanoseconds. After the diode stored charge is depleted, the diode turns off quite quickly. This abrupt change in current causes EMI that extends well into the GHz region. To reduce this effect, use Shottky diodes, which do not store charge in the same way. Or, use fast recovery time diodes, which are designed to not store as much charge. For more detail see the diode manufacturer application notes about this problem, for example AND9071 — Noise Management in Motor Drives with IGBTs.
Diagnose EMI Problems
A Spectrum Analyzer is essential. It needs to have a maximum frequency that is greater than the highest frequency radio in the system. An antenna probe connected to the analyzer can ‘sniff out’ and problems in the design, and track down sources of interference.
This same type of instrument is used in EMI testing for FCC compliance. They can track down compliance problems as well as radio performance problems.
Start Early
Early in the design process, fix sources of interference with better ground plane connections, more vias, PC board-mounted filter components, and other low-cost or zero cost solutions. These fixes are better than a spool of copper tape and a collection of clamp-on ferrites. Don’t throw out your copper tape, though, because you might need it anyway!
An earlier version of this article was published in the Tempo Automation blog. Check out the Tempo web site to see my latest articles about electronic design and manufacturing. Starting with nothing but your CAD files and a credit card, Tempo can build you a fully loaded printed circuit board assembly in 3 days!