RF sins that may not be your fault
Many years ago, I knew an engineer who used to chuckle when he used the term “compromising emanations” to describe unintentional RF output. Today, most of us use the less colorful term “interference” to refer to signals that appear where they should not, or are more powerful than regulations allow. We’re likely more concerned about coexisting in the RF spectrum or violating a standard than we are about revealing something.
Wireless systems seem infinitely capable of generating unintended signals, and one of the more interesting is the rusty bolt effect.
I recently ran across a discussion of this when looking into the causes and consequences of imperfect connections in RF systems. Though I’ve previously written about connections of various kinds, including coaxial connectors, cables, adapters and waveguide, I’ve focused more on geometry and impedance than metal-to-metal contact.
Dealing with the wrong impedance is one thing, but for some time I’ve wanted to better understand why so many bad electrical contacts tend to be rectifying rather than Ohmic. Not surprisingly, it involves semiconductors. Accidental semiconductors, but semiconductors nonetheless.
Some oxides are conductive and some are insulating, but a number of common metal oxides are semiconductors. Oxidation or other corrosion—say from skin oils—makes it easy to produce a metal-to-semiconductor contact and a resulting nonlinearity.
Nonlinear connections are problematic in wireless, primarily because of the RF distortion products they produce. In the simple sinewave case, they create energy at harmonic frequencies, and when multiple signals are present they produce intermodulation distortion. The intermodulation distortion is particularly troublesome because it can appear in-band or in nearby bands, and at many frequencies at once.
Modern multi-channel systems, including base stations and carrier-aggregation schemes, create many simultaneous signals to “exercise” these nonlinearities and create distortion products. The distortion may be described as passive intermodulation (PIM) because it’s generated without powered elements. The rusty bolt example involves high currents through imperfect antenna or metal structure connections, though wireless systems offer many other opportunities for nonlinear mischief.
One of the most maddening characteristics of this phenomenon is its elusive nature. Outdoor antennas are subject to strain from wind and temperature changes as well as weathering from salt air or acid rain. Nonlinearities can appear and disappear, seemingly at random. Even indoor wireless transmitters have to contend with mechanical stresses, changing humidity and temperature, and contamination of all kinds.
In many cases, astute mechanical design and mitigation of oxidation or contamination will help eliminate nonlinear connections. Because Ohmic metal-to-semiconductor connections are essential to their products, semiconductor manufacturers are a good source of information and techniques.
At some point, of course, you need to make spectrum measurements to find intermodulation problems or verify that emissions are within limits. Signal analyzes do the job well, and many measurement applications are available for popular standards to automate setup, perform measurements, and provide pass/fail results. They’re the most efficient way to ensure you avoid sins that you’d rather not be blamed for.