By Myk Dormer, Senior RF Design Engineer
First published in 'Electronics World' Magazine
In modern electronic design, the use of DC-DC converters to efficiently step supply voltages up or down, has become commonplace. Off line switch mode designs have replaced the line-frequency transformer linear supplies in the common plug top power supplies, while on-board both module and IC based buck or boost converters generate stable, high current, logic supplies, with minimum size and heat dissipation.
But there has to be a ‘down-side’. For the user of low power wireless modules the ever-present switching regulator or voltage converter is a constant source of anxiety, and frequently a serious limitation on radio performance.
At a very simplistic level, the potential difficulties a switch-mode supply are obvious: it’s inherently an oscillating circuit, and all AC signals have the potential to cause interference. At the core of any such supply design is a switch device, driven by an alternating waveform, that is handling a significant amount of power. The switch waveform itself, harmonics of it, and secondary effects (such as self resonant ringing in reactive components and mixing in non-linear elements, such as rectifiers) are potential interferers, and can affect the radio module by direct radiation, by conduction through power supply rails or ground-plane (loop) effects, or even by magnetic induction.
Equally as obvious, however, is the fact that the environment is full of periodic waveforms, from logic signals to the domestic mains, and untold numbers of radio links continue to provide excellent performance, despite the proximity (in may cases) of DC-DC converter circuitry.
The issue with any design that integrates a switch mode supply with a sensitive radio element is therefore down to identifying the sometimes subtle ways that radio performance can be degraded and by minimizing or eliminating the possible interference issues. Receivers are inherently more at risk than transmitters (owing to the minuscule signals they handle, and the large amounts of circuit gain), but even if they remain apparently functional, transmitters can suffer sufficient interference to become non-compliant with relevant legal regulations.
These interference effects are not always immediately obvious. An on-channel (either RF input of IF frequency) interferer will reduce receiver sensitivity, and hence degrade range, but unless a careful measurement is made with proper RF test equipment this can be overlooked if the effect is not extreme.
If the interference causes degradation of a rejection characteristic, the effect will be invisible in initial single signal on-the-lab-bench tests, and will only become apparent when real interferers are present in the final application, at which point the performance penalty will be obvious to the customer
With transmitter applications, the situation is even more serious, as a non-compliant spurious output is a legal, regulatory issue, with potentially worse consequences than just embarrassingly poor product performance.
So what can be done to eliminate such problems (beyond eliminating the DC-DC converter from the design altogether, which is frequently not an option)
Make as much use of decoupling as possible. Avoid running the wireless device from ‘raw’ switch-mode output rails: include passive decoupling, or intermediate linear regulators, to ensure the supply is as ‘clean’ as possible. Be aware of the major current paths to and from the supply (including earth and ground-plane currents), and do not let them pass ‘through’ the radio.
Minimise radiated effects: do not locate the switch-mode close to the radio. Within the physical limits of the design, place it as far away as possible. Include shielding to attenuate radiated energy, and consider making that shield of a ferrous material to prevent magnetic coupling.
Keep the power levels low. The less power it handles, the less interference it is generating. Use the DC-DC converter to supply only those elements of the circuitry that actually need it. Run everything else directly from the unswitched supplies. Use low power design methods throughout.
Make best use of timing Where possible, time-slice the operation of the DC-DC supply and the radio system (this is especially applicable to systems using short, defined radio communications bursts, where a momentary interruption of user interface or display functions would not be noticed)
Optimize the switching regulator itself. Topologies are divided between constant frequency designs (where the mark:space of a constant switching frequency varies to achieve regulation) and constant on-time (where the switching waveform on-time is fixed and the frequency is varied with the load).
Generally, the constant frequency design is to be preferred, as the actual interfering signal is then known, and can (within limits) be placed where it will do least harm. Frequencies in the baseband frequency range of the link should be avoided, as should frequencies approximating to the channel offset and the IF frequencies, and obviously (for high frequency switchers) the channel frequency and subharmonics of it are also forbidden. Some regulator designs allow the switching frequency to be locked to an external source, which then reduces worry over ‘drift’ of the switching frequency over temperature, or manufacturing spreads.
Layout techniques should follow good RF practice, and all tracks carrying high power alternating waveforms (including the ground returns) need to be short, and low impedance (wide). Low ESR decoupling capacitors will be needed, and both low and high frequency decoupling must be provided. Lastly, the power inductor should be chosen for lowest external magnetic field (toroids or closed magnetic circuit ‘pot’ types are superior to drum or spindle types)
Be prepared to try several different designs. Effects can be unpredictable, and committing to a single approach too early in the design/testing process can be fatal.
Switch mode regulators are a necessary part of modern electronic design, and with a certain amount of care, good results can be achieved from a design incorporating them with low power wireless hardware. Be aware, however, that there are pitfalls awaiting the unwary, and as ever: test everything!