benz

Making Sense of Battery Power and RF Power

Blog Post created by benz on Dec 14, 2017

  Understand, anticipate, and respect your power limitations

Are IoT and other smart/connected devices the biggest wireless opportunity right now, or the biggest source of hype? I suppose they can be both at the same time, and it’s clear that lots of devices will be designed and sold before we know the true magnitude of this wireless segment.

A crucial element of many wireless devices is operation on battery power. In recent years, this has often meant lithium ion batteries that are recharged once every day or two. These days, however, lots of design effort is transitioning to devices that use primary batteries, ranging from traditional alkaline cells to buttons and lithium-coin cells. These power sources are expected to last months, if not a year or more, despite their small size.

Meeting these power demands will require careful engineering—both RF power and DC power—and a holistic approach, to give you confidence that you’ll get the needed combination of performance and real-world functionality. This is a field with lots of investment and competition, meaning you may not have a second chance to fix a design failure or a development delay.

Exceptional power efficiency doesn’t happen by accident, and it isn’t a result of some tuning or tweaking at the end of the design process. Instead, it starts when devices are being designed, and overall success stems from a sustained process of measuring and optimizing. Two aspects of test & measurement are worth special note in designing for very low power: 

  • Using a power source with realistic limitations
  • Precisely measuring power consumption in all modes of operation, and during transitions

When powering a device or circuit, using a benchtop power supply can actually hide problems from you. Primary cells, especially when they’re very gradually going flat, can be highly imperfect power sources, and their imperfections can change with aging and temperature. Some precision power sources are now available to emulate real-world cells.

 

Diagram shows voltage-current emulation capabilities of Keysight B2961A/62A low-noise power sources

Keysight’s B2961A/62A low-noise power sources can emulate the DC voltage/current output characteristics of many different power sources, providing insight into real-world behavior in limited power conditions.

These advanced power sources can give you early warning of DC power problems while there’s still time and flexibility to design around them. They can also emulate power sources such as solar cells, with their very non-battery characteristics.

As always, if you’re going to optimize something, you have to measure it. On the power measurement side, extended battery life may require the ability to measure small currents, and perhaps a form of power scheduling to avoid excessive demand from simultaneous digital and RF activities. Whether you’re using a real battery or an emulator, instruments such as a DC power analyzer can tell you how much power is being used, and just when it’s needed.

Short term and long term current measurements from Keysight N6075C DC power analyzer

On the Keysight N6705C, the dynamics of current consumption are shown over 30 ms in scope view (left) and over 30 seconds in data logger view (right). Measurements such as these provide a more complete understanding of the real-world power demands of a device or subsystem.

The use of periodic quiescent states is one proven technique for extended battery operation, and it presents its own measurement challenges. Extremely tiny currents must be assessed to understand cumulative consumption, and recent products, such as the Keysight CX3300A device current waveform analyzer, are meant for just that. These analyzers have both analog and digital inputs, and the ability to time-align measurements of both.

In this post I’ve drifted away from my usual focus on RF measurements but, of course, the core concern for us in these DC power issues is to ensure that RF matters are proceeding as they should, no matter the state of DC power. Fortunately, there are ways to use the new power analyzers to trigger RF signal analyzers and thus correlate DC power with RF power and modulation, and that’s a subject for a future post.

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