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Saving Power in Low-Power Wireless Radio Systems Cheat Sheet (DRAFT) by [deleted]

Tips for Saving Power in Low-Power Wireless Radio Systems

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Introd­uction

When developing IoT solutions, these 11 consid­era­tions are always taken into account by the embedded design consul­tants at ByteSnap

#1 System choice

The type of radio to use depends on several factors — princi­pally, data rate, commun­ication range, and operating costs. The range of options is consid­erable, from short-­range ZigBee, Thread, Bluetooth, and Wi-Fi to long-range low-power radio like LoRa and SigFox or cellular solutions.

#2 Wavelength

Some radio wavele­ngths propagate much better than others, and this propag­ation may be correlated with increased power effici­ency. Typically, lower freque­ncies propagate better than higher freque­ncies, but the compromise is that the possible data rate is lower. For example, the 2.4-/5-GHz bands have distances measured in tens of meters but can transmit hundreds of kilobits of data per second, whereas sub-GHz bands have distances measured in hundreds of meters, or even kilome­ters, but may only transmit a few kilobits of data per second. Wireless mesh networks manage to stretch the transm­ission distance by hopping the data between nodes.

#3 Microc­ont­roller sleep power

There is a wealth of microc­ont­rollers claiming to be low-power. Some, however, have very low-power sleep states only, whilst others are low-power when running also. How often a device is asleep affects which is the key parameter. If your device is normally asleep, look to the deep-sleep power consum­ption; if, however, it is always awake — for instance, to listen to a network — then it is the operating power consum­ption that is key.

#4 Power source

Power is a key consid­eration in restricted devices. Once you’ve decided if user-r­epl­aceable (AA, AAA, etc.) or rechar­geable cells such as Li-Po are approp­riate for the product, there’s plenty of optimi­zation to do. Enviro­nmental factors (intense cold or heat) will have a big impact on life, as will the battery discharge curve. Low-power radios tend to use very little power whilst sleeping and then use large pulses of current for receiving and transm­itting when awake — some types of battery may not handle this use model well.

#5 Pull-ups and other design tips

Low-power wireless radio system design requires attention to detail outside of just the radio circuit, such as pull-up resistors. These need to be optimized to maximize your battery life; for example, by adding active components such as FETs to turn devices and pull-up resistors on and off.

If you decide to use FETs in your design, then choose them carefully. Even though you may only be consuming a few milliamps, you’ll find that a higher­-power device rated for a few amps will have a lower Vf (forward voltage), meaning less power is wasted in the FET when it is on.
 

Battery Power

#6 Spread­sheets

Using a spread­sheet can be incredibly useful to determine which compro­mises should be made to make the design workable.

A spread­sheet can be used to see how compro­mises in transmit or receiver timing can affect the battery lifetime and how long the transm­ission should last as well as determine effici­encies of regulators over the voltage range and current requir­ements.

#7 Power measur­ement

Standard multim­eters have low sample rates that will miss the short RF bursts associated with transm­ission. Either use an oscill­oscope with a math function or a high-f­req­uency multimeter for power measur­ement. Altern­ati­vely, sum the power over a long period with a watt-hour meter.

#8 Antenna tuning

If range is important, remember to tune your antenna. This maximizes what you’ve got without adding to the power budget of your system.

#9 TX power

Don’t unnece­ssarily boost your output power beyond what you need. If your radio link has to span only 10 meters, for instance, then 5-dB output power is unlikely to be required and will just waste power, thereby reducing battery life.

#10 Batch testing

With batter­y-p­owered low-power devices, you can be operating close to the limits of component perfor­mance. In the case of FETs and other active devices in which you rely on a low voltage drop, for example, there will always be a variation in the charac­ter­istics between devices that may affect the perfor­mance. This means that it’s worthwhile to perform batch testing with a good-sized batch to ensure that any variation will not compromise operation of the final systems.
Simulation of some simpler aspects of the design using a SPICE simulator — running tests across temper­ature and voltage extremes — can similarly save pain in mass produc­tion.

#11 Transm­itter pulses

When the transm­itter is on, a low-power radio is at its maximum power state. It makes sense to minimize that ‘on’ time by minimizing the amount of data that has to be transm­itted, possibly by using binary or by compre­ssing larger files using .zip format