Bright future for ultra-low-power RF

Even though the chips ship in their tens of millions each week, the market for short-range, low power RF technologies operating in the globally popular 2.4GHz ISM band - such as Wi-Fi, Bluetooth, ZigBee and a slew of proprietary solutions - is far from maturity. In the next few years, many impressive developments will emerge and wireless connectivity will pervade every aspect of our lives.

In particular, ultra low power (ULP) wireless applications – using tiny RF transceivers powered by coin cell batteries, waking up to send rapid “bursts” of data and then returning to nanoamp “sleep” states – are set to increase dramatically. For example, according to analysts ABI Research, the wireless sensor network (WSN) chips market grew by 300 percent in 2010. And the same company forecasts that no less than 467 million healthcare and personal fitness devices using Bluetooth low energy chips will ship in 2016.

ULP wireless connectivity can be added to any portable electronic product or equipment featuring embedded electronics, from tiny medical and fitness sensors, to cell phones, PCs, machine tools, cars and virtually everything in between. Tiny ULP transceivers can bestow the ability to communicate with thousands of other devices directly or as part of a network – dramatically increasing a product’s usefulness.

Yet, for the majority of engineers, RF design remains a black art. But while RF design is not trivial - with some assistance from the chip supplier and a decent development kit - it’s not beyond the design skills of a competent engineer. So, in this article I’ll lift the veil from ULP wireless technology, describe the chips, and take a look at how and where they’re used.

Inside ULP wireless
ULP wireless technology differs from so-called low power, short-range radios such as Bluetooth technology (now called Classic Bluetooth to differentiate it from the recently released Bluetooth v4.0 which includes ultra low power Bluetooth low energy technology) in that it requires significantly less power to operate. This dramatically increases the opportunity to add a wireless link to even the most compact portable electronic device.

The relatively high power demand of Classic Bluetooth - even for transmission of modest volumes of user data – dictates an almost exclusive use of rechargeable batteries. This power requirement means that Classic Bluetooth is not a good wireless solution for ‘low bandwidth - long lifetime’ applications and it’s typically used for periods of intense activity when frequent battery charging is not too inconvenient.

Classic Bluetooth technology, for example, finds use for wirelessly connecting a mobile phone to a headset or the transfer of stored digital images from a camera to a Bluetooth-enabled printer. Battery life in a Classic Bluetooth-powered wireless device is therefore typically measured in days, or weeks at most. (Note: There are some highly specialized Classic Bluetooth applications that can run on lower capacity primary batteries.)

In comparison, ULP RF transceivers can run from coin cell batteries (such as a CR2032 or CR2025) for periods of months or even years (depending on application duty cycle). These coin cell batteries are compact and inexpensive, but have limited energy capacity, typically in the range of 90 to 240mAh (compared to, for example, an AA cell which has 10 to 12x that capacity) - assuming a nominal average current drain of just 200µA.

This modest capacity significantly restricts the active duty cycle of a ULP wireless link. For example, a 220mAh CR2032 coin cell can sustain a maximum nominal current (or discharge rate) of just 25µA if it’s to last for at least a year (220mAh/(24hr x 365days)).

ULP silicon radios featuring peak currents of tens of milliamps - for example, current consumption of Nordic Semiconductor’s nRF24LE1 2.4GHz transceiver is 11.1mA (at 0dBm output power) when transmitting and 13.3mA (at 2Mbps) when receiving. If the average current over an extended period is to be restricted to tens of microamps, the duty cycle has to be very low (around 0.25 percent) with the chip quickly reverting to a sleep mode, drawing just nanoamps, for most of the time.