Achieving ultra-low-power RF in short-range apps

Achieving ultra-low-power RF in short-range apps

Ultra-low-power RF technology is critical in applications where power is at a premium and payload is greater than 10 bits/s

BY DIDIER SAGAN
Zarlink Semiconductor
Ottawa, Ontario, Canada
http://www.zarlink.com

Until recently, radio frequency (RF) wireless telemetry in ultra-low-power systems was only feasible for very low data rates. Most radios were designed for broad markets or standards, with peak power consumption often one or two orders of magnitude higher than the target applications.

These technologies enabled RF telemetry to be implemented only by a heavy duty-cycling (increasing the ratio of the off-period to total cycle-time) of the radio to limit its average power. The resulting throughput of a few samples per minute was suitable for building automation or industrial control applications, but was too low when the observed parameters varied over the range of a few seconds or less.

With the advent of new short-range wireless communications applications, including medical telemetry systems, innovative RF transceiver products must support high data rates while consuming minimal power. Ultra-low-power performance for RF telemetry is not dictated by a single parameter, and is possible through the right design choices at the transceiver IC and network levels.

The baseline current consumption without the load-dependent power amplifier (PA) current is of primary importance. It is a function of design choices and know-how covering carrier frequency, frequency range, modulation scheme, full tolerance to variations versus trim, settling times, programmability, and more. And not all radios are created equal; there is more than one order of magnitude of power between different IC offerings.

The supply voltage, PA power consumption (at comparable range), and link data-rate are often ignored when comparing different solutions. However, all three have a substantial impact.

A radio operating at 2.5 V consumes twice as much power as a radio with the same current consumption but operating at 1.25 V. Operating at a higher voltage is only required when output power in excess of 5 dBm is needed. This is not the case for short-range applications as output power is rarely over 0 dBm.

Low supply voltage is an easy way to reduce power consumption at the system level, but it requires an RFIC designed for low voltage operation. Only two radios available today can operate at 1.2 V, while the rest need at least 1.8 V, resulting in a power consumption range of up to 50 times (see Fig. 1 ).

Fig. 1. Supply voltage, baseline Tx power, and Rx power (L to R) range for various devices.

For a transmitter, the power consumption of the PA can be very large. Many 802.15.4 or Bluetooth radios consume 25 to 40 mW for a 25-m free-space range, wasting over 95 percent of the power. The three main factors impacting power consumption are receiver sensitivity, carrier frequency, and output impedance. They are additive, and together can represent more than two orders of magnitude in PA power consumption variation for an identical range.

The principal parameter from a transmitter PA point of view comes from the receiver. Its sensitivity defines, for a given range, how much power must be radiated. Most radios fall into the –85-dBm to –95-dBm sensitivity range, resulting in a factor 10 in PA power consumption.

Carrier frequency is the second most important parameter defining the PA current consumption. The free-space-path loss is 8.7-dB higher at 2.4 GHz than at 900 MHz. Also, losses through obstacles or absorption, as seen in communication through or around the human body, are larger for higher frequencies. As a result, the radiated power will need to be at least 7.5 times higher for the same range.

Third, but not least, in the list of parameters affecting PA power consumption is output impedance. Most radios have an impedance less than 100 Ω. Low impedance is only required for high output power (that is, long range) but it results in up to five times higher current consumption than higher output impedance options.

Overall, assuming a similar receiver sensitivity and PA efficiency, a high-impedance 900-MHz radio would use only 1 mW in its PA to achieve the same range as a 50-Ω 2.4-GHz radio using 25 to 40-mW power.

Finally, data rate is one of the most important factors defining the power consumption in duty-cycled wireless links. The average power is almost inversely proportional to the link data rate. A 100-kbit/s radio will consume almost half the power of a 50-kbit/s radio for the same payload.

For a given payload, a higher data rate can be seen as a way to improve the energy efficiency (see Fig. 2 ). When comparing RF transceivers, “energy-per-bit” is a better indicator than current consumption.

However, high-data-rate radios are often those with the higher peak currents. These are highly undesirable for most small batteries or energy harvesters as they result in large, leaky storage capacitors, generally a few hundred microfarads.

Fig. 2. Energy-per-bit versus peak-power of some available solutions for 25-m free-space range. (a) Transmitter energy/bit versus peak-power;(b) Receiver energy/bit versus peak-power. For battery-powered or energy-harvesting-powered systems, the optimum combination would be close to the lower left corner.

Other IC-level specifications such as leakage current and wake-up time also affect the power consumption. However, while they are critical for very low-payload data rates, their importance diminishes past 10 bits/s.

At the network level, the protocol has a major impact on the average power budget. Today’s standards, like 802.15.4 (ZigBee) or Bluetooth, offer highly sophisticated link and network layers. But these stacks amount to 50% to 75% of the radio power-consumption and larger overheads. For ultra-low-power systems, the “one-size-fits-all” standardized option is rarely the optimum solution. Instead, ultra-low-power applications should consider using a protocol optimized to their need.

The latency requirement of the network has a significant impact as well. The amount of time nodes spend listening, or “sniffing.” is a function of latency. Low latency means continuous or frequent sniffing. In highly duty-cycled systems, the receiver power is the largest portion of the power budget due to sniffing. For example, in 802.15.4 mesh networks about 90% of the system power is used for receive. In higher payload systems, sniffing may not be as dominant but receive-power will still be over 50% of the RF budget. The lowest possible receiver power consumption is often essential to achieving ultra-low-power RF telemetry.

Zarlink’s ZL70250 ISM transceiver has been designed following state-of-the-art ultra-low-power techniques. With only 13 nJ/bit and less than 2.5-mW peak in both transmit and receive, it is one of the lowest-power radios available today (see Fig. 3 ). Above 100 bits/s, the next best available solution consumes up to five-times more power for the same range.

Fig. 3. Average power versus payload data rate of a wireless sensor using the ZL70250 ISM transceiver.

Despite its very low power, the ZL70250 still has enough data rate to also support voice or sound communications. With over 186 kbits/s, it has enough bandwidth to carry a telephone-quality voice link or higher-quality sound with some ultra-low-power signal processing. With an overall power consumption of about 4 to 5 mW, a headset small enough to fit in the ear could operate for over 10 hours and a wireless stethoscope patch using a thin-film battery could continuously monitor chronic respiratory diseases or support the study of sleep apnea.

Ultra-low-power RF technology is critical in applications where power is at a premium and payload is greater than 10 bit/s. Where previous body-worn wireless sensors could only be used for slowly varying parameters, new RF technologies can help observe faster changing physiological parameters, like heart and brain electrical activity or blood oxygenation, that require on the order of 0.5 to 5 kbits/s to extract meaningful waveforms. A wireless body sensor based on the ZL70250 would consume on average less than 100 µA, making thin-film batteries or even thermo-electric energy harvesters viable power options.

The ultra-low-power performance of these new RF technologies, in combination with emerging energy harvesting techniques, has the potential to further our ability to accurately monitor and ultimately treat a range of diseases and conditions.

For more on ultra-low-power RF, visit http://www2.electronicproducts.com/RF-Microwave.aspx.