Design Article
Sorting out ZigBee power options
Cirronet
7/3/2007 6:30 PM EDT
The emphasis is for good reason: Designed to deliver wireless networking to even the most humble devices—including devices that run on batteries lasting for years—ZigBee has to be extremely power-miserly to meet its objectives.
Yet manufacturers are introducing ZigBee products distinguished by power ratings, offering radios, for example, with 1 mW RF power and "high-power" versions with 100 mW RF power.
Are these manufacturers deviating from the ZigBee specification? A little clarification is in order. Manufacturers of ZigBee radios with higher RF power create them for very good reasons grounded in the realities of successful wireless applications; they are also faithful to the ZigBee specification and what it means by "low power."
By separating product lines along RF power differences, manufacturers give design engineers an important option for ZigBee implementations. Choosing relative to that option is a matter of matching power capabilities to application needs.
What ZigBee's "low power consumption" means
ZigBee's low power consumption is rooted not in RF power, but in a sleep mode specifically designed to accommodate battery powered devices. Any ZigBee-compliant radio can switch automatically to sleep mode when it's not transmitting, and remain asleep until it needs to communicate again. For radios connected to battery-powered devices, this results in extremely low duty cycles and very low average power consumption.
When a radio is in sleep mode, its RF power rating is irrelevant; it's only when transmitting that its RF power affects power consumption. In the case of Cirronet's ZigBee solutions, for example, a radio with 100 mW RF power will typically consume 150 mA at 3.3 V when transmitting, compared to 75 mA at 3.3 V for a radio with 1 mW RF power.
The 100 mW radio consumes twice as much power—but only when actively transmitting. As long as the high power radio's low noise amplifier is turned off, power consumption while sleeping is roughly equivalent to that of a low power radio.
If the high RF power radio is awake and transmitting 5% of the time, which would be a very active radio, the extra average power consumption is roughly 5%. This additional power consumption means that a battery that would last for five years with a 1 mW radio would last four years and nine months with a 100 mW radio.
As this illustrates, ZigBee radios with higher RF output ratings are still excellent candidates for use with battery powered devices.
It's important to note that the ZigBee Alliance doesn't itself specify anything for RF power. ZigBee's RF power specification comes from IEEE 802.15.4, which specifies a minimum power output rating of 1 mW, with no specified maximum. The de facto 100 mW "high power" level relates to the European limit of 100 mW EIRP, including antenna gain.
Why we have 1 mW RF power ZigBee products
The reason for having ZigBee radios with lower RF power is simple: They meet many application needs without adding unnecessary expense.
ZigBee chipsets all come with an RF power rating of around 1 mW, and creating a ZigBee module or packaged radio with 100 mW RF power requires the addition of an RF power amplifier. While the added cost of the amplifier is modest—for modules, it typically accounts for about $10 per unit at low quantities—the per-module cost can add up in a multi-node network, and that expense that should obviously be avoided unless required by the application.
ZigBee products with 1 mW RF output power are typically deployed where nodes are all in close proximity. Radios of this type have an indoor transmission range of around 100 feet (30 m), which is ample for applications in which transmission is unobstructed and in close range, such as home automation and some closely contained industrial applications.
See part 2
