Unlicensed 915-MHz Band Fits Many Applications and Allows Higher Transmit Power

Since wireless LANs, Bluetooth, and ZigBee have been introduced, much of the design effort within the unlicensed spectrum in the U.S. has been focused on the 2.4-GHz band. But other unlicensed bands that the Federal Communications Commission (FCC) set aside for industrial, science and medical (ISM) applications also have a lot to offer design engineers who need wireless links for short-range audio and video transmission, as well as a variety of remote control, metering and sensing applications.

FCC regulations for the 915-MHz ISM band, for example, place no restrictions on type of application or duty cycle. In addition, the power output permitted by the regulations is considerably higher than it is in other portions of the ISM spectrum.

915 MHz is the center frequency of the band bounded by 902 and 928 MHz. Within this band, FCC regulations allow 50 mV/m electrical field strength, at a distance of 3 meters from the transmitting antenna.

Although the regulations specify field strength, a more useful metric for designers is effective isotropic radiated power (EIRP). Expressed in decibels, it is the power needed by an ideal antenna (one that radiates uniformly in all directions) to generate the same electrical field strength that the actual device produces at a particular distance. EIRP is particularly useful for comparing design options. It can be calculated using the formula in Figure 1.

Figure 1: Effective isotropic radiated power (EIRP) is a function of electrical field strength (E), supply voltage (V), and distance from the antenna (r).

Using the allowable field strength of 50 mV/m at a distance of 3 m gives an EIRP of -1.23 dBm. Regulations allow higher output power if the system uses spread-spectrum techniques, such as frequency hopping or direct-sequence spread spectrum. Higher field strengths are allowed because spread-spectrum systems are less likely to interfere with other systems compared to single-frequency transmitters. Being more immune to interference from other systems offers design engineers an added advantage.

The many limitations and qualifications for a spread spectrum transmitter are defined in FCC section 15.247. Using spread spectrum techniques will be well worth the effort. Figure 2 shows a comparison of maximum transmit power . EIRP values are also shown for the fundamental frequency’s harmonics because they are subject to regulation.

Figure 2: Transmit power limits for the 915-MHz band are improved by spread-spectrum techniques.

Two significant design issues in the 915-MHz band are:

1. The third, fourth, and fifth harmonics all fall in restricted bands, which imposes some design constraints on output filtering.
2. Although it is unlicensed in North America, Australia and South Korea, the band is more strictly regulated in other parts of the world. Chipmakers have made this limitation easier to deal with by designing radio chips that are tunable to different bands.

Design resources

Systems for monitoring and collecting highway tolls in the U.S. use the 915-MHz band as do home weather stations, automated meter reading, industrial monitoring and home security. RFID tag technology for monitoring and recording the movements of pharmaceuticals in the distribution chain also uses the 915-MHz band.

Designers interested in learning more about the basic constraints and opportunities of the 925-MHz band should read, “ISM-Band and Short Range Device Regulatory Compliance Overview,” posted on Texas Instruments’ web site. In addition, two Product Training Modules posted on DigiKey’s website, from Semtech and Analog Devices, include brief introductions to ISM band design as well as product descriptions.

Several chipmakers offer products – including evaluation kits – for 915-MHz designs.

The ADF7020BCPZ transceiver from Analog Devices integrates a frequency agile PLL that allows for use in frequency-hopping spread-spectrum (FHSS) systems. The VCO operates at twice the fundamental frequency to reduce spurious emissions. The transmitter output power is programmable in 63 steps from −20 to 13 dBm. The transceiver’s RF frequency, channel spacing and modulation are programmable using a simple three-wire interface.

Semtech’s SX1211 transceiver is optimized for asynchronous sensor network designs where low receiver power consumption is critical to battery life because the receiver is constantly waking up to sniff for signals. Key applications include security and alarm systems, submetering, industrial monitoring and control, building automation and KONNEX systems.

The MC33696 transceiver from Freescale Semiconductor has a context-switching feature that enables it to receive communications from either remote keyless entry (RKE) or tire-pressure-monitoring systems (TPMS). This allows optimization of the TPMS, RKE or passive entry receivers in the car body.

Texas Instruments’ CC1101 integrates the RF transceiver with a highly configurable baseband modem. The modem supports various modulation formats and has a configurable data rate up to 500 kbaud. Its main operating parameters and the 64-byte transmit/receive FIFOs can be controlled via an SPI interface. In a typical system, the CC1101 will be used together with a microcontroller and a few additional passive components.