ISM Bands Supplement Wireless Communications

Utilizing frequency bands typically occupied by higher-power applications has enabled many RF/microwave communications transceivers to provide reliable short-range communications.

Bandwidth is valuable wherever it can be found, and sometimes it can be found right alongside something as common as the microwave oven. That is the essence of Industrial-Scientific-Medical (ISM) band communications devices, which "piggyback" on the bandwidths occupied by typically higher-power applications (such as microwave ovens in the 2.4-GHz band), carving out enough operational bandwidth at low transmit power levels for reliable short-range communications for a variety of applications.

Global definitions of the ISM frequency band are established by the International Telecommunication Union (ITU), The basic premise for the use of these frequency bands is that communications devices can use the bands, but must tolerate any interference from ISM equipment already operating in them. In 1985, the United States Federal Communications Commission (FCC) opened the use of frequency bands previously reserved for ISM equipment. Some of the ISM bands include 902 to 928 MHz, 2.4 to 2.5 GHz, 5.725 to 5.875 GHz, 24.000 to 24.250 GHz, 61.0 to 61.5 GHz, and 122 to 123 GHz.

Many manufacturers have developed handy transceiver integrated circuits (ICs) for short-range monitoring and communications applications in these bands, especially at the lower frequencies. In some cases, the FCC sets transmit-power limits on these devices or requires that they employ spread-spectrum techniques. One such technique is frequency hopping, or digital modulation to ensure that the average power across the occupied frequency band is considerably less than the peak power at any frequency and time.

At the lower frequencies, for example, Analog Devices offers the model ADF7020 transceiver IC which operates in frequency bands of 431 to 478 MHz and 862 to 956 MHz. it can achieve bit rates of 0.15 to 200 kb/s with frequency-shift-keying (FSK) modulation and 0.15 to 64 kb/s with amplitude-shift-keying (ASK) modulation. The device operates on a supply from +2.3 to +3.6 VDC and consumes only 19 mA current in receive mode and only 26.8 mA in transmit mode. The IC features programmable output power, which can be set from -16 to +13 dBm in 0.3-db steps, and receiver sensitivity as good as -112 dBm with FSK at 9.6 kb/s and -106.5 dBm with ASK at 9.6 kb/s. The IC's transmitter section contains a voltage-controlled oscillator (VCO) and low-noise fractional-N phase-lock-loop (PLL) circuit with frequency agility that supports frequency-hopping, spread-spectrum use.

Similarly, the Si4320 FSK transceiver IC from Silicon Labs, a member of the company's EZRadioPRO product family, is designed for unlicensed ISM applications at the frequency bands located at 315, 433, 868, and 915 MHz. It features a completely integrated PLL and can achieve bit rates to 115.2 kb/s in digital mode of operation and as high as 256 kb/s in analog mode.

At higher frequencies, the CC2500 radio transceiver from Texas Instruments is enclosed in a 20-pin QlP 4 x 4 mm package for applications from 2.400 to 2.4835 GHz. It provides -30 to +1 dBm programmable output power and can achieve data rates from 1.2 to 500 kBaud. The transceiver IC features -104 dbm radio sensitivity at 2.4 kbaud. the model RF2948b spread-spectrum transceiver IC from RF Micro Devices delivers as much as +6 dBm output power across the 2.4-GHz band while operating on supplies from +2.7 to +3.6 VDC with only 70 mA current consumption during receive mode.

The model RF70 GFSK 2.4-GHz ISM-band transceiver from HOPE Microelectronics offers -40 to +5 dBm programmable output power from 2.4000 to 2.4835 Ghz while operating on +1.9 to +3.6 vdC supplies. It is housed in a 20-pin, 4 x 4 mm QFN package and uses burst-mode transmissions, achieving over-air data rates of 1 and 2 Mb/s. The transceiver IC is ideal for wireless control of computer mice and keyboards and for operating remote controls and home automation systems. The MC1319x family of 2.4-GHz ISM-band transceivers from Freescale Semiconductor provides similar functionality across the frequency range from 2.4000 to 2.4835 GHz.

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