Chapter 8
to perform the tests. On passing compliance tests, the FCC issues an ID num-
ber to the product.
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Designs that don’t require licensed operators can use any of several RF bands. In
general, to transmit a particular distance, a lower frequency requires less power
but a longer antenna. The maximum range varies with the design of the trans-
mitter and receiver and the environment.
The transmitter modulates an RF carrier frequency with the data. The receiver
demodulates the carrier to extract the data. The bands described below are suit-
able for transmitting asynchronous serial data..
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Two popular bands for unlicensed communications are 902–928 MHz and
2400–2483.5 MHz (called the 2.4 GHz band for short). FCC regulations spec-
ify the maximum transmitted field strength, harmonic levels, and spurious radi-
ation for the bands.
Because these bands are available for just about any use, interference from other
devices is more likely in these bands compared to others. Wireless networks
(802.11b and 802.11g), Zigbee and Bluetooth systems, and many cordless
phones use the bands. Part 18 of the federal regulations also allocates these
bands for use by industrial, scientific, and medical devices..
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The band 260–470 MHz is available for unlicensed use with some restrictions
on the content, duration, and scheduling of transmissions. Devices that can
operate within the restrictions have access to a generally less crowded portion of
the RF spectrum.
Section 15.231 of the FCC regulations specifies the allowed types of transmis-
sions. Subsections a–d allow these uses:- Control or command signals such as alarm data, door openers, and
remote switches. - ID codes that identify a system component.
- Variable data accompanied by a control or ID code.