B&B Electronics Zlinx Xtreme ZXT24-RM, Руководство пользователя

Страница: 159 / 163

Document Number: ZXTxRM-5110m
Page
159
Appendix D – Radio Frequency Basics
D. Appendix D – Radio Frequency Basics
D.1 What is dBm?
Radio Frequency (RF) power is measured in milli-Watts (mW) or, more usefully, in a logarithmic
scale of decibels (dB), or decibels referenced to 1 mW of power (dBm). Since RF power attenuates
as a logarithmic function, the dBm scale is most useful. Here are some examples of how these
scales relate:
1mW = 0dBm A 2-fold increase in power yields 3dBm of signal.
2mW = 3dBm A 10-fold increase in power yields 10dBm of signal.
4mW = 6dBm A 100-fold increase in power yields 20dBm of signal.
10mW = 10dBm
100mW = 20dBm
1W = 30dBm
D.2
Lower Frequencies = Better Propagation
Industrial applications typically operate in “license free” frequency bands, also referred to as ISM
(Industrial, Scientific and Medical). The frequencies and power of these bands varies from country
to country. The most common frequencies encountered are:
2.4 GHz – nearly worldwide
915 MHz band – North America, South America, some other countries
868 MHz band – Europe
As frequency rises, available bandwidth typically rises, but distance and ability to overcome
obstacles is reduced. For any given distance, a 2.4 GHz installation will have roughly 8.5 dB of
additional path loss when compared to 900 MHz. However, lower frequencies require larger
antennas to achieve the same gain.
D.3
Range is not just a function of transmitter power
The more sensitive the radio, the lower the power signal it can successfully receive, stretching right
down to the noise floor. There is so much variety in “specsmanship” for radio sensitivity, that it is
difficult to make a meaningful comparison between products. The most meaningful specification is
expressed at a particular bit error rate and will be given for an ideal environment shielded from
external noise. Unless you’re in a high RF noise environment (typically resulting from numerous
similar-frequency radio transmitters located nearby), the odds are good that the noise floor will be
well below the receive sensitivity, so the manufacturer’s rated receive sensitivity will be a key factor
in your wireless system and range estimates.