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About RF
General physics of radio signals
RF communication works by creating electromagnetic waves
at a source and being able to pick up those electromagnetic
waves at a particular destination. These electromagnetic
waves travel through the air at near the speed of light.
The wavelength of an electromagnetic signal is inversely
proportional to the frequency; the higher the frequency,
the shorter the wavelength.
Frequency is measured in Hertz (cycles per second) and
radio frequencies are measured in kilohertz (KHz or thousands
of cycles per second), megahertz (MHz or millions of cycles
per second) and gigahertz (GHz or billions of cycles per
second). Higher frequencies result in shorter wavelengths.
The wavelength for a 900 MHz device is longer than that
of a 2.4 GHz device.
In general, signals with longer wavelengths travel a greater
distance and penetrate through, and around objects better
than signals with shorter wavelengths.
How does an RF communication
system work?
Imagine an RF transmitter wiggling an electron in one
location. This wiggling electron causes a ripple effect,
somewhat akin to dropping a pebble in a pond. The effect
is an electromagnetic (EM) wave that travels out from the
initial location resulting in electrons wiggling in remote
locations. An RF receiver can detect this remote electron
wiggling.
The RF communication system then utilizes this phenomenon
by wiggling electrons in a specific pattern to represent
information. The receiver can make this same information
available at a remote location; communicating with no wires.
In most wireless systems, a designer has two overriding
constraints: it must operate over a certain distance (range)
and transfer a certain amount of information within a time
frame (data rate). Then the economics of the system must
work out (price) along with acquiring government agency
approvals (regulations and licensing).
How is range determined?
In order to accurately compute range – it is essential
to understand a few terms:
dB - Decibels
Decibels are logarithmic units that are often used to
represent RF power. To convert from watts to dB: Power
in dB = 10* (log x) where x is the power in watts.
Another unit of measure that is encountered often is
dBm (dB milliwatts). The conversion formula for it is
Power in dBm = 10* (log x) where x is the power in milliwatts.
Line-of-site (LOS)
Line-of-site when speaking of RF means more than just
being able to see the receiving antenna from the transmitting
antenna. In, order to have true line-of-site no objects
(including trees, houses or the ground) can be in the
Fresnel zone. The Fresnel zone is the area around the
visual line-of-sight that radio waves spread out into
after they leave the antenna. This area must be clear
or else signal strength will weaken.
There are essentially two parameters
to look at when trying to determine range.
Transmit Power
Transmit power refers to the amount of RF power that
comes out of the antenna port of the radio. Transmit power
is usually measured in Watts, milliwatts or dBm. (For
conversion between watts and dB see below.)
Receiver sensitivity
Receiver sensitivity refers to the minimum level signal
the radio can demodulate. It is convenient to use an example
with sound waves; Transmit power is how loud someone is
yelling and receive sensitivity would be how soft a voice
someone can hear. Transmit power and receive sensitivity
together constitute what is know as “link budget”.
The link budget is the total amount of signal attenuation
you can have between the transmitter and receiver and
still have communication occur.
Example:
Maxstream 9XStream TX Power: 20dBm
Maxstream 9XStream RX Sensitivity: -110dBm
Total Link budget: 130dBm.
For line-of-site situations, a mathematical formula can
be used to figure out the approximate range for a given
link budget. For non line-of-site applications range calculations
are more complex because of the various ways the signal
can be attenuated.
RF communications and data rate
Data rates are usually dictated by the system - how much
data must be transferred and how often does the transfer
need to take place. Lower data rates, allow the radio module
to have better receive sensitivity and thus more range.
In the XStream modules the 9600 baud module has 3dB more
sensitivity than the 19200 baud module. This means about
30% more distance in line-of-sight conditions. Higher data
rates allow the communication to take place in less time,
potentially using less power to transmit.
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