User manual

Page 16 of 26 A2500R24x – Users Manual
Release Date 01/10/11
2.4. Potential Interference Sources
The radio modules operate in the unlicensed ISM band, in which there are many other
applications e.g. WLAN, Bluetooth
®
and microwave ovens. This can cause interference that
may, in certain conditions, prohibit radio communication. The user is encouraged to implement a
number of channels and to alternate between those channels based on one of the below
methods.
• Sense if a particular channel is clear prior to transmitting (clear channel assessment)
• Pseudo Random channel change
• Select channels with the least interference (site survey)
Some of the most common interferences are listed below.
2.4.1. WLAN
Wireless Local Area Networks (WLANs) are divided into a number of overlapping channels.
Figure 5 below shows the channels used by WLAN; for the US and Canada, only channels 1-13
may be used. Predominantly, channels 1, 6 and 11 are used to achieve non-overlapping
channels. The user is therefore encouraged to select a channel scheme that also has channels
falling in between, or entirely outside, WLAN channels. Using a detection scheme (clear channel
assessment) will enable the user to find channels with minimal interference from WLAN devices.
Figure 5 WLAN channels
2.4.2. Bluetooth
®
Bluetooth
®
divides the 2.402-2.480 GHz band into 79 channels, each of which is 1MHz wide.
However, since Bluetooth
®
changes channels rapidly (up to 1600 times a second), it is unlikely
to cause continuous interference in a specific channel and can be overcome by implementing a
simple acknowledge/retransmit functionality.
2.4.3. Microwave ovens
A typical microwave oven uses a self-oscillating magnetron with a duty cycle below 50%,
because the tube is completely off for half of every AC mains cycle (8.33 ms in 60 Hz countries
and 10 ms in 50 Hz countries). The frequency of the microwave oven changes cyclically within
each AC mains synchronized on period, and is generally not predictable.