Sierra Wireless MC5720, Hardware Integration Manual, Page: 18 / 30

MC5720/8755/8765 Hardware Integration Guide
Note: The module can cause
interference with other devices
such as hearing aids and on-
board speakers.
Wireless devices such as the
MiniCard transmit in bursts
(pulse transients), for set
durations (RF burst frequencies).
Hearing aids and speakers
convert these burst frequencies
into audible frequencies,
resulting in audible noise.
Device-generated RF
All electronic computing devices generate RF interference that
can negatively affect the receive sensitivity of the module
(RF desense).
The proximity of host electronics to the antenna in wireless
devices can contribute to RF desense. Components that are
most likely to cause RF desense include:
• Microprocessor and memory
• Display panel and display drivers
• Switching ‐ mode power supplies
These, and other high ‐ speed devices (in particular, the
processor) can cause RF desense because they run at
frequencies of tens of MHz. The rapid rise and fall of these
clock signals generates higher ‐ order harmonics that often fall
within the operating frequency band of the module, causing
RF desense.
Example
On a sub ‐ system running at 40 MHz, the 22nd harmonic falls
at 880 MHz, which is within the cellular receive frequency
band.
Note: In practice, there are usually numerous interfering frequencies
and harmonics. The net effect can be a series of desensitized receive
channels.
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