User's Manual

MC5720/5725/8755/8765 Hardware Integration Guide
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Device-generated RF
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.
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.