User's Manual

RF Integration
Rev 1.0 Apr.02 Proprietary and Confidential 85
Device generated RF
All electronic computing devices generate radio
frequency (RF) interference. You should pay
particular attention to RF noise as it can impact
the sensitivity of the SB555 modem’s radio
receiver.
The proximity of the host’s electronics to the
antenna and radio have an effect on the radio
sensitivity. There are many high-speed devices
(in particular the processor itself) running at
frequencies of 10’s of MHz. Higher order
harmonics of these frequencies caused by the
rapid rise and fall times, often fall within the
operating frequency band of the radio.
For example, if we have a sub-system running at
40 MHz, the 22nd harmonic falls at 880 MHz,
which is within the cellular forward channel
frequency band. In practice there is more than
one interfering frequency harmonic, and the net
effect is a series of desensitized communication
channels. This energy leaks out of the computer,
and is received by the antenna, masking the
desired signal.
Most device designers are familiar with having
to pay attention to radiated emissions in order to
meet the FCC part 15 rules. The major culprits in
causing RF desensitivity have been found to be
the microprocessor and memory, display panel
and display drivers, and switching mode power
supplies.
Some or all of the following techniques may be
followed to mitigate RF desensitivity:
• Keep the antenna as remote as possible. By
moving the antenna further away from the
cause of the interference, the effect of the