Interference And Sensitivity; Power Supply Noise; Device-Generated Rf; Rf Desense Mitigation - Sierra Wireless MC5720 Hardware Integration Manual

Interference and sensitivity

Several sources of interference could impact the RF performance of the EM.
Common sources are discussed below.
Most carriers require a certain level of receiver performance to ensure proper
functioning of the device on the their networks. Although the EM has been
designed to meet these carrier requirements, it is still susceptible to various
performance inhibitors. As part of the Engineering Services package, Sierra
Wireless offers modem OTA (Over-The-Air) sensitivity testing and
desensitization (desense) investigation. For more information, contact your
account manager or the Sales Desk (see page 4).

Power supply noise

Noise in the power supply can lead to noise in the RF signal. The EM
specification contains limits for power supply ripple, which should be no more
than 200 mVp-p 1 Hz to 100 kHz. This includes voltage ripple due to transmitter
burst activity.

Device-generated RF

All electronic computing devices generate RF interference. You should pay
particular attention to RF noise—it can impact the receive sensitivity of the EM
wireless modem.
The proximity of host electronics to the antenna can have an adverse effect on the
radio's sensitivity. There are many high-speed devices (in particular the
processor itself) running at frequencies of 10s of MHz. These signals generate
higher order harmonics due to the rapid rise and fall of the clock signal. These
harmonics often fall within the operating frequency band of the EM, causing a
decrease in receiver sensitivity.
For example, on a sub-system running at 40 MHz, the 22
MHz, which is within the cellular receive frequency band. In practice, there are
usually numerous interfering frequencies and harmonics. The net effect can be a
series of desensitized receive channels. Most device designers are familiar with
having to pay attention to radiated emissions in order to meet the FCC Part 15 (or
equivalent) rules. The components most likely to cause RF desense are: the
microprocessor and memory, display panel and display drivers, and switching-
mode power supplies.

RF desense mitigation

To mitigate RF desense, some or all of these techniques can be used:
•
•
Rev 1.2
A p r - 0 5
Locate the antenna as far as possible from sources of interference. The
drawback is that the modem may be less convenient to use.
Shield the host device. The EM itself is well shielded to avoid interference.
However the antenna cannot be shielded for obvious reasons. In most
instances, it is necessary to employ shielding on the components of the host
P r o p r i e t a r y a n d c o n f i d e n t i a l
R F I n t e g r a t i o n
nd
harmonic falls at 880
1 3