User's Manual

Boomer II User Manual & Integrator’s Guide ______________________________ APPENDIX F – Guide to Desense
BM210012WT37 133 Copyright Wavenet Technology © November 2003
Consistent and reliable reception occurs when a safety margin dictated
by co-channel rejection is maintained. For example, if the co-channel
rejection is 10dB, all unwanted signals must be 10dB below the
receiver’s sensitivity level. Some modems and networks have different
rejection levels. Use the rejection level appropriate for your modem
(typically –10dB). This means an interference signal that is more than
10dB below the wanted signal has little impact on the data receiver’s
data recovery. Any interfering source above this level creates desense,
reducing the radio’s sensitivity for data reception. For every one dB
above the threshold level, one dB of desense is created.
Noise Sources
CPU clocks, address and data buses, LCD refresh, switching power
supplies, and peripheral drivers are the primary contributors of EMI.
The frequencies of these emissions are often unstable. One reason for
this instability is that high stability clock sources are not a requirement
in host/terminal designs.
The frequency of sources drifts as a function of temperature, time, and
aging. Other sources by nature move within the frequency spectrum as
a function of time. The edges of clock signals create detectable
harmonics well into the 1GHz band. This presents a challenge in
measuring the effects of the emission, as one must first determine
where the emission exists in the frequency spectrum.
Noise from the host/terminal can conduct through the
electrical/mechanical interface or radiate electromagnetic fields that are
received by the modem antenna and impact the modem. The Boomer-II
OEM modem is specifically designed to minimize conducted noise.
Radiated electromagnetic fields emanating from the internal circuitry
are incident on the modem antenna. These fields then are converted to
noise power by the antenna and are incident on the receiver. The
physical interface signalling connection has less impact on the receiver
performance and can be electrically decoupled using passive
components.
Receiver Susceptibilities
The receiver is susceptible to being desensed within the channel
bandwidth and at intermediate frequencies used for down conversion.
Excessive noise on power supply pins can also create sensitivity
problems.
Measurement Techniques
Desense can be measured in one of the following ways:
 Indirectly by recording the emission level from the
host/terminal and then calculating the effect on the modem.