Hearing Aid Considerations; Radiated Sensitivity Measurement; Physical Configuration - Sierra Wireless MC5720 Hardware Integration Manual

P C I E x p r e s s M i n i C a r d H a r d wa r e I n t e g r a t i o n G u i d e
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It is important to investigate sources of localized interference early in the design
cycle.

Hearing aid considerations

Just as outside frequencies can interfere with the modem's sensitivity, the modem
can cause noise in hearing aids due to the keying of the transmitter.
Most digital wireless technologies do not transmit radio frequencies
continuously. They transmit in bursts, usually of specific durations, referred to as
RF burst frequencies.
Unfortunately, most hearing aids are not immuned to RF; they convert the burst
frequencies into audible frequencies. This causes unpleasant noise for hearing aid
users in close proximity to transmitters, as is the case with digital wireless
phones.

Radiated sensitivity measurement

A wireless device contains many sources of noise, such as the LCD, the digital
processor and memory, and any high-speed digital bus (due to the fast rise and
fall times of the digital signals). These sources can generate noise over a wide
range of frequencies, with significant energy in the receive band. If noise
generated in the receive band is allowed to radiate from the device and the
antenna detects it, the noise may mask low-level desired signals, thereby
decreasing the sensitivity of the device. To determine the extent of any
desensitization of receiver performance due to self-generated noise in the host
device, Over the Air (OTA) or radiated testing is required.

Physical configuration

To make OTA measurements, a test chamber is required. A full-size anechoic
chamber is not necessarily required. Figure 1 shows a small anechoic chamber
manufactured by Lindgren. This does not provide power to the same accuracy as
a full-size anechoic chamber, but is sufficient for this application. A base station
simulator, in this case an Agilent 8960, is used to provide FER (Frame Error
Rate) measurements.
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device (such as the main processor and parallel bus) that have the highest RF
emissions.
To filter out unwanted high-order harmonic energy, use discrete filtering on
low frequency lines.
To form shielding layers around high-speed clock traces, use multi-layer
PCBs.
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