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36
EMC Considerations
36
Intermittent interference (µs-scale) has little impact if its duty cycle is below 10%. For
example, these short pulses from a digital circuit close to the antenna are essentially
harmless.
If interference is detected, look for the root cause by switching off devices. Typical sources
of interference are:
•
Unshielded flat cables carrying digital signals or power signals towards digital
circuits. Particularly, cable joints tend to radiate.
•
High-speed digital devices, such as application processors, modems and cameras.
•
Digital signals on the application board (e.g. clock signals, SDIO signals).
If spectral peaks are observed in the spectrum, this usually relates to radiated harmonics.
The source can be identified by looking for an integer relation between the observed
spectral peaks and the system frequencies. For example, peaks at 1200 and 1248 MHz
are an indication of an interfering source at 48 MHz as this maps to the 25
th
and 26
th
harmonic of a 48 MHz signal. This may correspond to the frequency of a microcontroller
in the application.
Integration cross-talk can be solved in a number of ways:
•
Shift the clock frequency of the interfering signal to avoid the GNSS bands.
•
Use shielding tape with conductive adhesive.
•
Shield radiating circuits, preferably all around.
•
Put digital signals in inner layers of the application board.
•
Change the antenna location by experimentation.
•
Enable the interference mitigation feature of the AsteRx-m3. Narrow spectral
peaks can be eliminated with the notch filters (see the
setNotchFiltering
command). Intermittent wide-band cross-talk can often be eliminated with the
wide band interference canceller (see the
setWBIMitigation
command).
AsteRx-m3 has been designed to minimize radiation and can be used close to an antenna
without additional shielding.
