Echelon Series 6000 Chip databook - Manual (Page 100)

Avoiding Magnetic Field Interference
All transformer-based transceivers are vulnerable to stray magnetic fields that can interfere
with the transformer coupling. In most environments, stray magnetic field noise is not a
concern for FT 6000 Smart Transceivers or Neuron 6000 Processors with external
transceivers. However, high frequency external magnetic fields can couple sufficient energy
into the transceiver to cause erratic network performance or, in some cases, cause data
traffic to cease.
One possible source of stray magnetic fields is a DC-DC switching power supply, or regulator,
using unshielded switching inductors located in close proximity to the transceiver. To
minimize magnetic coupling into the transceiver, the switching magnetics of power supplies
should be kept at least 7.5 cm (3 inches) from the FT 6000 Smart Transceiver or Neuron
6000 Processor with external transceiver. Otherwise, noise induced into the receiver could
affect communications.
If you suspect magnetic field interference, you should measure the level of noise at the FT
6000 Smart Transceiver or Neuron 6000 Processor with external transceiver. You can
measure the noise induced by a switching power supply by using an oscilloscope with
differential probes having at least 5 V common-mode range with 50 dB of common-mode
rejection to measure the voltage between the Series 6000 chip’s NETP and NETN pins
during an interval when there is no packet activity. You can also identify the source of the
magnetic noise by moving the suspected source away from the FT 6000 Smart Transceiver or
Neuron 6000 Processor with external transceiver by connecting it to at least 15 cm twisted-
pair wires and monitoring the noise at the NETP and NETN pins.
The noise between the NETP and NETN pins should be less than 15 mV peak-to-peak
differential. For noise that is magnetically coupled from a switching power supply, the noise
will be synchronous with the power supply switching frequency. The worst case occurs when
the coupled noise is between 10 kHz and 300 kHz, the center of the network data
communication band. If the noise is greater than 15 mV peak-to-peak, you should take steps
to reduce the coupling effect. The easiest solution is to provide more distance between the
power supply and the FT 6000 Smart Transceiver or Neuron 6000 Processor with external
transceiver.
For noisy power converters, shielded inductors can provide a solution to magnetic field
interference. In the commonly used buck DC-DC converter, a shielded inductor can be used
instead of an open-slug inductor to significantly lower the amount of stray magnetic field
generated by the DC-DC converter. For example, Taiyo Yuden’s LHFP-series inductor can
be used instead of their LHL-series, and TDK’s FS-series inductor can be used instead of
their EL or ELF-series. In transformer-based DC-DC converters, the power supply designer
should take care to choose a transformer style that generates minimum external stray
magnetic fields. For example, a pot-core DC-DC transformer will generally produce less
stray magnetic field than will an E-E core transformer, and the stray fields that it does
generate are vertical when they go through the plane of the PCB. A common way to
minimize magnetic fields emitted from power supply transformers is to wrap a “shorted-
turn” of copper tape around the transformer in the same direction as the transformer
winding.
In addition to power supplies, other noise sources can include DC motor controllers and
industrial ovens or heaters. For these noise sources, shielding the emitting device is often
the most effective approach to solving magnetic field interference problems.
88 Design and Test for Electromagnetic Compatibility