Echelon FT 3150 Smart Transceiver - Manual (Page 106)

Chapter 4 - Hardware Design Considerations
100 FT 3120 / FT 3150 Smart Transceiver Data Book
current flows during V
gate
transitions, and more common-mode RF current couples into the network twisted pair.
This common-mode RF current can generate EMI in the 30-500MHz frequency range in excess of FCC/CISPR “B”
levels, even when C
leak,SIGNAL
from a clock line to earth ground is less than 1pF. Guarding of clock lines is essential for
meeting Level “B” limits.
From this discussion, it should be apparent that minimizing C
leak,SIGNAL
is very important. By using 0.1µF or 0.01µF
decoupling capacitors at each digital IC power pin, V
CC
and logic ground noise can be reduced. Logic ground can
then be used as a ground shield for other noisy digital signals and clock lines.
Since the FT 3150 Smart Transceiver based-device has an external memory interface bus, there are many more traces
that need to be guarded by logic ground in a FT 3150 Smart Transceiver based-device. In addition, the V
CC
noise
generated by the memory interface and external ROM/RAM components requires more V
CC
decoupling, and may
require a four-layer PCB to maintain an RF-quiet V
CC
and logic ground.
Some FT Smart Transceiver-based devices with fast digital circuitry, such as MIP-based devices, DSP engines, and
large memory arrays, may require extra RF attenuation between the FT-X1 or FT-X2 transformer and the twisted pair
network in order to meet FCC/CISPR level “A” or “B”. This extra attenuation can be provided by a ferrite bead
(muRata BLM11A601 or equivalent) in series with each network line adjacent to the network connector. Each of
these ferrite beads must have an inductance of no more than 30µH. Alternately, a common mode EMI choke (muRata
PLT1R53C or equivalent) could be used in place of the two ferrite beads. If a common-mode choke is placed in series
with the network connection, it must have less than 40pF of differential capacitance. If the Network Isolation Choke
of the FTT-10A Transceiver was used in the past between the transceiver and the network for EMI attenuation on a
particular device, then that choke can still be left in place with the FT Smart Transceiver and the FT-X1 transformer.
However, the FT Smart Transceiver and FT-X1or FT-X2 transformer generally do not gain any benefit from the
common-mode choke for EN 61000-4-6 compliance.
Some amount of filtering may also be required on an FT Smart Transceiver device's power supply input, depending
on the level of noise generated by the application circuitry. This is best accomplished by placing ferrite chokes in
series with the power input traces adjacent to the power connector. A typical power supply circuit illustrating the
placement of these ferrite chokes is shown in Figure 4.6.
Figure 4.6 Illustration of Power Supply Input Filtering Using Ferrite Chokes
3-Terminal Voltage
Regulator
IN OUT
GND
+5V
OUTPUT
Power
Connector
Input
L3
L4
L3, L4 Suppliers & Part Numbers:
Associated Components WB2-30T
Fair Rite 294366631