Test Environment Integration Guidelines
10-6
Take care regarding the trace length and routing for the two differential pairs, TX and RX.
Neither of them may cross or run in parallel with any digital signal. Neither of them may
cross or run in parallel with any digital signal nor run through a digital ground or power
plane. The trace length inside of the unit running from the CoBox to the RJ45 should be as
short as possible. The trace length may have an impact on signal quality (i.e. link length),
especially if internal ambient noise is a factor.
If trace length cannot be shortened or the internal noise frequencies are hitting the carrier
frequency, or the multiples of these (depending on the product and operating mode either
10Mhzor100Mhzanduptothe11
th
overtone), we suggest a different strategy. Use a
multi-layer board and a separated shield layer on the solder and assembly sides of the board
which are routed in the inner layers. (Refer to the following figure.) These shield layers can
either be connected to the RJ45s shield orto a virtual ground signal provided by the CoBox.
(See also next section and product specific section.)
Figure 10-1: Multi-layer board strategy
Virtual Ground
The CoBox units provide a virtual ground at a (plated) mounting hole. It is a ground
imitation. It uses the tap of two capacitors (ceramic 0.1uF) conducted symmetrically to
ground and VCC. In absence of a solid ground (earth) this can be used for shielding or
balancing metal parts of the case.
Serial Signals
CoBox TTL level serial input and output signals are protected by 220 Ohm resistors. These
resistors provide a simple output shortage protection for infinitive duration (by limiting the
current). They also reduce conducted interferences at higher frequencies to the base board
Additional Emission Improvements
Depending on the used voltage regulator and base board design, conducted interferences
can sometimes be emitted by the power supply cord. If the voltage level there is low,
common mode chokes are the appropriate barrier to avoid these frequencies being emitted
via the power cord as an antenna.
RJ45 s
shield
CoBox n
inner layer signal traces
digital ground &
power plane
(inner layer)
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