Tait TM8000 Series Applications Manual page 92

Shielding
Cable Shielding
92 Creating Your Own Options Board
For low-speed digital designs or audio designs, a two layer board with plated
through holes is usually sufficient. Low-speed digital devices have relatively
long rise and fall times, this includes standard CMOS logic gates and low
power 5V 8-bit micro-controllers. Tracked earthing is usually sufficient but
ground fill should be used where possible.
For high-speed digital designs or RF designs it is strongly recommended that
a PCB with four or more layers is used. High-speed digital devices have
short rise and fall times, this includes most Digital Signal Processors and 16/
32-bit micro-controllers. The board should have one layer reserved as a
ground plane. No signal tracks should be placed on this layer.
The internal options connector has separate analog and digital earth pins.
These are connected together on the radio PCB through a low impedance
ground plane. Separate ground signals allow digital I/O and analog ground
current to flow in different wires on the loom. This is important because the
loom wire has relatively high impedance and so significant earth noise
voltage due to digital I/O activity can be developed across the length of the
wire. Having two earth wires also halves the impedance of the earth
connection where the earths are common at the internal options board end.
On the internal options board, the earth signals can either be connected
together or kept separate and fed to the appropriate digital and analog
circuitry. For low-speed designs it is practical to keep them separate but for
high-speed design it is not usually the case due to ground plane
requirements.
It is recommended that the internal options board is earthed to the chassis
lid as close as possible to the external options connector. This can be
achieved via the mounting screws closest to the external options connector.
For the screw hole, use a plated through hole diameter 3.5mm with pad
diameter 7mm on both sides. The resist should be cleared from the pad and
the pad connected to analog earth or the ground plane. Other mounting
screws may also be connected to the chassis lid but this is not essential.
For low-speed designs shielding is usually not necessary. For high-speed
designs shielding may be necessary. For RF designs it is usually essential.
If the external options cable is longer than 1 metre it is recommended to
shield the cable and connector backshell.
recommended shielding arrangement. The earth braid wire (bare copper)
and aluminium foil should only be earthed at the radio end of the cable.
For RF signals, coaxial cable must be used and the shield must be earthed at
both ends of the cable.
Figure 4.8
TM8000 3DK Hardware Developer's Kit Application Manual
September 2003 © Tait Electronics Ltd
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