Tait TM8000 Series Applications Manual page 93

Input/Output
Filtering
TM8000 3DK Hardware Developer's Kit Application Manual
September 2003 © Tait Electronics Ltd
Figure 4.8 Recommended auxiliary cable and connector shielding
metal D-range shroud in
contact with backshell
The recommended filtering for input and output (I/O) lines from the
internal options board is shown in
The component values shown are for guidance but should be suitable for
most applications.
For the I/O lines to or from the radio, filtering is usually not necessary.
The exception is when the internal options board contains high-speed
digital circuits. In this case, the outputs to the radio should be RC-filtered
on the internal options board as close as possible to the connector to
minimise noise on the loom.
It is recommended that filtering is applied on all I/O signals of the external
options connector. They also need ESD protection. The filtering shown in
Figure 4.6 and Figure 4.7 on page 91
high frequency filtering. For the audio, 10nF capacitors are recommended
because they are large enough to keep the voltage developed by an
electrostatic discharge to a safe level while not significantly affecting audio
frequency response. The capacitors are earthed to the chassis to provide a
low impedance return path for large ESD currents. High frequency filtering
is provided by the series resistance and 10nF capacitor.
Large decoupling capacitors cannot be used for digital signals because they
round the wave form edges to an unacceptable extent. Therefore for ESD
protection, a small 470pF decoupling capacitor in parallel with a zener diode
clamp is recommended. The capacitor reduces the slew rate of the ESD
pulse so that the zener diode clamps without overshoot. Again, the capacitor
and zener diode are earthed to the chassis to provide a low impedance return
path for large ESD currents. It is also recommended that a zener diode is
placed on the digital supply as some current will flow back into the supply
via the series resistance and digital IC clamping diodes during an ESD event.
The zener voltage should be approximately 0.5V higher than the supply
voltage. High frequency filtering is provided by the series resistance and
470pF capacitor.
It is essential that all I/O filter components are located as close as possible to
the connector. This minimises the possibility of noise bypassing the filters.
metal backshell
signal earth wire
aluminium foil
metal cable clamp
earth braid wire
analogue ground pin
Figure 4.6
provides both ESD protection and
Creating Your Own Options Board
cable insulation
and Figure 4.7 on page 91.
93