EMC Design Guide for Printed Circuit Boards
Frame 10 of 78
Printed copies are uncontrolled
Radiation due to radio transmitters and similar nearby electrical or electronic
Transient environment caused by electrical switching operations, electrostatic
discharge and lightning
2.2. The Environment
There are two fundamental classes of transfer types:
The difference is not only due to the information coding but with regard to EMC the main
difference is due to the quality and vulnerability.
Analogue circuitry reacts immediately on perturbations but the effects remain within
relatively small limits, they might cause a rectification and possibly a drifting of the
operation point. Typically, analogue circuitry recovers from the perturbation by turning
back towards the regular operation. The operational safety margin corresponds to the
signal to noise ratio.
In contrast to the above, digital circuits provide a 'large' safety margin because of the
switching thresholds for the different states. Hence a digital application appears more
robust than an analogue one. However, the move towards low voltage logic, 3V and even
less, will reduce these margins. Another difference lies in the quality of failure which
might be quite unpredictable for digital application a bit might switch and cause a
system to malfunction in the case of switching to a defined state or to hang because of
turning into an undefined state.
Most problems associated with digital circuits are due to the high bandwidth inherited
from the high-speed clocks and edge rates. Rise times in the realm of a few nanoseconds
are equivalent with bandwidths well above 300 MHz range and the increase of the clock
rates will drive this into the microwave range. In other words, higher bandwidths increase
both emissions and the susceptibility of the circuitry.
This issue is fundamental to the functioning of the designed circuitry and comprises
mainly the aspects of internal or intra-system EMC and Signal Integrity. Intra-system
EMI includes problems due to mixed technologies, e.g. analogue and digital, or
electromechanical and digital. In the former case, the noise created by the digital circuitry
due to the impulsive nature of the power demands might cause some jamming of the
analogue circuits. In the latter case, the noise due to motors and switching relays typically
causes jamming of the digital circuits. In the case of high speed digital application the
digital circuitry might also cause some malfunctions due to crosstalk between such high
speed applications and reflections on the interconnects. A particular characteristic of
analogue components is that they typically operate at low frequencies and low levels and
in addition show very high input impedances.