The interface ground

To prevent incoming interference (RF, transients, ESD) from propagating through the digital circuit ground tracks and corrupting the digital operation, the I/O cables should be grouped together and the filtering of such cables referenced to a separate interface ground area. This ensures that common-mode noise present on the digital ground is not fed out onto the cables. Filtering and shielding techniques to reduce common-mode currents on cables both require a “clean” ground point, not contaminated by internally generated noise. Capacitive filtering at high frequencies is next to useless without such a ground. The critical parameters for this “quiet ground” are that it should:

provide a filtering termination for all off-board connections, power supply included;

be referenced to the RF ground of the rest of the unit, either the chassis or a ground plate;

not have circuit currents flowing in it.

These requirements can be ensured by devoting an area along one edge of the PCB to the quiet ground plane, including the I/O resistive, capacitive and inductive filtering and suppression elements on it (but no other circuits), and separating the circuit 0V from it. RF-critical circuitry in the main part of the circuit should be widely separated from the I/O area. The quiet ground plane itself should be strapped to the chassis ground, if it exists, by one or more low-inductance links, e.g. the mounting pillars of the PCB. Or in some mechanical designs, the quiet ground can be implemented on a completely separate interface PCB which is positioned behind the connectors, so allowing optimum placement of all the filtering and protection components.

Layout of filters

Protection of the interfaces is a vital part of EMC. Normally LC or RC filters, perhaps in combination with transient suppressors, are used. These must be positioned immediately adjacent to the interfaces they are protecting, with short, direct tracks to the interface connector, not allowing any tracks between the protection and the connector to couple by inductive, capacitive or common impedance paths to other tracks on the board. With incoming frequencies up to 1GHz and transient rise times of the order of 1ns, only a centimetre or two of track - 10nH inductance - may have a critical effect if it is poorly laid out.

Differential mode filtering is easily arranged by placing the filter components directly across the signal and return or power and return terminals. Common mode filtering requires that the filter components must be referred to a “clean” ground reference plane (see above), which will not normally be circuit 0V although it may be electrically tied to it at some point.

Wireless on board

Segregation on the PCB is often important when wireless is on board. Wireless functions are typically sourced and integrated within IT products as separate modules, often with a built-in antenna. Since this implies close coupling between the antenna and likely sources of interference within the product, the principal methods for control of coupling will be near field solutions, mainly E- and H-field shielding and separation distance. Separation is always the most effective and the cheapest; mutual coupling in the near field will fall off initially proportional to the cube of distance, so doubling the separation will give an 18dB reduction in coupling. While space and layout constraints within the product often force a compromise on separation, it is worth ensuring at the start of a project that there is the flexibility to gain maximum distance between receiving antennas and noise sources.

Near field coupling is not the only threat. Common impedance coupling, typically via the power supply, can also cause problems. It is dangerous, for instance, to supply power for the wireless module directly from the supply rails on a digital PCB, if this carries hazardous clock or data harmonics. A separately-routed supply directly from the PSU is safer, and good high frequency filtering and decoupling of the supply is essential. This normally means series ferrites and 3-terminal surface mount capacitors, optimised for the receive frequency range. A 0V signal return connection between the processor section and the wireless module is normally necessary, but there should be no opportunity for noise currents to flow across this connection – i.e. no accidental ground loops – and resistive or ferrite buffering of signal and data lines is advisable.