Twisted pair

Twisted pair is a particularly effective and simple way of reducing both magnetic and capacitive interference pickup or emission. Only circuit pairs should be twisted together, i.e. signal and its return, or power plus and minus; the configuration relies on the two wires carrying equal and opposite currents, that is, a single differential mode signal. It is wrong to twist together wires that are carrying different circuits.

Twisting is most useful in reducing low-frequency magnetic pickup because it reduces the magnetic loop area to almost zero. Each half-twist reverses the direction of induction so that, in a uniform field, two successive half-twists cancel the wires’ interaction with the field. Effective loop pickup is now reduced to the small areas at each end of the pair, plus some residual interaction due to non-uniformity of the field and twist irregularity. If the field is localised along the cable, performance improves as the number of twists per unit length increases. Inter-pair magnetic crosstalk is reduced by randomizing the twist rate or twisting adjacent pairs in the opposite sense.

But the external coupling to any cable, whether or not it is twisted pair, has both capacitive (electric) and inductive (magnetic) components, although in any given situation or environment, one or other will be dominant. The equivalent circuit (see diagram) models each complete twist with the capacitive coupling as a current source I_C onto each conductor half-twist while the inductive coupling is a voltage source V_H in series with each conductor, with an opposite sign at each half-twist. Then interference induced on the complete length of cable is the sum of induced signals over the number of twists.

The effectiveness of twisting a signal/return pair depends on the impedance and the balance or unbalance of the signal circuit.

For unbalanced circuits, capacitive coupling dominates at high impedances and is not reduced by twisting. As the circuit impedance drops so capacitive coupling reduces and the inductive part becomes dominant, so that twisting becomes progressively more beneficial. Twisting together power conductors (circuit impedances of a few ohms) is therefore good practice.

Balancing the circuit eliminates (to a first order) the effect of capacitive coupling; because there is equal coupling to both sides of the circuit, the capacitive injection is entirely in common mode. interaction with external fields is determined purely by residual inductive coupling. This will be sensitive to an odd or even number of half-twists, or more properly to the differences in voltages induced in the enclosed area at each termination.

Ribbon cable

Ribbon is widely used for parallel data transmission within enclosures. It allows mass termination to the connector and is therefore economical. It should be shielded if it carries high frequency signals and is extended outside a screened enclosure, and within the enclosure should be routed near to the metallic structure and not across apertures or seams.

The performance of ribbon cables carrying high frequency data is very susceptible to the configuration of the ground returns within the cable. The cheapest configuration is to use one ground conductor for the whole cable (a), but this creates a large inductive loop area for the signals on the opposite side of the cable, and hence crosstalk and ground impedance coupling between signal circuits. At the very least, if ground return pins or wires are limited, they should be next to the highest di/dt signals in the cable, typically clocks or the highest speed data lines.

The preferred configuration is a separate ground return for each signal (b). This gives almost as good performance as a properly terminated ground plane cable, and is easier to work with. Crosstalk and common impedance coupling is virtually eliminated. Its disadvantage is the extra size and cost of the ribbon and connectors.

An acceptable alternative is configuration (c), two signal conductors per return. This improves cable utilization by 50% over (b) and maintains the small inductive loop area, at the expense of possible crosstalk and ground coupling problems between adjacent signals.

Better performance still is gained using a flexible ground plane integrated within the ribbon; in this case the performance of the cable is limited by the way in which the plane is terminated. Because of its inductance, a single pin carrying the plane termination is not a good compromise and largely negates the usefulness of the plane structure. Multiple pins as in (b) would be best.

Ground plane flexi

A particularly effective way to carry high frequency signals between boards within a product is the ground plane flexi connector assembly. The double-sided flexi has one side dedicated to a ground plane while the other side carries the signal tracks. Alternate pins on the surface-mount connector at each end take the 0V and are via’d through to the ground plane. This low-impedance ground return ensures that the minimum of ground noise is developed between the two ends of the circuit.

This construction gives a simple, cheap and effective mass-termination connection system. Flexis can be designed for any physical configuration; some suppliers will do simple designs as standard parts. Multi-layer flexis can be built so that both sides are covered in a ground plane, giving an effective total shield. The method of construction and assembly is exactly like that of a standard PCB except that the substrate is thin and flexible rather than rigid; and of course it can be extended to gain the advantages of both, without connectors, in the flexi-rigid construction. The method is very suitable for wideband digital bus connections such as to TFT displays, off-board memories and communication ports.

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