User:Rob Kam/sandbox1/Crosstalk

Crosstalk When more than one signal is run within the same cable bundle for any distance, the mutual coupling between the wires allows a portion of one signal to be fed into another, and vice versa. This phenomenon is known as crosstalk. Strictly speaking, crosstalk is not only a cable phenomenon but refers to any unwanted interaction between nominally un-coupled channels. The coupling can be predominantly either capacitive, inductive, or due to transmission-line phenomena.

The equivalent circuit for capacitive coupling at low-to-medium frequencies where the cable can be considered as a lumped component is as shown in Figure 1.24.

In the worst case where the capacitive coupling impedance is much lower than the circuit impedance, the crosstalk voltage is determined only by the ratio of circuit impedances.

Digital crosstalk Crosstalk is well known in the telecomms and audio worlds, for example where separate speech channels are transmitted together and one breaks through onto another, or where stereo channel separation at high frequencies is compromised. Although digital data might seem at first sight immune from crosstalk, in fact it is a serious threat to data integrity as well. The capacitive coupling is all but transparent to fast edges with the result that clocked data can be especially corrupted, as Figure 1.25 shows. If the logic noise immunity is poor, severe false clocking can result. A couple of worked examples (see Figure 1.25) demonstrate the nature of the problem.

Crosstalk can be combated with a number of strategies, which follow from the above examples. These are:

Reduce the circuit source and/or load impedances. Ideally, the offending circuit’s source impedance should be high and the victim’s should be low. Low impedances require more capacitance for a given amount of coupling.

Reduce the mutual coupling capacitance. Use a shorter cable, or select a cable with lower core-to- core capacitance per unit length. Note that for fast or high-frequency signals this won’t solve anything, because the impedance of the coupling capacitance is lower than the circuit impedances. If you use ribbon cable, sacrifice some space and tie a conductor to ground between each signal conductor; another alternative is ribbon cable with an integral ground plane. Best of all, use an individual screen for each circuit. The screen must be grounded or you gain nothing at all from this tactic!

Reduce the signal circuit bandwidth to the minimum required for the data rate or frequency response of the system. As can be seen from (b) above, the coupling depends directly on the rise time of the offending signal. Slower rise times mean less crosstalk. If you do this by adding a capacitance in parallel with the input load resistor (across RL2 in Figure 1.24) this will act as a potential divider with the core-to-core capacitance, as well as reducing the input impedance for high-frequency noise.

Use differential transmission. The bogey of crosstalk is a major reason for the popularity of differential data standards such as EIA-422 (RS-422), and other more recent ones, at high data rates. Coupling capacitance is not necessarily reduced by using paired lines, but the crosstalk is now injected in common mode and so benefits from the common-mode rejection of the input buffer. The limiting factor to the degree of rejection that can be obtained is the unbalance in coupling capacitance of each half of the pair. This is why twisted pair cable is advised for differential data transmission.

Voltage breakdown and crosstalk Track spacing is also determined by production capability and electrical considerations. Minimum spacings similar to track widths as shown in Table 2.2 are achievable by most PCB manufacturers. Crosstalk and voltage breakdown are the electrical characteristics which affect spacing. For a benign environment – dry and free from conductive particles – a spacing of 1 mm per 200 V, allowing for manufacturing tolerances, is adequate for preventing breakdown. BS 6221 Part 3 gives greater detail. When mains voltages are present, wider spacing is normally set by safety approval requirements. Spacings less than 0.5 mm risk solder bridging during wave soldering, depending on the transport direction, if solder resist is not used.

Crosstalk (see Section 1.2.7) is likely to be the limiting factor on low-voltage digital or high-speed analog boards. The mechanism is similar to that for cables; calculating track-to-track capacitance is best performed by electromagnetic field solvers for individual tracks working on the actual board layout. The simplest rule of thumb is that a track spacing greater than 1 mm will result in crosstalk voltages less than 10% of signal voltages for most board configurations. Electrically short connections can be spaced much closer than this without undue concern. Crosstalk can be reduced by routing ground conductors between pairs of signal lines considered susceptible.

References

A Reference Guide to Basic Electronics Terms by F. A. Wilson, Babani, 1992, ISBN 0859342328, p. 78

External links

• Crosstalk, Wikipedia

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