Cross-Talk In Transmission Media
Any disturbing signal produced by transfer of unwanted power from one transmission path (called disturbing circuit) to another transmission path (called disturbed circuit) is known as cross talk.
Cross talk may be produced by
- Galvanic, capacitive or inductive couplings between transmission media (Linear cross-talk) e.g. between pairs of a VF (voice frequency) cable system.
- Poor control of frequency response i.e. defective filters or poor filter design is the cause.
- Non-linear performance in analogue (FDM) multiplex systems. A signal transmitted on one circuit or channel of a transmission system (multi-channel) creates an undesired effect in another circuit or channel (non-linear cross talk)
Types of cross talk Cross-Talk In Transmission Media
Broadly speaking, cross talk is of six types.
- Near-end cross- talk (NEXT).
- Far-end cross talk (FEXT).
- Intelligible cross-talk
- Unintelligible cross-talk
- Interaction cross-talk
- Reflected cross-talk
Near-end cross talk (NEXT)
Near-end cross talk occurs if the cross talk power in the disturbed channel propagates in the direction opposite to the propagation of useful power in the disturbing channel. Refer to figure for illustration of near-end cross talk.
The terminals of the disturbed channel, at which the near-end cross talk is present, and the energized terminal of the disturbing channel, are usually near each other. The near-end cross talk is much stronger than far-end cross talk because the magnetic (or galvanic) and electrostatic inductions are additive in the case of near-end cross talk and the inducing current in the disturbing circuit is much stronger.
Far-end cross-talk (FEXT)
It occurs if the cross talk power in the disturbed channel propagates in the direction of the propagation of the useful power in the disturbing channel. Refer to Fig. 2 for illustration of far-end cross talk.
The terminals of the disturbed channel, at which the far-end cross talk is present, and the energized terminals of the disturbing channel, are usually remote from each other. Far-end cross talk is less effective in impairment of the original signal in the disturbed circuit because the magnetic and electrostatic inductions are subtractive. Also the inducing current in the disturbing circuit gets very much attenuated after it has travelled to the far end. Cross-Talk In Transmission Media
Intelligible cross talk
The cross talk is intelligible when the whole or an important part, of the speech on the disturbing circuit is intelligible on the disturbed circuit. Between circuits transmitting the same frequency band or working without frequency translation (audio-frequency) only intelligible cross talk can arise. As the secrecy of the conversation is affected by intelligible cross talk, steps should be taken to see that intelligibility of sentence articulation of the cross talk should be less than 10%.
Unintelligible cross talk (also called noise)
The cross talk is unintelligible when the disturbing circuit gives rise only to noise in the disturbed circuit. It decreases the intelligibility but does not endanger the secrecy of conversation. Unintelligible cross talk occurs
- Between carrier channels having different frequency allocations.
- Between carrier channels having virtual carrier frequencies essentially differing from each other and
- In consequence of non-linear distortion.
Interaction cross talk (Indirect cross talk)
Interaction cross talk conveyed by a third circuit from the disturbing circuit to the disturbed circuit, where it causes far end cross talk (fig.3). This type of cross talk is also called double near-end cross talk. It occurs mainly in two-wire carrier systems fitted with intermediate repeaters.
Indirect cross talk caused by reflection due to mismatch of the circuit is called reflected cross talk.
I – Disturbing Circuit.
II – Disturbed Circuit.
(a) Reflected near-end cross talk causes far- end cross talk.
(b) Near-end cross talk caused by reflected wave at far-end causes FEXT.
Cross talk is mainly caused by two types of induction viz., Magnetic and Electrostatic.
It is well known that a change in magnetic lines of forces is associated with the flow of electric currents. The magnetic lines of forces due to currents flowing through circuit A will also embrace the wires of circuit B. As the current in circuit A alternates, the magnetic field also alternates, and according to Faraday’ law it induces e.m.fs in the wires of circuit B. Cross-Talk In Transmission Media
Electrostatic induction occurs due to the capacitance between four wires of the two circuits that are built side by side.
Practically it is noted that the current due to magnetic induction flows in one direction in the entire circuit, whereas that due to the electric induction flows through the two sections in opposite directions.
There are a number of methods for eliminating or at least substantially reducing cross talk in open-wire lines. Since the cross talk reduction in open wire lines depends upon three factors viz. Wire configuration, transposition and resistance unbalance. One possible method is to arrange the wires in such a configuration that the effect of the electric and magnetic fields of one pair will be the same on both wires of the disturbed pair, thus leaving no residual difference to cause currents in the disturbed circuit.
Another method to reduce the cross-talk is to reduce the separation between the wires of either or both disturbing and the disturbed pairs and, if possible, to increase the separation between the pairs themselves. Cross-Talk In Transmission Media
The most commonly used method is the use of ” transpositions”. Transposition means interchanging the position of the two wires forming the pair at regular intervals on the pole route, right through the length of the pair. The transposition is by far the most effective and practical method of reducing cross talk.