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My understanding of crosstalk is how a radio transceiver works. You basically tune your transmitter and receiver to a similar frequency in order to get a cross-coupling of energy (the transmitter's energy gets coupling over to the receiver, hence the receiver is able to listen to what is being broadcasted). Is this analogous true for crosstalk in parallel unshielded, untwisted wires where the 2 wires have similar inductance and capacitance?

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Your question is a bit confusing since your title doesn't quite match what you asked. You also makes few statements that are incompletes.

You basically tune your transmitter and receiver to a similar frequency in order to get a cross-coupling of energy

Receiver and transmitter are often complex system including many subssystems. They includes modulator/demodulator, antennas, filters etc. So "tuning" them is a complex task that requires many parts to interract together.

Is this analogous true for crosstalk in parallel unshielded, untwisted wires where the 2 wires have similar inductance and capacitance?

A capacitance involve two elements. A single wire doesn't have a capacitance. It has a capacitance with something else.

This being said, to answer your title question, no, two wires doesn't need to share similar charateristic (I assume here that you refer the resistance and inductance) to have crosstalk.

See Wikipedia's definition :

In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, inductive, or conductive coupling from one circuit, part of a circuit, or channel, to another.

You need some sort of coupling between the two elements. In the case of long unshielded, untwisted wires, there is severals sources of coupling :

  1. Capacitive coupling. The 2 wires act as a capacitors. Capacitor have low impedance at high frequency. A fast time-varying signal like an aggressive digital edge or a high frequency radio signal in one conductor would affect to other conductor. Capacitive coupling has nothing to do with electromagnetic waves.
  2. Inductive coupling. Long wires next to each other also have an inductive coupling, so they act as transformer. Big currents in one conductor will create a magnetic fields that will induce an electromagnetic force in the other conductor.
  3. ElectroMagnetic coupling (or interference). EMI will moslty be present if you have big changes of current in time. The length, material, shape of the wire will all affect its effectiveness to transmit/receive EM waves. Altough you seemed interested in "tuning" your wire to avoid interference, EMI is not restricted in bandwidth. A current surge in a wire will generates noise in a very wide frequency range. Even if you have an antenna tuned for a specific frequency, it will receives noise anyway.

Hope I could make things a little clearer.

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Lets put some numbers into an example of magnetic coupling, between an rectangular loop of some size: Area, and a long straight wire in the plane of the loop.

Vinduce = { MU0 * MUr * Area / (2 * pi * Distance) } * dI/dT.

The loop Area can be 1cm long with 1/3 of 1/16th inch height above GND plane; this would be any 2 adjacent layers of a 4 layer PCB of total thickness 1/16 ".

The "long straight wire" could be ANY trace on the PCB, containing the aggressor (transmitter of interference) energy.

Suppose your loop is 100cm from the 2,000 amp bus of an electric speed controller, switching in 1uS. Thus dI/dT = 2e9 amps per second.

Consider the entire PCB.....the GND structure (chopped up badly, or a clean plane) to be 4" by 4".

Vinduce = 2e-7 * Area/Distance * dI/dT Vinduce = 2e-7 * 0.1meter * 0.1meter / 0.1meter * 1e+9 Vinduce = 2e-7 * 0.1 * 1e+9 = 2e-7 e-1 e+9 = 2e1 = 20 volts induced in the GND plane...........massive eddy current.

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No, there does not need to be any similarity between wires.

There doesn't have to be any similarity between radios, either — radios include tuned circuits or other such mechanisms not because it is necessary but because it is useful, so that multiple transmitters can avoid interfering with each other and receivers can select just one transmission.

(Strictly speaking, any possible radio is somewhat frequency-selective, if nothing else due to the size of its antenna. But this is a much, much weaker effect than deliberate filtering.)


You specified untwisted parallel wires, but I'll mention anyway that there is an interesting case in twisted pair wiring. As you may know, the purpose of twisted pair is to ensure that the two wires in the pair are (approximately) equally influenced by reasonably uniform outside fields, so that the differential signal carried on the pair is unaffected.

But if multiple twisted pairs are bundled into one cable, there can be crosstalk because both pairs are alternating. Therefore, each pair in such a cable is twisted at a different rate, so that the same effect applies (the lineup of the conductors goes from A+ A- B+ B- to A+ A- B- B+ and back) though over longer distances than the basic twist rate.

So, that's a case where a deliberate dissimilarity prevents crosstalk. But it only applies when you have wires.

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