Crosstalk (Noise) in Digital Systems

Ivor Catt, IEEE Trans. Com. Vol. EC-16 No. 6, dec1967, pp743-63.

Je viens d’ajouter deux et deux at je suis arrive a quatre.

Je vois que peut-etre il y a un peu du Francais atour de vous; votre nom, et votre pays.

I number of factors come together. You have French connections. You said you were involved in Electricity Board power lines.

I will take you through the way I came upon the idea of two velocities of propagation.

In Motorola Phoenix Arizona in 1964 I investigated the problem of interconnecting high speed (1 nsec) logic gates. To avoid interconnection delays, the system had to be reduced in size, leading to a 13 layer printed circuit mother board. Alternate layers had signal lines and voltage planes.

I decided that one key problem was to determine the amount of crosstalk between two parallel lines, either surface or buried. First, I found a flat topped pulse getting onto the other line, which disproved the idea that dv/dt or di/dt created the noise. However, there was a further puzzle; a big spike at the end of the flat topped noise pulse.

My mentor Ken C Johnson, whom I had left when I moved from Ferranti in Manchester England, happened to run in to me in the corridor of Motorola Phoenix. I grabbed him, and he told me there were two modes involved. He then shot back to England. However, he got them wrong. If the active and passive lines are A and P, and their mirrors (below the voltage plane, which acts as a mirror) are B and Q, he said the modes were, firstly, between A and P on the one side to B and Q on the other; and the other mode was between A and B on the one side, and P and Q on the other. This second mode he gave incorrectly. However, he gave me the important clue, to look for two modes. (See pp32, 33, figs. 39 and 40 in my 1995 book which you bought from me, for the correct two modes.)

In due course I worked out the correct two modes, A and P to B and Q (Even Mode) and A and Q to B and P (Odd Mode). I proved my theory with high speed pulses and photography, see my 1967 paper.

Before Ken turned up, I read Jarvis, who had brewed up the correct equations. However, then, to “simplify” the problem, he removed certain “minor” terms. This was a disaster, because it unbalanced the equations and prevented him from getting the correct result. In fact, his resulting formula was ridiculous, with noise increasing more and more until it could be larger than the incident signal. I said either he was wrong, or we had an amplifier!

I worked out my own equations, and solved them, see p30 of my 1995 book, now at www.ivorcatt.com/em.htm .

Jarvis referenced Cotte, and after I had worked it all out (as I remember) I got two articles by the Frenchman Cotte, dated 1947 and 1954. He had the equations, but upside down, and he did not resolve them as I did (see p30 of my 1995 book). I am writing all this from memory, because the Cotte papers are somewhere in the roof here. I am certain he did not “solve” the equations, and that he could not do so because, as with Jarvis, they were upside down.

Some time later I discovered that in Microwave, the two modes were known. However, working in digital electronics, knowledge in the Microwave camp was not available to me. I reference Oliver as my 4th reference at the end of my 1967 paper. I reference Cotte, who wrote in French, at my 5th and 17th references. (I spoke French at that time, and do so now.)

As I remember, Cotte worked on overhead power lines.

I wrote this because (1) you have a French name and are in Canada; (2) Cotte, who wrote in French, was involved in the problem of crosstalk early on; (3) my memory says that, like you, he was involved in overhead power lines.

It is likely that you are not concerned with crosstalk, but merely with transients (switch on/off of lightening) in overhead power lines. However, since Cotte got into crosstalk, and (as I remember) worked on overhead power lines, then crosstalk in that situation may be of importance.

Ivor Catt    25jan02