Design Simulation Systems Ltd


PCB Crosstalk Simulation Toolkit


Mark Sitkowski

Overview

The simulation primitive elements in the library "pcb" enable simulation of the PCB track elements described in this paper.
The test circuits in adt/lib/pcb will enable predictions to be made of the crosstalk between adjacent PCB tracks. There are two tests available, one on a three track topology, with a ground conductor between the two tracks being tested, and one on a two-track topology, where there is no ground conductor.

The PCB track is represented by a lumped equivalent circuit, after the paper by Ladd & Costache, IEEE Trans. on Circuits & Systems, Vol 39 No 6 June 1992.

The transmission lines formed by PCB track can be broken up into electrically short segments, such that the segment length is less than or equal to one-tenth of the wavelength of interest. Then, the distributed parameters of each segment can be replaced by a series inductor or shunt capacitor, as may be seen in the drawing '2cm'.

The crosstalk effects modelled, include electrostatic and electromagnetic coupling between the tracks. Also, since the current in each segment can be measured, the current distribution along the line can be simulated. This means that the electric field emanating from the track can also be estimated, at various distances from the board.

The base module has been chosen to be two such lumped (1cm) segments, representing a 2 cm length of two parallel tracks.

Two forms of this are supplied. One is '2cm', which has a third (ground) conductor midway between the transmitting track and the receiving track. The other is '2seg', which omits the third conductor.

The examples, '10seg' and '10cm', represent a 10cm section of parallel track, made up of five 2cm segments. They differ in that '10seg' is a two-track test circuit, while '10cm' is a three-track circuit.

The tracks are 2.5mm wide, and have (w / h) set at 1.58 such that the characteristic impedance is 58 ohms. The tracks are set 7.5mm apart, from inside edge to inside edge, on a glass epoxy PCB, with a dielectric constant of 4.7.

The circuit values have been calculated from distributed parameter data, obtainable either from the PCB manufacturer, or from finite-element analysis.

The drawing 'via' is a plated through hole, assumed to be solid copper, of height 1.588mm, and radius 0.1mm. The equations for calculating the five terms of the LR series representing the hole are on the drawing. The via is for use with 2cm, for grounding the central conductor at the appropriate intervals which, for optimum crosstalk suppression, are:

                       Dvia < 1 / (4 * fMAX * sqrt(Ue))

MODEL PARAMETERS

The lumped model parameters for the PCB tracks are derived from the distributed parameters by multiplying by the line length and dividing by the number of segments.

The per-unit-length parameters for tracks of the above dimensions and spacing are as follows:

Component2 track3 track
C1095.3 pF/m91.9 pF/m
C120.459 pF/m0.185 pF/m
C13-4.30 pF/m
C2095.3 pF/m91.9 pF/m
C23-4.30 pF/m
C30-79.5 pF/m
L11374.2 nH/m363.7 nH/m
L1213.22 nH/m12.90 nH/m
L13-42.20 nH/m
L22374.2 nH/m363.7 nH/m
L23-42.20 nH/m
L33-405.2 nH/m

Parameters For Three Track Topology

From the above, the value of inductors L1, in the transmitting line:

   L1 = (L11 / 100) / 2 = 3.637 / 2 nH (as there are 2 per cm)
                       = 1.8185nH 

For all inductors L2, in the receiving line:

   L2 = (L22 / 100) / 2 = 3.637 / 2 nH (as there are 2 per cm)
                       = 1.8185nH

And for inductors L3, in the ground conductor:

   L3 = (L33 / 100) / 2 = 4.052 / 2 nH
                       = 2.026nH

The inductive coupling coefficients, from each line to the ground conductor, are given by:

           kij = Lij / (sqrt(Lii * Ljj))

Thus,      K13 = L13 / sqrt(L11 * L33)
               = 42.2e-9 / sqrt(363.7e-9 * 405.2e-9)
               = 42.2e-9 / 383.8896e-9
               = 0.109927

And

           K23 = L23 / sqrt(L22 * L33)
               = 42.2e-9 / sqrt(363.7e-9 * 405.2e-9)
               = 42.2e-9 / 383.8896e-9
               = 0.109927

Since there is only one capacitor per 1cm segment, the capacitor values are:

 C1 = C10 / 100 = 0.919pF
 C12 = C12 / 100 = 0.00185 pF
 C13 = C13 / 100 = 0.043 pF
 C2 = C20 / 100 = 0.919 pF
 C23 = C23 / 100 = 0.043 pF
 C3 = C30 / 100 = 0.795 pF

Parameters for Two Track Topology (No Gnd Line)

With just two tracks, the values all need to be recalculated from the figures in the first column:

 L1 = (L11 / 100) / 2 = (374.2 / 200) nH = 1.871 nH
 L2 = (L22 / 100) / 2 = (374.2 / 200) nH = 1.871 nH

The only inductive coupling is now from L1 -> L2, so

         K12 = L12 / sqrt(L11 * L22)
               = 13.22e-9 / sqrt(374.2e-9 * 374.2e-9)
               = 13.22e-9 / 374.2e-9
               = 0.0353287

THe new capacitances are:

 C1 = C10 / 100 = 0.953pF
 C12 = C12 / 100 = 0.00459 pF
 C2 = C20 / 100 = 0.953 pF

Simulation

There are two meaningful analyses which may be performed on the circuit as it stands.

AC Analysis

The frequency range of interest is from 5 MHz to 1 GHz, and the resolution should be set to around 200 points per decade. Driving with a 1 volt AC source provides a reference level to which the output crosstalk can be related.

It is instructive to plot the voltages at both ends of all tracks, but the only important waveforms are at the near and far ends of the receiving track.

The response shows a rising characteristic, with two points of inflection.

The first is situated at approximately 600MHz for the near-end response, while the second occurs at around 930 MHz.

The far-end curve rises more steeply, the first inflection point being at around 420 MHz, and the second at 800 MHz.

Note that the curves for two-track topology will show more crosstalk than those for three-track.

A special drawing, '10cm2via' simulates the case where the ground conductor is only provided with a via at each end. An analysis of this circuit shows a large resonance peak at around 800MHz, this being the frequency for which 10cm is a quarter wavelength.

Taking the propagation velocity as 3.16e10 cm/sec:

                  3.16e10 = f * 40
                        f = 790 MHz

Transient Analysis

The time-domain measurements are made on the near end of the receiving track. The analysis is set up to sweep 0 to 80n in 10ps steps.

The transmitting track is driven by a rectangular pulse, defined by

     pwl 0 0 1n 0 6n 1 40n 1 46n 0 80n 0

The results show two pulses in the near-end waveform, both 5ns wide, coincident with the rising and falling edges. The magnitudes are about 400-800uV for the positive pulse, and 500-650uV for the negative one.

IMPORTANT NOTE:

To speed up the analysis, you should not include the parameter TMAX in the specification of the .TRAN line Also, the use of ".options method=gear" will remove the spurious oscillations.

Although this is a well-known standard precaution for LC simulations, it does no harm to mention it.