Electronic Environment #4.2017 Spatial shifted differential pairs to minimize differential crosstalk; Sometimes one sees recommendation to spatial shift traces to minimize differential crosstalk or to place vias and pins in an orthogonal configuration. Fig. 5 shows two examples of this type of arrangement. Both have low differential crosstalk but high common mode crosstalk. If for example the shifted differential pairs are used in a PCB layout and the length of the coupled section is 10 mm. The differential mode crosstalk will be low but the common mode crosstalk will be very high. Fig. 6 shows the differential near end crosstalk, NEXT, with 40 mm traces between the aggressor and the coupling section and 40 mm traces between the coupling section and the receiver. The skew, of 10 ps or 20 ps, are placed equally on both the aggressor side and on the receiver side. The differential crosstalk, at close to half baud rate, increase with almost 24 dB with 10 ps of skew and almost 30 dB with 20 ps of skew compare to the situation with no skew. For such layout, for high speed links, will reduce the operational margin for a system when skew is introduced. Some connectors use this shift arrangement of pins inside the connector to minimize the differential crosstalk. The connector will then suffer from very high common mode crosstalk.
Fig. 4. Link with skew and common mode crosstalk
In this case will the signal convert to common mode in the aggressor skew part and feed the connector with common mode signal. This common mode signal will leak into the victim channel and at the skew part in the victim channel, after the connector, will this common mode signal convert back to differential mode and show up as a differential crosstalk at the receiver. A differential crosstalk, coupled as common mode crosstalk, added to the pure differential crosstalk. If for example the common mode crosstalk is 20 dB higher than the differential mode crosstalk and a skew of 10 ps exist at both sides of the connector in a 25G link (about -8 dB in mode conversion at half baud rate, see Fig 1.) then will the differential crosstalk, caused by the skew, be -8 dB + 20 dB -8 dB = +4 dB. This crosstalk caused by the skew will be 4 dB higher than the pure differential crosstalk and then dominate the crosstalk picture. The skew effects can not be neglected if the common mode crosstalk is high compare to the differential crosstalk. In a crosstalk section is normally also a mode conversion taking place, a conversion from differential mode to common mode. For some skew settings on the aggressor channel will this mode conversion and common mode crosstalk be in phase and add up. For other skew settings they will cancel out each other and reduce the output common mode signal. The skew on the receiver channel will also be in phase or out of phase of the common mode crosstalk signal. This crosstalk, caused by the skew, is added to the pure differential coupled crosstalk. The phases of the crosstalk caused by the skew and the pure differential coupled crosstalk will strongly influence the total differential crosstalk. To find the worst crosstalk situation different combinations of the aggressor skew and the receiver skew must be investigated. One skew combination will give highest crosstalk in one frequency area and another combination might give highest crosstalk in another frequency area. The mode conversion from skew takes place at the edge of the signal transition. If there are two skew sections together with a common mode crosstalk coupling in between and the skew is short compare to the pulse width. Then will the differential crosstalk signal, out from this skew section, be a sinusoidal pulse shaped signal generated on each signal transition. The period time of the pulse will be close to the bit length. The spectra content of this pulse train will be high in frequency. The majority of the crosstalk noise power will be around 10 GHz in a 25G link. The level of common mode crosstalk around 10 GHz is then very important for the crosstalk coupling caused by skew.
Fig. 5. Example of differential pair, pin and via arrangement to minimize differential crosstalk
IV. Structures with high common mode crosstalk The common mode crosstalk takes place in different types of structures. Listed below are some structures that suffer from high common mode crosstalk. Microstrip routing; Tight routed differential microstrip traces will have
a rather high common mode crosstalk much higher than stripline. Microstrip routing needs then larger spacing to minimize common mode crosstalk. Microstrip has also different propagation velocity for differential mode and common mode signals. A length compensation is then needed to be placed close to where the length mismatch occurs otherwise will the differences in speed make the length compensation not to work.
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Fig. 6. Crosstalk in a 10 mm length of shifted differential pairs with and without skew
Fig. 7 shows a rather common used via pattern for connectors. Here is the signal vias as red and ground vias as black. To the right is the differential mode NEXT and common mode NEXT plotted. The common mode NEXT is about 30 dB higher than differential NEXT in a large frequency range. The thickness of the backplane is 4 mm.
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