Coupling methods for electrical connections when tracing differential pairs on printed circuit boards

The publication describes the method of interfacing electrical connections when tracing differential pairs on printed circuit boards. The method is based on the technique of generating and applying patterns of connecting printed conductors of a differential pair to traceable contacts of electronic components with minimization of the length of non-conjugate sections.

Introduction

Differential pairs are widely used for transmitting high-frequency and noise-proof signals in electronic devices implemented on printed circuit board designs. Each such signal is transmitted by a pair of equivalent signals of opposite polarity ( Signal + / Signal- ), and the receiver responds exclusively to the difference between their values. In this case, external interference affecting the transmission lines (printed conductors) of the pair is added to the two signals that are inverted with respect to each other without changing their difference and, thus, do not appear in the resulting signal on the receiver side.



The need to trace differential pairs in automatic or interactive modes has led to the implementation in modern CAD of electronic equipment of laying electrical connections (printed conductors) between given pairs of outputs — sources and receivers of signals, taking into account the specified rules on the topology and parameters of these connections. The list of rules for tracing compounds in differential pairs is quite wide and diverse, the main ones among them are the ones listed below.
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Taking into account the specified rules and limitations of the differential pair tracing means, conductor laying is performed with the maximum possible pairing and ensuring the difference in length not exceeding the specified design rules.

If necessary, the transition from layer to layer with the construction of the corresponding pair of interlayer transitions is allowed.

Connection pairing when tracing a differential pair

The sizes of the contact pads of sources and receivers of signals, as well as the distances between them, significantly exceed the specified value of the nominal gap between the conjugate conductors in the differential pairs.

The first problem to be solved when tracing a differential pair is to pair the conductors, that is, to lay their sections from the terminals of the components to the interface point. The quality of the coupling results is determined by the difference in the lengths of the constructed conductors and their symmetry. A symmetric pairing consists of a pair of conductors, in which one of them is a copy of the other, obtained as a result of applying to it a mirroring operation relative to the axis of symmetry of the contacts.


Asymmetric mating consists of a pair of conductors of different lengths and with a different topology.


The proposed method of coupling junctions for tracing differential pairs is based on the use of junction patterns. Each template contains a parameterized description of a pair of contacts and mating conductors emanating from these contacts.

It is assumed that pairs of contacts can be represented by the following objects of printed wiring:

• round pads - planar or cross-cutting conclusions of electronic components, as well as interlayer transitions
• rectangular contact pads - planar outputs of electronic components
• printed conductor segments.

The method of conjugation of conductors is considered on the example of round contact pads with preservation of the common approach with respect to rectangular and other convex forms of contact pads.

Each mate pattern is formed on the basis of the source, containing descriptions of a pair of contacts and mating conductors. All descriptions in the source template are presented in the local coordinate system and zero orientation. When placing each copy of the template on the board, it is applied the operations of transferring to a given position Mv (X, Y) , turning by a given angle Rot (θ) and mirroring RflOX () and RflOY () relative to the OX and OY axes, respectively, as shown in picture below.


The description of a pair of contacts in the original mate pattern is performed using parametric approximation of the shape of the pads by closed polygons (for round pads, regular octagons are used as shown in the figure).


Each instance of the original pattern is formed by a combination of a pair of edges of the polygon intersected by the mating Signal + and Signal-conductors. An enumeration of all possible combinations of edges forms a complete set of patterns of conjugations of conductors.
In the following presentation, only the templates of the base subset are presented, the application to which the transformation operations RflOX () and RflOY () form a complete set of conjugation patterns.

Symmetric Blend Patterns

An important advantage of symmetric mating patterns is that the mating sections of conductors have the same length. Further, the following parameters of each mate pattern are estimated:

• the value of the parameter settings of the template (λ) to ensure the specified design rules
• maximum length of mating conductors (Lmax)
• difference in length (modulo) of mating conductors (Δ)

The two symmetric mating patterns presented below are of limited use in cases where any elements of printed wiring are directly placed in front of a pair of pads that prevent the junction of conductors from minimizing their total length.

Asymmetric Blend Patterns

When tracing high-density printed circuit boards, the use of symmetric patterns in many situations is impossible without violating design rules. For this reason, their set expands with asymmetric patterns, in which the mating conductors have different lengths. Next, left-sided variants of asymmetric mating patterns are presented, right-sided patterns are obtained as a result of applying them to the mirror image operation relative to the OY axis, for example: A'B = RflOY (AB ') .








The presented set of conjugation patterns (including also the derivatives obtained as a result of applying the transformation operations RflOX () and RflOY () to the basic set) is applicable to any pair of traced contacts.
In the process of tracing a differential pair of contacts, for each interface point (determined by the current position of the mouse cursor or automatically calculated) the following operations are performed:


Below are illustrations of the choice of interface templates by regions when tracing a pair of contacts of a differential circuit.




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A few examples of laying differential pair connections using mating patterns.

Brief conclusions

1. Conjugation of conductors when tracing differential pairs requires the use of special methods that minimize the total length of non-conjugate sections.
2. The solution of the problem of pairing conductors is performed both on the “ side ” of a pair of contacts - signal sources, and on the “ side ” of contacts - receivers.
3. The use of wire interface templates is not the only method for solving this problem, but it has high speed (due to localization of the analyzed area of ​​the printed circuit board mounting space) and provides the choice of the most suitable wire interface options.
   
   
4. The composition of the mating templates used can be expanded with options that, if necessary, compensate for the total difference in length of the Signal + / Signal- connections in the differential pair - by forcing the length of the short connection of the pair to be extended.