This application claims priority to foreign French patent application No. FR 09 04348, filed on Sep. 11, 2009, the disclosure of which is incorporated by reference in its entirety.
The invention relates to a printed circuit for high-frequency signals, and more particularly to interconnect means between transmission lines situated on different faces of the printed circuit. The invention is of particular use for the transmission of radio frequency signals in the X band, in particular for frequencies of 9 to 10 GHz. The invention can be adapted to other frequency bands.
In order to convey high-frequency signals, a so-called microstrip technology implemented on a printed circuit is, for example, used. Printed conductive tracks are produced on faces of the printed circuit. These faces may be external or internal and may be separated by one or more conductive planes. The microstrip lines have particular dimensions so that, once associated with the conductive planes, they form impedance-matched lines. This matching makes it possible to ensure a certain transparency of the line with regard to the signal conveyed. In other words, the aim is to minimize the electrical power losses of the signal along the line.
The transmission line may have to change from one face to another of the printed circuit. For this, vias, also known as metallized holes, are used that ensure electrical contact between the tracks of different faces. The vias are generally of circular section. The simplest way of producing these vias is drilling of the printed circuit followed by metallization of the interior of the hole.
A simple radio frequency interconnect via in a multilayer structure, as illustrated in
It is, in this case, found that the via 15 exhibits a matching level of −14.5 dB for the useful working frequency of 9.3 GHz, and insertion losses of 0.49 dB for this same frequency. Given these results, the interconnect is far from optimal.
The invention improves the transparency of a via-type interconnect in a printed circuit between two transmission lines positioned on different faces of the circuit. The invention seeks to increase the matching level and to reduce the transmission losses.
To this end, the invention is a printed circuit for high-frequency signals, comprising at least two faces each bearing at least one transmission line for the signals and means for interconnecting the transmission lines between the two faces, a transmission line of one face being linked to a transmission line of another face by means of two vias each extending along an axis, wherein, in the vicinity of the interconnect means, the transmission lines each extend in a main direction, wherein in a plane containing the main direction of a first of the transmission lines and perpendicular to the face bearing the first transmission line, an orthogonal fix is formed whose abscissa is borne by the main direction of the first transmission line, and wherein the abscissae of the axes of the vias or of their projection on the plane, perpendicularly to the plane, are separate.
The invention will be better understood and other advantages will become apparent from reading the detailed description of an embodiment given by way of example, the description being illustrated by the appended drawing in which:
a and 10b represent a distance variation between two vias producing the interconnect of
In the interest of clarity, the same elements are given the same identifiers in the various figures.
This interconnect structure with two vias, called structure with two parallel vias, is used to improve the transparency of the interconnect. It has already been seen that a via has mainly an inductive nature leading to losses in the interconnect. In a first approach, the two parallel vias can be modelled by two parallel inductances, which provides better transparency for the interconnect due to a lower interconnect inductance than for a single via. In reality, couplings between vias exist and the results of an electromagnetic simulation are presented in
The insertion and matching levels are better than those measured with a single via. The aim is, however, to further improve the transparency of the interconnect.
In other words, in a plane 30 containing the main direction 22 and perpendicular to the face 11, an orthogonal fix is formed whose abscissa is borne by the main direction 22. In this fix, the abscissae of the axes 24 and 25 of the vias 20 and 21 or of their projection on the plane 30 perpendicularly to the plane 30 are separate.
The improvement in transparency is of particular interest when the main directions 22 and 23 of the two transmission lines 13 and 14 are parallel. This cascaded via structure also offers an advantage when there is a limitation imposed by the width of the transmission lines. In practice, this width is an important parameter in defining the characteristic impedance of the transmission line. Placing two vias in cascade configuration makes it possible not to widen the transmission line in order to place the two vias 20 and 21 therein.
The invention is not limited to two cascaded vias. It is also possible to produce the interconnect for the two transmission lines 13 and 14 by means of more than two vias which, when cascaded, do not exceed a volume defined by the overlapping of the two transmission lines 13 and 14 when the axes 22 and 23 are parallel.
To improve the transparency of an interconnect, the dimensional parameters of the interconnect are important. These parameters must be adapted according to the frequency band of the electrical signal. The cascaded via structure allows for a wide choice of dimensional parameters and within a greater range of variation than for parallel vias.
The distance between two vias form a first possible adjustment parameter.
In a numerical example with figures, at the frequency of 9.3 GHz, the microstrip lines have an impedance of 50Ω for a width of 1.91 mm and for a substrate with a thickness of 0.813 mm and a relative permittivity equal to 3.38. The two vias 20 and 21 have a diameter of 600 μm.
For example, for a frequency of 9.3 GHz and a distance d of 100 μm, the matching level is −21.4 dB and the insertion losses are 0.33 dB. For this same frequency and the distance d of 200 μm, the matching level is 18.9 dB and the insertion losses are 0.36 dB. Finally, still at 9.3 GHz, for the distance d of 300 μm, the matching level is −17 dB and the insertion losses are 0.39 dB.
Another dimensional parameter that can be adjusted to improve the transparency of the interconnect is the diameter of the cascaded vias 20 and 21.
Advantageously, the printed circuit comprises at least one conductive plane positioned between the two faces 11 and 12. In the examples represented in
It will be readily seen by one of ordinary skill in the art that embodiments according to the present invention fulfill many of the advantages set forth above. After reading the foregoing specification, one of ordinary skill will be able to affect various changes, substitutions of equivalents and various other aspects of the invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by the definition contained in the appended claims and equivalents thereof.
Number | Date | Country | Kind |
---|---|---|---|
09 04348 | Sep 2009 | FR | national |
Number | Name | Date | Kind |
---|---|---|---|
6023211 | Somei | Feb 2000 | A |
6933450 | Okumichi et al. | Aug 2005 | B2 |
7336502 | Goergen | Feb 2008 | B1 |
20050202722 | Regnier et al. | Sep 2005 | A1 |
20080191362 | Chuang et al. | Aug 2008 | A1 |
20090015345 | Kushta et al. | Jan 2009 | A1 |
20090033442 | Zhao et al. | Feb 2009 | A1 |
20090107716 | Pai et al. | Apr 2009 | A1 |
20090315158 | Maetani | Dec 2009 | A1 |
Number | Date | Country |
---|---|---|
2008105478 | Sep 2008 | WO |
Entry |
---|
Scott A. Wartenberg, et al., “A Coaxial-to-Microstrip Transition for Multilayer Substrates”, IEEE Transactions on Microwave Theory and Techniques, Feb. 2004, pp. 584-588, vol. 52, No. 2, XP11107716. |
Number | Date | Country | |
---|---|---|---|
20110215882 A1 | Sep 2011 | US |