SE500982C2 - Microwave circuit with impedance interface interface - Google Patents

Abstract

The invention relates to a microwave circuit comprising a two-sided substrate (1) with microwave components, a ground plane (5) and at least one interface means impedance-matched for external connection of the microwave circuit. The interface means has an external-connection point (6). The impedance matching of the interface means comprises a matching pattern (11) arranged on one side of the substrate (1) and connected to a ground plane (5) arranged on the opposite side. The matching pattern provides a circuit with excellent matching, thus giving low power losses at significantly higher frequencies than the prior art.

Description

500 982 10 15 20 25 30 35 2 When high-frequency signals, such as microwaves, pass through an interface of some kind, for example between two circuits, it is important that the two channels that meet in the interface are electrically matched, ie have the same impedance (usually 50 ohms). If this is not the case, part of the signal is reflected, which means a loss of power.

Since the power at these frequencies is very expensive, especially in applications where a high output power is desired, great importance is attached to the adaptation. The losses due to poor adaptation increase with increasing frequency.

It is generally quite difficult to obtain good adaptation. This may be particularly true of the double-sided structure (US-4,626,805) described above, to which the present invention is primarily directed.

The path of a signal fed to or from the circuit can be divided into three main parts, which can be said to constitute the above-mentioned interface, namely the connecting leg, the through hole and the input or output conductor. The design of the main parts is crucial for the quality of the adaptation.

The part which in the known microwave circuits has the greatest negative effect on the adaptation, and thus determines the upper limit frequency of the circuits, is the input / output conductor.

Dominant factors for a conductor's impedance are its width and its distance to the ground plane. In the double-sided construction, it is possible to control the impedance of the conductor with great precision except in a portion adjacent the through hole. The reason for this is that an area around the through hole lacks a ground plane, which is a matter of course if it is not to be earthed. This lack of ground plane causes a different impedance in the part of the conductor which is on the upper side of the substrate opposite this area, and thus lacks a ground plane below it, than in the rest of the conductor. This party gives an extra inductance.

The through hole also adds a certain inductance, but considerably less because it has been possible to solve the adaptation of this reasonably well. The compensation for the inductance of the third part, i.e. the connecting leg, is relatively simple and is made with a suitably wide gap between the leg and the ground plane.

In circuits that work with small effects and at frequencies below about 8 GHz, the effect of the mismatch in the interface is in most cases acceptably small, but at higher frequencies and / or higher effects, the power loss becomes so noticeable that measures must be taken to improve the adjustment.

A certain improvement can be obtained by increasing the width of the conductor considerably in said portion, which has no ground plane below it. However, this does not solve the problem completely and can also be a completely impassable road, for example when the space does not allow the required widening.

U.S. Pat. No. 4,626,805 discloses another way of improving the fit. At the entrance on the underside of the substrate, a part of the ground plane is replaced by a coplanar conductor (30), in one end (34) of which the through hole, or via hole (40), is placed and in the other end (32) of which the connection for connecting a another microwave circuit is located. The via hole connects the coplanar conductor to the input conductor (36) on the top of the substrate. A gap (6) surrounding the end (34) of the coplanar conductor and the via hole is formed. In this case, a capacitance is introduced which compensates for the inductances at the end of the input conductor and in the via hole, whereby the width of the gap is dimensioned to obtain the correct capacitance. The result of this solution is not satisfactory. Simultaneously with the compensating capacitance, the gap gives rise to an inductance in the part of the input conductor which is located above the gap.

The design of a coplanar conductor on the underside of the substrate means that the via hole is placed far in from the edge of the substrate and thus the entrance occupies a considerable surface on the upper side of the substrate, which surface is unusable.

U.S. Pat. No. 4,626,805 also discloses another embodiment in which an attempt is made to compensate for the inductance obtained in the part of the input conductor which is located above the gap arranged on the underside. A portion (T) of 500 982 10 15 20 25 30 35 4 the input conductor is greatly widened, which provides a capacitive addition. However, this compensation is not satisfactory because it is not made at the source of the maladaptation. It is also not possible with this type of compensation, since an increase in the capacitive connection through widening of the conductor presupposes an underlying ground plane, which is thus not where the maladaptation arises.

The result in the form of circuit performance will not be acceptable with the measures, as above, described in US-4,626 805. Although the losses increase only slowly up to 20 GHz, this constitutes a breaking point over which the losses increase very rapidly. The circuit is therefore virtually unusable for frequencies above 20 GHz.

An object of the present invention is to provide a microwave circuit with better adaptation in the interface of the circuit than is the case in known microwave circuits.

Another object of the present invention is to provide a microwave circuit which has small power losses for frequencies up to more than 26 GHz. The objects are fulfilled with a microwave circuit of the kind initially stated, which is given the features which appear from the characterizing part of claim 1 in the appended claims.

This solution to the adaptation problem is applicable to microwave circuits with virtually any design and independent of component content and placement, design principle, etc. The arrangement of the adaptation pattern on the substrate on the opposite side to a ground plane means that the mismatch can be attacked at the source in contrast to the solutions according to the prior art.

Furthermore, the adaptation pattern has only positive effects and introduces no problems, as the solutions according to US-4,626,805 do. The matching pattern according to the present invention thus makes it possible to achieve low losses very high up in frequency by a suitable design of the matching pattern. Hereinafter, a microwave circuit according to the present invention will be described in more detail in the form of exemplary embodiments with reference to the accompanying drawings, in which: Fig. 1 shows a part of a substrate with an adaptation pattern. according to the invention; Fig. 2 shows the fitting pattern in detail; and Fig. 3 shows a piece of the underside of the substrate.

Figure 1 shows a broken away and enlarged part of a substrate 1. The substrate 1 is in reality of the order of 15-20 mm long and 10-15 mm wide. The substrate 1 is arranged on a bone frame and is covered by a lid, which hermetically encloses the substrate 1 and also provides shielding. The leg frame and lid are not shown, as they are of secondary importance to the invention. On the upper side 2 of the substrate 1, microwave components and conductor patterns 3 are arranged (the majority of which are not shown for the purpose of clarification) and on the underside 4 of the substrate a ground plane 5 and connections 6 are applied. Penetrations in the form of through holes, so-called via holes, 7 connect the upper side 2 and the lower side 4 to each other. The via holes 7 are fully plated or completely filled with conductive material. These via holes 7 are used for transmitting signals between the connections 6 and conductors 3 or components and for direct earthing of the top components and more. The ground plane 5 covers most of the underside 4 but leaves a gap 8 with bare substrate material around each connection 6, which is not to be earthed. On the upper side 2, Fig. 1 shows a grounded frame 9, to which the cover is connected by soldering, an input (or output) conductor 10 and an embodiment of an adaptation pattern 11 according to the present invention.

These details are also shown in Fig. 2, which is a broken away and, in relation to Fig. 1, further enlarged piece of the substrate 1 seen from above. Fig. 3 shows a broken away piece of the substrate 1 seen from below. Fig. 3 has the same scale as Fig. 1. 500 982 10 15 20 25 30 35 6 As has been mentioned above, it is of great importance that the microwave circuit is well adapted, ie that the impedance in the inputs and outputs of the circuit is the same as in the connected, external the leaders. A common impedance is 50 ohms. If the impedance is not equal, signal reflections are obtained in the transition, which means power losses. In the embodiment shown of a microwave circuit according to the invention, the critical transition constitutes the connection 6, the via hole 7 and a part of the input conductor 10, which will be explained in more detail below.

The input conductor 10 extends, in this embodiment, perpendicular to an end edge 12 of the substrate 1 from the via hole 7 and inwards to some input component in the circuit.

The factors that in the type of circuit structure shown have the greatest significance for a conductor's impedance are the conductor's distance to the ground plane and the conductor's width. Since substantially the entire underside 4 of the substrate 1 is covered by the ground plane and the thickness of the substrate is well defined, it is relatively easy to dimension the input conductor 10 so that its impedance becomes 50 ohms. This is true with one important exception, namely the following.

An end portion 14 of the entrance conductor 10 and a pattern portion 13, in the form of a rectangular collar, which surrounds the opening of the vi-hole 7 and which is wider than the entrance conductor 10, lack ground planes directly below them due to the necessary gap 8, see above all Fig. 2. The collar 13 is otherwise obtained when plating the via hole 7. This results in a mismatch which above all has a large negative effect at frequencies higher than about 8 GHz, whereby the power losses increase sharply with increasing frequency. In this end portion 14, the input conductor 10 constitutes an additional inductance, which gives an overall too high impedance. In addition, there are inductances due to the collar 13, in the via hole 7 and in the connection 6.

This entire non-completely adapted part, ie the connection 6, the via hole 7, the collar 13 and the end portion 14 is referred to in the following interface. The inductance in the connection 6 is compensated according to the prior art by dimensioning the gap 8 10 15 20 25 30 35 500 982 7 for suitable capacitive connection to the ground plane 5. In accordance with the invention, the problem of mismatch in the rest of the interface in the almost fully through the application of the fitting pattern 11. The fitting pattern introduces a capacitive counterweight to the inductors throughout the interface, although it mainly eliminates the mismatch in the end portion 14 and the collar 13.

The adaptation pattern 11 is connected to the ground plane 5 via via holes 15 and can thus be seen as an extra ground plane placed on the upper side 2. The adaptation pattern 11 consists of two parts, which are symmetrically arranged on each side of the input conductor 10 and which are stepped, ie they have a step-shaped boundary line facing the input conductor 10 with two steps. Study the part of the fitting pattern at the bottom of Fig. 2. What is said in the following about this part applies through the symmetry in a corresponding way also to the other part. The first and second ledges 16 and 17, respectively, of the steps are parallel to the entrance conductor 10 and are located at different distances from the entrance conductor 10, to form a first and a second gap 18 and 19, respectively. A first connecting edge 20 perpendicular to the ledges 16, 17 joins the ledges 16.17 with each other. The first ledge 16 extends along the end portion 14 from the edge of the opposite gap 8 towards the pattern portion 13 and leaves only one portion right next to the pattern portion 13, where the second ledge 17 takes over. The second ledge 17 extends to an imaginary straight line, which is perpendicular to the ledge 17 and passes through the middle of the via hole 7.

A second connecting edge 21, which is parallel to the first connecting edge 20, connects the second ledge 21 to a "floor" 22, which extends to the frame 9.

This matching pattern 11 thus introduces a capacitive counterweight to the above-mentioned inductance in the end portion 14 and the pattern portion 13 by the introduction of capacitive coupling to the ground plane. Alternatively, the adaptation pattern 11 may be designed by minor changes so that it also compensates for the inductance in the via hole 7. If this should be desired, the adaptation pattern 11 can also be designed so as to compensate for a part of the connection 6. inductance / capacitance, whereby the gap 8 is adjusted to a corresponding degree.

The width of the columns is essential for the impedance in the area. In this embodiment of the microwave circuit according to the invention, the width of the input conductor is 0.24 mm; the width of the first gap 18 is 0.1 mm and the width of the second gap 19 is 0.3 mm. Also important is the distance between the first connecting edge 20 and the pattern portion 13, measured perpendicular to the first connecting edge 20. This distance is 0.1 mm.

The via hole 7 has a diameter of 0.25 mm and its center is located 0.25 mm from the nearest side edge of the end portion 14.

In this embodiment, the length of the first ledge 16 is 0.45 mm and the length 17 of the second ledge is 0.35 mm.

The distance between the end portion 14 and the nearest bushing 15 is 1.0 mm in this microwave circuit, which is intended to be used at about 10 GHz. This distance can be arbitrarily short but should not be longer than 1/8 wavelength (ie A / 8). In this example, it is advisable to keep the distance shorter than about 2.0 mm. area where the microwave circuit according to the invention is intended that within the frequency response, the upper limit of this distance is in practice at approximately 3.0 mm.

Most of the above dimensions depend on the thickness of the substrate 1 and many also depend on the width of the conductor 10. The thickness of the substrate 2 is usually 0.25 m, 0.38 mm or 0.64 mm, and here it is 0.25 mm. This thickness controls the width of the conductor 10, which in turn primarily controls the width of the gap 18. Depending on the thickness of the substrate 1, the width of the conductor 10 and the length of the ledge 16 (the ledge 16 may extend along a smaller part of the conductor 10 with a simultaneous reduction of the width of the gap 18), the width of the gap 18 may vary between almost 0 and about 0.5. mm, where the lower limit is essentially determined by what is technically possible (today approx. 0.01 m).

The above-described embodiment of the microwave circuit according to the invention is of course not the only possible solution but modifications can be made as long as these are within the scope of the invention, as defined in the claims. For example, the design of the adaptation pattern can be changed according to the conditions regarding the design of the conductor, the thickness of the substrate, the extent and shape of the groundless portion on the underside of the substrate and more.

Thus, the fitting pattern need not be connected to the frame. The number of via holes in the fitting pattern can be varied. As mentioned above, the matching pattern can be designed to provide a higher capacitance than in the above-described embodiment to compensate for the inductance in the via hole. On the other hand, the fitting pattern can, if desired, be designed, for example, not to compensate for the mismatch in the pattern portion around the via hole. However, this gives a worse result at the higher frequencies, but may nevertheless have advantages in some respects. The symmetry that the adaptation pattern shows in the depicted embodiment is also not necessary. The step-by-step shape of the adaptation pattern should also only be seen as an exemplary embodiment.

The periphery of the adaptation pattern can of course be given another shape provided that the result is that intended by the invention.

Claims (10)

500 982 10 15 20 25 30 35 10 PATENT REQUIREMENTS Microwave circuit comprising a double-sided substrate (1) with microwave components, ground plane (5) and at least one interface means impedance-adapted for external connection of the microwave circuit, which comprises an external connection point (6), characterized in that the impedance means means an adaptation pattern (II) arranged on one side of the substrate (1), which is connected to a ground plane (5) arranged on the opposite side. A microwave circuit according to claim 1, wherein the interface means further comprises a conductor (10) and a bushing (7) connecting the conductor (10) to the external connection point (6), a portion (14) of the conductor (10) runs over a groundless area (8) arranged on the opposite side of the substrate, characterized in that the adapting portion (11) is arranged at least at said portion (14) of the conductor (10) and primarily compensates for the absence of ground plane (5) in the ground levelless area (8). Microwave circuit according to Claim 2, characterized in that the groundless area (8) consists of a gap (8) which separates the external connection point (6) from the ground plane (5). Microwave circuit according to one of Claims 2, characterized in that there is a gap (18) portion (14), which gap (18) has a width which is in the range 0 <width ¿0, 5 mm. Microwave circuit according to any one of claims 2-4, between the matching pattern (11) and said characterized in that the matching pattern (11) is further arranged at a pattern portion (13), which surrounds the bushing included in the interface means and that there is a gap. (19) and the pattern portion (13), which gap (19) has a width between the fitting pattern (ll) 10 15 20 25 30 35 500 982 ll which is in the range 0 <width 50.5 mm. Microwave circuit according to any one of claims 2-5, characterized in that the adaptation pattern (II) consists of two parts, which are arranged on each side of said portion (14). Microwave circuit according to one of the preceding claims, (II) is connected to the ground plane (5) via one or more bushings (15). k ä n n e t e c k n a d of that adaptation pattern Microwave circuit according to one of the preceding claims, characterized in that the adaptation pattern (11) is connected to the ground plane (5) via several bushings (15) comprising through, plated holes or completely or partially filled bushings, or combinations thereof. . Microwave circuit according to one of Claims 7 to 8, characterized in that the distance between the portion (14) and the nearest bushing (15) in the adaptation pattern is 33.0 mm. Microwave circuit according to one of the preceding claims, characterized in that the adaptation pattern (11) is connected to a frame (9) of conductive material arranged on the substrate (1).