MICROSTRIP CIRCUITS

Abstract: Impedance adjusting elements for a microstrip circuit employing a signal transmission line (6) connected to an active circuit element and at least one impedance matching element, such as a parallel-connected open-ended stub (11), are formed of one or more wire elements (21, 22) arranged in proximity to the transmission line (6) and the open-ended stub (11) and connected at one end to a ground plane on a side of a substrate opposite to the transmission line (6). The free ends of the wire elements (21, 22) can be freely moved, thereby to adjust the spacing of the wire elements from the signal transmission line (6) and open-ended stub (11) and adjust the angle of intersection of the wire elements and the signal transmission line and open-ended stub, so that the effective impedances of the circuit can be controlled.

Description: This invention relates to microstrip circuits. Transmitting and receiving information signals via satellite generally involves the use of high frequency signals in the microwave region of the frequency spectrum. As satellite broadcasting becomes more and more prevalent, and becomes more accessible to the home-owner or individual end-user, it becomes necessary also to produce microwave circuitry for processing the microwave signals received from satellites, both economically and in substantial quantities. Such microwave circuitry typically employs what are known as microstrip circuits, which form a basic building block for hybrid microwave circuits. For example, high frequency amplifiers are typically employed to process the received microwave signals, and circuits must be provided to match the input and output impedances of a semiconductor used as an amplifying element in the high frequency amplifier. By providing such impedance matching, the overall circuit characteristics, such as the noise factor (NF), are improved. Additionally, microstrip circuits are also typically used to provide impedance matched interconnections between various passive components, including resonators and filters, and are used as integral parts of phase shifters, oscillators, and circulators. One previously proposed microstrip circuit, in which the input and output impedances are controlled by adjusting the dimensions of the microstrip circuit, includes a field effect transistor employed as a high frequency amplifier in a converter for converting super high frequency (SHF) signals to ultra high frequency (UHF) signals. As is known, a microstrip is typically formed as a planar structure having a dielectric substrate and conducting strips forming a conductor pattern on one side of the substrate with a conductive ground plane on the other side of the substrate. The impedances of the microstrip circuit can be controlled by altering the physical dimensions of the conductive strip and, in that respect, one previously proposed practice involves depositing a plurality of small conductive elements appearing substantially as a pattern of dots in the vicinity of various tuning stubs of the microstrip circuit, and then connecting together various ones of the dots in the pattern and to the stub by hand soldering to custom match the impedances of the circuit. In view of the increasing demand for microstrip circuits and the requirement to mass produce the circuits with a relatively low unit cost, the above-explained technique of individually adjusting the impedance is not suitable. US Patent No. US-A-3 925 740 discloses a microstrip circuit comprising a signal transmission line formed on a substrate. A stub strip conductor is formed on the substrate at a position spaced from the signal transmission line so that there is a gap between the signal transmission line and the stub strip conductor. A tuning member, which may be metallic (electrically conductive), is suspended above the gap and is movable towards and away from the gap, by virtue of the member being threaded and being screwed into a threaded aperture in a cover of the circuit, so that the capacitance across the gap may be varied whereby the total stub impedance is varied to enable tuning of the circuit. US-A-2 794 174 discloses a microstrip circuit comprising a signal transmission line formed on one side of a substrate and a ground plane formed on the other side of the substrate. To tune out impedance mismatches, a wire may be soldered to the signal transmission line so as to extend perpendicularly to the line and parallel to the plane of the substrate. The possibility is mentioned of the wire also being connected to the ground plane whereby the signal transmission line and ground plane are short-circuited together by the wire. US-A-2 897 460 discloses a microstrip circuit comprising a signal transmission line formed on one side of a substrate and a ground plane formed on the opposite side. For impedance matching, two sector-shaped insulating pieces are mounted on the substrate with a quarter-wavelength separation and so as to be each rotatable in a plane parallel to the substrate. Curved conductive surfaces on the insulating pieces can therefore be brought closer to or further away from the transmission line, thereby changing the capacitive coupling between the transmission line and curved conductive surface, and between the ground plane and curved conductive surface. This causes the impedance of the microstrip circuit to change. According to the invention there is provided a microstrip circuit comprising: a signal transmission line; a circuit element connected to the signal transmission line; an impedance matching element in the form of an open-ended stub connected in parallel with the signal transmission line; a ground plane; and an electrically conductive wire element formed of an electrically conductive metal wire covered by an insulative sheath and arranged in proximity to the signal transmission line or the open-ended stub, the wire of the wire element being soldered at one end to the ground plane whereby the wire element is freely movable in space above the transmission line or the stub with said one end of the wire acting as a supporting point, whereby the spatial position of the conductive wire element relative to the signal transmission line or the stub can be varied to vary an effective impedance of the signal transmission line relative to the circuit element. With such an arrangement, the impedance can be adjusted easily and economically. Further, the impedance can be adjusted in a non-permanent fashion. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a previously proposed high frequency amplifier microstrip circuit; Figure 2 is a graphical representation of a Smith chart for use in setting the input impedance of a field effect transistor as might be employed in the circuit of Figure 1; Figure 3 is a schematic representation of a previously proposed approach to adjusting the impedance of a microstrip amplifying circuit, such as that shown in Figure 1; Figure 4 is a schematic representation of a microstrip circuit embodying the present invention; and Figure 5 is a cross-sectional view taken along a section line A-A' in Figure 4. Figure 1 schematically illustrates an example of a previously proposed microstrip circuit employed as a microwave amplifier. Figure 1 shows a conductive strip pattern or signal line pattern that would be formed on a dielectric substrate that has a conductive ground plane on the other side thereof, neither the substrate nor the ground plane being shown in Figure 1. A field effect transistor (FET) 1 has source leads 2 and 3 thereof fed through the dielectric substrate for connection to the ground plane. A gate lead 4 of the FET 1 is connected to a microstrip circuit or signal transmission line 6 and a drain lead 5 of the FET 1 is connected to another microstrip circuit or signal transmission line 7. The transmission line 6 is connected with a d c return circuit choke pattern 8, which is employed to apply a negative bias voltage to the gate lead 4 of the FET 1. Similarly, the transmission line 7 is connected with a respective d c return circuit choke pattern 9, which is employed to provide a positive bias voltage to the drain lead (circuit) 5 of the FET 1. The d c return circuit choke pattern 8 is formed of a series of high-impedance and low-impedance conductive strips. More specifically, the choke pattern 8 is formed to have high-impedance line segments 8A, which are of a width determined to be one quarter of the wavelength of the frequency of the signal of interest, and low-impedance line segments 8B, which are relatively wide compared to the high-impedance line segments 8A. A number of the high-impedance line segments 8A and low-impedance line segments 8B are connected alternately, depending upon the required impedance. Similarly, the choke pattern 9 also is formed of high-impedance line segments 9A and low-impedance line segments 9B connected alternately to provide the required output impedance match. Both the d c return circuit choke patterns 8 and 9 are dimensioned and constructed so as to present an infinite or open circuit impedance to the frequency of the signal of interest fed to the signal transmission lines 6 and 7, in order to prevent such signal from being adversely affected by the bias voltages being applied. Accordingly, an input signal that is supplied through the signal transmission line 6 to the gate lead 4 of the FET 1 is amplified by the FET 1 and is fed out from the signal transmission line 7 connected to the drain lead 5 of the FET 1. In order further to tune and control the input impedance of the stripline circuit, open-ended stubs 10 and 11 are connected in parallel with the signal transmission line 6 to adjust the circuit impedance as seen by the gate circuit of the FET 1. The stubs 10 and 11 are open-ended and are in parallel with the signal path of the transmission line 6. The lengths d₁ and d₂ of the open-ended stubs 10 and 11, respectively, and their arrangement along the strip signal transmission line 6 at distances l₁ and l₂, respectively, are determined by the impedance parameters of the FET 1. In other words, the pattern dimensions of the microstrip circuit as seen in Figure 1 are determined in order to match the impedance parameters and, once determined, the microstrip circuit is manufactured using conventionally known etching methods. One known technique for determining such pattern dimensions involves the use of a Smith chart as represented in Figure 2. Referring to Figure 2, the impedance of the microstrip transmission line 6 is set at an impedance point represented by an encircled X, and the desired impedances can be obtained by determining the respective dimensions l₁, l₂, d₁, and d₂ in order to established the relationship as represented in the Smith chart of Figure 2. An open-ended stub 12 may be connected in parallel to the signal transmission line 7 that is connected to the drain lead 5 of the FET 1 and the length of and arrangement along the transmission line of the open-ended stub 12 are similarly determined, in the same fashion as were those of the open-ended stubs 10 and 11. In this way, it is possible to control or adjust the impedance at the output side of the FET 1. Nevertheless, even though the pattern dimensions can be determined as described above to provide impedances that match the input and output impedances of the microwave semiconductor, because of variations in the real-world characteristics of semiconductors, as well as parametric variations caused when the semiconductor is mounted onto the microcircuit, the actual impedances will quite frequently be moved from the optimum points. Therefore, it is necessary to provide some manner of further adjusting the impedances of the open-ended stubs. It has previously been proposed to provide some impedance adjusting means, such as represented in Figure 3, in which impedance adjusting patterns 13 and 14 formed of a plurality of conductive elements are employed. The adjusting patterns 13 and 14 are metal conductors of the same material as the transmission line 6, for example, and are arranged on the substrate at the free or open ends of the stubs 10 and 11 so that several of the elements or pieces forming the patterns may be connected together and to the stubs by hand soldering, thereby adjusting the effective lengths d₁ and d₂ as well as the effective locating distances l₁ and l₂ of the stubs 10 and 11. As might be imagined, this impedance adjusting technique requires troublesome hand labour and therefore is not suitable for low-cost mass production. An embodiment of the present invention shown in Figure 4 has the same basic structure as the circuit of Figure 1. However, the circuit of Figure 4 differs from that of Figure 1 in that adjustable conductive wire elements are provided to control accurately the impedances provided by the transmission line and open-ended stubs. More specifically, conductive wire elements 21 and 22 are provided for impedance adjustment and are arranged near the signal transmission line 6 and the open-ended stub 11, respectively. Both the conductive wire elements 21 and 22 are formed of substantially the same materials and, as is shown more clearly in Figure 5, which is a cross-sectional view taken through a section line A-A' in Figure 4, the wire element 21 comprises an inner, metallic conductive material or wire 23 having a non-conductive cover or sheath 24 arranged around it. The insulative cover or sheath 24 can be of polytetrafluoroethylene (for example that sold under the trademark "Teflon") or similar insulative material having a low high-frequency loss. One end of the conductive element 21 is bared of its insulative sheath 24 so that the inner wire or conductor 23 is exposed and this exposed end is soldered or otherwise electrically connected to the conductive ground plane 16 arranged on the side of the dielectric substrate 15 opposite the conductive strip pattern or signal transmission line 6. Thus, the orientation of each of the wire elements 21 and 22 can be adjusted freely, using the soldered end as a supporting point. Accordingly, the distances from the wire elements 21 and 22 to the signal transmission line 6 and open-ended stub 11, respectively, can be made smaller or larger by movements as shown by an arrow A in Figure 5, and the angular positions at which the wire elements 21 and 22 intersect the signal transmission line 6 and open-ended stub 11, respectively, can be changed by movements in the directions represented by arrows B and C, respectively, in Figure 4. Thus, if the wire elements 21 and 22 are arranged to be closer to the transmission line 6 and open-ended stub 11, a parallel capacitance is added to the transmission line 6 and open-ended stub 11, so that the effective length of each line can be changed equivalently, thereby carrying out an input impedance adjustment. Unlike the previously proposed approach, because the metallic conductors 23 are covered with insulative sheaths 24 there is no possibility of the transmission line pattern accidentally being circuited to ground. Although the present invention has been described above by way of example as being embodied in a high frequency amplifier circuit, the impedance adjusting measures described above need not be limited to such high frequency use but can be applied to any other use for microstrip circuits. Such other uses might comprise, for example, a mixer circuit utilised in a super high frequency to ultra high frequency converter, impedance adjustment at the output side of a local oscillator circuit, or impedance matching adjustment of a circulator. Note also that the impedance adjusting device need not be employed with each and every open-ended stub in the circuit, but can be employed as necessary to provide appropriate impedance matching adjustment. Because the impedance adjustment can be carried out simply by moving the free end of the conductive wire element, the other end of which is attached to the conductive ground plane of the microstrip circuit, and because such impedance adjusting element is provided in proximity to the signal transmission line or impedance stub, one need only change the spatial positions of the wire element relative to the signal transmission line or the open-ended stub in order to perform impedance adjustment, and bothersome and inefficient steps, such as soldering elements of an adjusting pattern, need not be performed. Thus, the impedance adjustment technique is specifically suited for low-cost mass production. That is, the impedance adjustment, or adjustment of the input/output voltage standing wave ratio (VSWR) of a high frequency microstrip amplifier, can be performed easily and the burdensome steps necessary in the above-described previous proposal are eliminated.

Claims: A microstrip circuit comprising: a signal transmission line (6); a circuit element (1) connected to the signal transmission line (6); an impedance matching element in the form of an open-ended stub (11) connected in parallel with the signal transmission line (6); a ground plane (16); characterized by an electrically conductive wire element (21, 22) formed of an electrically conductive metal wire (23) covered by an insulative sheath (24) and arranged in proximity to the signal transmission line (6) or the open-ended stub (11), the wire (23) of the wire element (21, 22) being soldered at one end to the ground plane (16) whereby the wire element (21, 22) is freely movable in space above the transmission line or the stub with said one end of the wire (23) acting as a supporting point, whereby the spatial position of the conductive wire element (21, 22) relative to the signal transmission line (6) or the stub (11) can be varied to vary an effective impedance of the signal transmission line (6) relative to the circuit element (1). A microstrip circuit according to claim 1, in which one said electrically conductive wire element (21) is arranged adjacent the signal transmission line (6) and another said electrically conductive wire element (22) is arranged adjacent the open-ended stub (11).

Inventors: KAJIWARA, TADASHI C/O SONY CORPORATION, KUMAMOTO, KENICHIRO C/O SONY CORPORATION, SATO, AKIRA C/O SONY CORPORATION, TSURUMARU, SHINOBU C/O SONY CORPORATION
Applicants: SONY CORPORATION
Application year: 1985
Main classification: H01P5
IPC codes: H01P1, H01P3, H01P5, H03F3