A COMPOUND WITH TRIMMABLE ELECTRIC IMPEDANCE I.E.A STUB

Abstract: A uniform electric component realizing a resistive-capacitive impedance, or a stub, comprising such a parallel connection of shorted and open structures of various lengths and widths of distributed RC-structures, where different structures are manifested as a steplike or smoothened shape of the line opposite the driving point of the component. The structures are also manifested as a mutual electric connection or disconnection between the resistor layer (13) and the ground plane (221) at various points of the said line. The measures of the stub are defined in a successive iteration process between the employed frequencies (f<u1>u, f<u2>u...) and the required admittances (Y<u1>u, Y<u2>u...). The stub is trimmed by shortening it or by removing or disconnecting one or more layers so that the endline of the remaining structure becomes perpendicular to the conduction band (211). The stub can also be manufactured as a reversibly trimmable structure. The stub can be used as part of a bandrejection filter.

Description: A COMPONENT WITH TRIMMABLE ELECTRIC IMPEDANCE i.e. A STUB The invention declared herein is an electric component consisting of distributed RC-structures and realizing a trimmable resistive-capasitive impedance, i.e. a trimmable complex impedance device or a stub. In the prior art are known resistors , capasitors and induct¬ ances as trimmable electric components. To realize a complex impedance, parallel or series connections of the above mentioned components or distributed RC-structures have been used. However, a trimmable complex impedance has not yet been presented and the trimming, in case of being unavoidable, is carried out by successive trimmings of a resistor and a capacitor connected in parallel or in series, the said trimmings taking place at different points of time. As the modern electronic circuits are often realized as integrated film circuits such as thick film and thin film circuits, where only one component can be t-rimmed at a time, only permanently and only to one direction, the trimming of a complex impedance or, in general, the trimming of an electric circuit by changing the complex impedance , is very difficult. That is the reason for aiming already during the design stage at solutions where a desired characteristic feature of the circuit could be adjusted by trimming only the resistor, that is to say, the resistive impedance. The distributed RC-structure which is the basis of the present invention has been presented, for example, in an article by W.M. Kaufman: "Theory of a monolithic null device and some novel circuits", pp. 1540-1545, Proc. IRE, September 1960, and in the British Patent Publication 1 149 764. The British Patent Publication, deals with two notch circuits. In this context, however, the trimming of a complex impedance cannot be referred to. The first notch filter consists of a coaxial transmission line, to which a grounding resistor has been connected. A good conductor in the middle of the coaxial transmission line is encircl by an insulation layer which in turn is encircled by a resistive outer layer. The notch is trimmed by shortening the transmission line gradually until the desired notch- frequency is achieved. It is pointed out ' in the same publication that a planar structure can be used instead of the coaxial transmission line. It must be noticed, however, that the notch describ above cannot be manufactured at an exact frequency becaus of the statistical divergency of the RC-product of the Re structure formed by the transmission line. Besides, the trimming must take place before the attaching •of * the connecting wires. In the above mentioned British Patent Publication is also presented a trimmerlike, adjustable notch filter. Therein the values of the two components, the resistive-capacitive an-d the resistive ones, can be adjusted reversibly and parallelly by using a slide. However, the manufacturing o such a circuit is difficult because the demanded ratio 17, between the resistivities of the components cannot be realized in a satisfactory manner. Thus the circuit does not function in the desired way. Besides, it would be impossible to use such a circuit in integrated circuits. Various kinds of electric circuits are presented in the US Patent Publication 3 109 983. The production of these circuits has not been practical because the manufacture divergence of their distributed RC-structures cannot be eliminated by trimming only the resistor or the capacitor. The trimmable impedance device of the present invention allows for the practical manufacturing of these circuits, too, which fact has been confirmed by various experiments. Clearly, there is a need for a component, in which both the resistive and the capacitive parts of the impedance are changed simultaneously in a fully predictable way and by a single trimming operation. The purpose of the present invention is to realize a component with a trimmable elect¬ ric impedance, i.e. a stub. This is achieved by the characteristic features of the invention, which are declared in the first claim. Earlier it has not been possible to produce a component, the impedance of which could be trimmed in an exact and predictable way. The advantages of the invented component are, above all, the accuracy of the trimming and the possibility to calculate the characteristics of the circuit exactly and in advance. These advantages are remarkable particularly when the component is part of an integrated circuit. The following figures are used to clarify the present invention: Figure 1 shows the distributed RC-structure in perspective. Figure 2 shows a longitudinal section of the RC-structure. of Figure 1. Figure 3 shows an analoguous circuit diagram of the distri¬ buted RC-structure of Figures 1 and 2. Figure 4 shows the symbol used for the distributed ' RC-structure. Figure 5 shows the circuit diagram for constructingthe stub. Figure 6 shows the physical structure of the stub. Figure 7 shows a cross-sectional drawing of a short circuited RC-structure. Figure 8 shows a cross-sectional drawing of an open RC-structure. Figure 9 shows the physical structure of a stub realized. Figure 10 shows the symbol for a stub. Figure 11 shows the principle of trimming the stub. Figure 12 clarifies the trimming process itself. Figure 13 shows the readjustable stub. Figure 14 shows a bandrejection filter or a notch. Figure 15 shows the use of the stub for realizing a band- rejection filter. The distributed RC-structure of Figure 1, forming the basi of the stub, consists of a conductor layer or secondarily of a resistor layer 11, on top of which are placed an insulation, layer 12 and a resistor layer 13 which is electrically connected with the terminating conductors 131 and 132. In most cases the film 11 is formed of a very goo electric conductor. The manufacturing of this structure is generally known, and some special methods have been described, e.g. in US Patent Publication 3 665 346 and in a German Patent Application DE 2 714 034. Figure 2 shows the longitudinal section of the distributed RC-structure. In Figure 3 there is an analoguous circuit diagram of the distributed RC-structure presented in Figures 1 and 2. Figure 4 represents the symbol for it. The layer 11 which is -connected with the end conductor 111 is called a ground plane. Futhermore, the structure is called an open one, if all the end conductors 131, 132 and 111 are unconnected with each other. If one of the end conductors 131, 132 of the resistor layer 13 is connected with the ground plane 11 or with its end conductor 111, the type of the structure is referred to as short circuited When thin film technology is used in order to realize a distributed RC-structure, the thickness of the layers 11, 12 and 13 is in general less than one micrometer, whereas it varies between 5 and 50 micrometers when thick film technology is used. Theoretical calculations have shown that the open distri¬ buted RC-structure can realize impedances, the phase angle whereof will vary from 43 to 90 degrees. A shorted structure is able to realize impedances, the phase angle of which will yary from 0 up to 47 degrees. After the required phase angle has been achieved, the desired absolut value of the impedance can be obtained by changing the shape and the measures of the structures of Figure 1. With an open structure the absolute value of the impedance decreases no more than in proportion to the increase of the frequency and no less than in proportion to the square root of the increase of the frequency. The corresponding decrease for the short circuited structure takes place in relation to the increase of the square root of the frequency at the most. Thus one distributed RC-structure can realize one impedance and only at one frequency. An impedance of this kind cannot be trimmed. A trimmable impedance can be realized, however, by the invented integrated connection of the distributed RC-structures, which is dealt with in the following by using the inverses if the impedances, that is, the admittances. The phase angles mentioned above are valid as such for the admittances, and the admittances increase along with the frequency in the same proportion as the impedances decrease in the above description. The realization of the stub i.s shown in Figures 5-9. The following is an example to clarify the method of forming a stub. Let the frequencies f, , f - , f,, f. represent equal or different frequencies of alternating current. The desired admittances at these frequencies are correspondingly Y- j, , Y 2 , Y 3 and Y^ . The distributed RC-structure 231 of Figure 5 is realized so that its admittance at the frequency £-. will be Y- j . Then the admittance Y f 2 of the structure 231 at the frequency _\~ are calculated. If it is smaller than the required Y ~ at this frequency, a distributed RC-struct- ure 232 with suitable dimensions is added between the point 21 and 22, that is, next to the structure 231, in order to eliminate the difference Y- - Y' ? * ^ n "this wa Y - frequency- admittance pairs are realized one at a time by always taking the former structures into account, until all the frequencies have been dealt with or till the task appears impossible, which may sometimes be the case. In general, the realization of the stub is possible, if both the resistive and the capacitive part of the admittance to be realized increase or decrease along the frequency. The additional structures 231, 232, 233 and 234 are eithe open (Figure 8) or short circuited (Figure 7) depending o the phase angles of their admittances. As they all have a common ground plane 221 and a common insulation layer 12, they can be prepared into a continuous structure by using the layer 221 as a ground plane. I order to avoid the electric interaction of the structures 231, 232,... of th stub connected with the conduction band 211, slots 141, 142,... are left between them as shown in Figure 6. When the adjoining parts 231, 232,... of the stub are approxim ately of equal length, which is generally the case, they can be merged without any considerable error to form a continuous structure according to Figure 9, where number 13 is the resistor layer in question and number 12 is the insulation layer. In the shorted structure (Figure 7) the resistor layer 13 crosses the edge of the insulation laye 12, and is thus electrically connected to the ground plane 221 as shown in the profile. The accurary of the line 230 of the resistor layer opposit to the driving point 211 of the stub can be increased by choosing more frequency points. This being done, the line acquires a more frequent stepping. When the stepping is frequent enough, it can be smoothened to form a curved lin without error. The stub is trimmed by shortening it from its most recently added structure, which is 234 in the above example, so that the layer 13 is removed or cut, or the whole structure is cut, the cut being rectangular to the conductor 221 of the driving point, as shown in Figure 11. When the resistor layer 13 of the stub is being remove with the A~0,-particles 5//of an air-abrasive trimmer4/;/ which is the most common trimming method of film resistors along with laser cutting, the admittance measured between the terminations of the stub changes along the line A as show in Figure 12. There is a definite difference between this /^& U RE OM and resistor trimming: in the latter the admittance moves along the real axis Re. In capacitor trimming, on the other hand, the admittance moves only along the imaginary axis Im. Thus it can be said that one measure carried out when shortening the stub corresponds to separate measures, that is, trimming both a resistor and a capacitor and these two even to the required magnitude. If it is desired to produce an admittance to be trimmed along the curve B, it is achieved by designing the form of the stub accord- ingly. The trimming can be made reversible with the arrangements of Figure 13. The different parts 231, 232 etc. of the stub are manufactured so that they will not interact electrically. In practice this means that the resistor layer 13 and the conduction band 212 of the driving point are disconnected between various parts (e.g. Figure 6). The trimming or the connection of the parts of the required structure as well as the disconnection is carried out by employing the slide 213 conducting electricity, which can be moved paral ' lelly to the conductor band 212, the slide 'being duly propped up. By constructing this kind of a stub to form a circle, a potentiometer adjusting complex impedance will be obtained. As the stub can be constructed so that its characteristic features do not change more than 0.1 % along with the time and temperature, it can be used as a trimmable element in the construction of various circuits, such as oscillators, filters and attenuators. A bandr ' e ectionfil ter is presented in Figure 14 as an application of this. The manufacturing of this filter has not succeeded in a satisfactory way, even though efforts have been made since 1960. The reason for the failure has been the fact that it has not been possible to trim the stop frequency of the filter which is the characteristic most often required, only by changing the value of the resistor or the capacitor. This being the case, the stop frequency will depend on the manufacturing distribution of the distributed RC-structure 1. Between the input 31 and the output 32 of the circuit, th transmission of the alternating current is completely hindered at the frequency 1,78/RC. Here R stand for the total resistance between the end conductors of the distri buted RC-structure, while C represents the total capacitan of the insulation layer 12 between the ground plane 11 an the resistor layer 13, if the value of the resistor 33 equals exactly the value of the distributed resistance divided by 17,8. The problem is that then the rejection frequency has the same statistical divergence as the distributed RC-product, which in practice means that they are distributed within 5 to 20 I on both sides of the desired value. However, the stop frequency of the circuit can be continuously adjusted by the capacitive-resistive admittanc 34 placed parallel to the resistor 33 as shown in Figure 15. By calculating values for this admittance .correspondi various stop frequencies and realizing them by using a st a circuit is obtained, . the stub frequency whereof can be - trimmed by shortening the stub. The attenuation taking place in the stop frequency can be trimmed from the resistor 33. A series of circuits has been manufactured, the stop frequency whereof could be trimmed 281 in relatio to the value 1,78/RC. The above description is only one example of the adapt¬ ability and the use of the stub in trimming an electronic circuit. Electric components of different kinds can be connected with the stub so that the desired trimming of the impedance (admittance) can be realized with the stub of the present invention. Thus the circuit can be modified with known components attached to the stub and other point of the circuit, in order to make the stub as suitable as possible for the required trimming of the circuit.

Claims: CLAIMS 1. An electric component consisting of distributed RC- structures, i.e. a stub realizing the resistive-capasitive impedance, c h a r a c t e r i z e d in that it comprises such a parallel connection of distributed RC-structures of rectangular or other similar shape which consists of conductor, insulation and resistor layers placed on top of each other, where the shorted (figure 7) and open (figure 8) structures of various lengths and widths form a continuous component, wherein various structures are realized as a steplike shape of the marginal line (230) opposite the driving point (211) of the component, or as its smoothened form and as an electric connection or disconnec¬ tion between the resistor layer (13) and<the ground plane (221) at different points of the said marginal line. 2. A method for the stub of claim 1, c h a r a c t e r ¬ i z e d in that while realizing the desired admittance (Y,) at the desired frequency (f-i) of the alternating current in certain length of the resistor layer (13) in the first distributed RC-structure (e.g. 231 figure 5 or figure 8), it also realizes another admittance (Y' 2 )" at another frequency (f 2 ), and if the admittance thus achieved is smaller than the required admittance (Y 2 ) , another distri¬ buted RC-structure (232) of proper size is added parallel to the. first distributed RC-structure (231) in order to eliminate the difference (Y 2 - Y' -) , and that the trimming described above can be performed step by step for several admittance values. 3. A method for the stub of claim 2, c h a r a c t e r ¬ i z e d in that the described trimming can be performed continuously within a certain admittance area. 4. A component of claim 1, c h a r a c t e r i z e d in that the trimming performed by shortening the component is accomplished by removing or disconnecting one or more - SRE layers, that the end line of the remaining structure is perpendicular in relation to the conduction band (211) of the driving point, and that the part of the structure lef on the eliminated side of the endline will not affect the electrical properties of the component. 5. The component of claim 1, c h a r a c t e r i z e d i that it is reversibly trimmable by connecting or disconnect ing the required number of structures by employing a slid (213) which moves in parallel with the termination conducto and which will short the parts of the stub from its starting point (23) to the terminal point (24) of the slid and that the trimming is possible when the distributed RC-structures (231, 232 etc) forming the stub are construct in such a way that they are electrically isolated. 6. The use of the stub of claim 1 as part of a bandrejection filter consisting of a distributed RC-structure (1 figure 14) and a ground resistor (33) c h a r a c t e r i z e d in that the stub (34 figure 15) is connected in parallel with the resistor (33) .

Inventors: LEPPAEVUORI S, RAPELI J
Applicants: LEPPAEVUORI S, RAPELI J
Application year: 1981
Main classification: H03H7
IPC codes: H01C10, H01C17, H01L27, H01P1, H03H1, H03H7