INTEGRATED ELECTRONIC CIRCUIT COMPRISING AN ELECTRONIC COMPONENT AND A DELAY ELEMENT WHICH IS CONNECTED TO THE COMPONENT

Abstract: An electronic circuit with at least one active electronic component (10) and a delay element connected to the component. The delay element comprises an electric conductor (13, 18) which is composed of a conductive material on an insulating layer (14) such that the conductor's capacitive coupling to the surrounding becomes low. The conductor (13, 18) is drawn such that its inductance is minimized. The electronic component (10) and conductor (13, 18) are integrated in an integrated circuit. Conductor (13, 18) is drawn in loops of some length, and ground conductor (35) of some length conected to a ground plane is provided between the loops. The delay element can be used in an oscillator coupling or in an electronically controlled antenna. In the oscillator coupling the electronic component (10) comprises an inverter coupling (11) with an input (11) and an output (12). The conductor (13) is provided to connect input (11) to output (12), thus creating an oscillator. The electronically controlled antenna comprises a receiver (17) and a number of antenna elements (16) connected to the receiver (17). Individual antenna elements (16) are connected to the receiver (17) via automatically selectable delay elements (18) such that electric signals from different antenna elements (16) are differentially delayed dependent on the position of the antenna elements in relation to the impinging electromagnetic wave fronts.

Description: Integrated electronic circuit comprising an electronic component and a delay element which is connected to the component SCOPE OF THE INVENTION The invention relates to a delay circuit with at least one active electronic component and a delay element connected to the component. STATE OF THE TECHNOLOGY In many different types of electronic circuits, components or configurations with a certain time constant or delay are used. In electronic circuits that are realized in integrated form there are delay components separated from the other components. An example of such an electronic circuit is the oscillator. A well-known way to set up an oscillator is to feed back a gate circuit with an inverter function, for example a simple inverter, via some form of time delay. The time delay can be implemented in various ways, for example by special delay circuits. In applications where high working frequencies are used, and particu- larly when the requirements of a frequency-stable oscillator are high, a crystal oscillator is used instead. An exactly embodied crystal oscillates mechanically at a well-defined and stable frequency, and the mechanical oscillation is converted to an electric oscillation. A problem with use of crystal oscillators is that they are relatively large. In connection with certain types of integrated circuits this becomes a problem in that a special circuit must be used with the oscillator or a crystal must be connected from outside the integrated circuit. With known oscillators the delay that arises in RC networks normally is used, i.e., electrical networks with both resistance and capacitance. Within higher frequency ranges antennas for transmission and reception of electromagnetic signals are used with a special embodiment for exploiting signals for strong directional effects. The currently most common type of antenna in this connection is the parabolic antenna. So-called elec- tronically controlled antennas or phase array antennas are becoming more common. A phase array antenna incorporates a group of identical radiation elements. By controlled feeding of the elements by means of a conductor network so that they act in phase, an electromagnetic beam with good direc- tionality can be achieved. The conductor network can also incorporate electronically controlled phase inverters and possibly amplifiers, one for each element, by means of which the direction of the beam can be varied without need of mechanically moveable components. The electronically controlled phase inverters are complicated and ex- pensive instruments, and for that reason this type of antenna has not enjoyed great commercial success. Another factor which limits the use of this type of antenna is the frequency dependency of the phase inverters. The foremost area of use for the phase array antenna is within the radar area. SUMMARY OF THE INVENTION One purpose of the invention is to produce an electronic circuit that can be embodied in a simple manner and that can be combined with other circuits in an integrated form. The electronic circuit enables electronic delay of an electric signal. This purpose is achieved by inclusion in the invention of the special features cited in Claim 1. According to one aspect of the invention an oscillator has been achieved that is realizable in all components in integrated form. According to another aspect of the invention an electronically controlled antenna has been achieved that lacks the disadvantages of the phase inverters. Electric signals from individual antenna elements are differentially delayed dependent on the position of the antenna elements in relation to the impinging electromagnetic wave fronts. Emanating from one matrix of the antenna elements that are distributed across a surface, the signals can be selectively delayed from indi- vidual antenna elements and thus affect the direction in which the antenna will be sensitive. The delay is preferably produced in integrated circuits, which are provided in connection with the antenna elements. According to one embodiment an installation of conductors of different length is provided between the antenna elements and a receiving unit. A conductor of a certain length is switched on by a control unit dependent on the position of the antenna element in relation to an impinging wave front of an electromagnetic wave. The given position affects the detectable characteristics of the electric signals that are received. The installation of conductors and the control unit are joined with other electronic components in a cell unit. The conductor is embodied in integrated form on an insulating layer. The following description, drawings, and dependent patent claims explicate further advantages and special features of the invention. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with the aid of examples of embodiments with reference to attached drawings, on which FIG 1 schematically shows an embodiment according to the invention of an electronic circuit in the form of an oscillator, FIG 2 schematically shows the distributed resistance and capacitance of a conductor, FIG 3 is a diagram of the delay in a conductor as a function of the length of the conductor, FIG 4 is a principle block diagram that shows an alternative embodiment according to the invention of an electronic circuit in the form of a cell unit with antenna peripherals, shows in principle how several cell units are provided on a portion of an antenna surface, schematically shows an embodiment of a delay element in the form of a conductor, which constitutes part of the invention, shows an embodiment schematically of a conductor that can be connected in different lengths, is a principal wiring diagram that shows a mixer included in an electronic circuit according to the invention, shows schematically a portion of a long conductor according to the invention with intermediately located ground conductors, is a principal wiring diagram that shows a digital amplifier included in an electronic circuit according to the invention, is a principle wiring diagram that shows an analog amplifier included in an electronic circuit according to the invention, is a cross-section view that shows a chip with conductors according to an embodiment of the invention, is a diagram that shows electromagnetic field curves around conductors provided in a dielectric with the dielectric constant ε = 35 in a delay circuit with a ground conductor according to the invention, is a diagram that shows electromagnetic field curves around conductors provided in a dielectric with the dielectric constant ε = 3.9 in a delay circuit with ground conductors according to the invention, is a diagram that shows electromagnetic field curves around conductors provided in a dielectric with the dielectric constant ε = 1 in a de- lay circuit with ground conductors according to the invention, is a diagram that shows electromagnetic field curves around conductors provided in a dielectric with the dielectric constant ε = 35 in a delay circuit without ground conductors, is a diagram that shows electromagnetic field curves around conduc- tors provided in a dielectric with the dielectric constant ε = 3.9 in a delay circuit without ground conductors, and FIG 18 is a diagram that shows electromagnetic field curves around conductors provided in a dielectric with the dielectric constant ε = 1 in a delay circuit without ground conductors. THE INVENTION In the embodiment according to FIG 1 an inverter coupling 10 is shown schematically, whose input 11 is connected to its output 12 via a long conductor 13. For a conductor of the length I, a time period T is given by the following formula where v c is the velocity of electromagnetic propagation in the conductor. T - ^- (1) V c At v c =200,000km/s and a given length I of 0.5m the frequency is 200MHz. This is under the condition that the delay in the inverter coupling itself is on the magnitude of some picoseconds. With low propagation in the inverter coupling's delay of, for example, 10% the inaccuracy is in the same class as that of a crystal oscillator. This length required for attainment of the desired delay acts in many cases as a deterrent. The actually delay in the conductor is also dependent on the conductor's distributed resistance and the conductor's distributed capacitance. In an ideal conductor any variations in time period and thus in frequency depend only on performance characteristics in the inverter coupling. If the delay in inverter coupling 10 is small relative to the delay in conductor 13, which primarily is dependent on the length of the conductor, the inverter coupling affects accuracy very little. The conductor is embodied as a metal conductor in an integrated process, for example a CMOS process. Conductor 13 is embodied with width w and rests on some form of insulating layer 14. The insulating layer contains both metal oxide and field oxide and has thickness b. The dielectric constant of the insulating layer affects the choice of the dimensions of the conductor and of other components. As the dielectric constant of the insulating layer increases, the dimensions can be reduced. A semiconducting material 15, for example silicium, surrounds the portion that holds the conductor. Inverter coupling 10 and other semiconductor circuits can be embodied in semiconducting materials, whereby these are given access to the oscillator. In one embodiment the insulating layer is constituted of silicon oxide. The insulating layer can also be embodied in glass or another ceramic material. As an alternative the insulating layer can be embodied as an air layer. In such an embodiment the conductor rests on stanchion-like formations that extend from the semiconductor or base material used. The conductor should be embodied so that it gives rise to as little in- ductance as possible. The conductor preferably runs in a loop with several parallel conductor sections so that the directions of the current in adjacent conductor sections are opposite to each other. It is also possible to provide two parallel loops in opposition or in double loops. According to an especially preferred embodiment of the invention the conductor is embodied in two loops running in parallel of which one is fed with the signal to be delayed and the other is fed with the inverse of the signal. See FIG 7 and FIG 9. In this way the influence of several different forms of interference is minimized. In an actual conductor, time delay occurs as a result of the conductor's distributed resistance and the conductor's distributed capacitance according to the formula below. The distributed characteristics can be illustrated according to FIG 2. The total length of the conductor is I. In FIG 2, C is the total capacitance of the conductor, and R is the total resistance of the conductor. The resistance is generally given by the following formula: p- l p - l K • / A a - w w p where K r = — and A w - / C = ε — = ε —— = K c - w / (3) d b ε where K c = — . For the formulas: b ε = the dielectricity constant for the insulating (oxide) layer, p = the resistance in the conductor material, a = the thickness of the conductor, b = the thickness of the insulating (oxide) layer, and w = the narrowest width of the conductor. From the relations above, the time delay is given: " R C t κr = V — • — when n → ∞ (4) RC o n n The time delay can be reformulated as time delay in small time segments, where the following obtains: Δt RC = ΔR - ΔC = K r • — - K. - w - Δ/ = K rc - Δ/ 2 (5) w The constants used previously are consolidated here in a new con- stant K rc , for which the following obtains: ε • p This constant is strongly dependent on temperature and voltage, for which reason it should be minimized. It should be noted that the time delay does not depend on the line width of the process. By means of transition to infinitely small time segments it can be determined that the result is a function that increases potentially. The diagram in FIG 3 with the curve t A shows the delay that depends on the velocity of propagation in the conductor. The curve t RC shows the de- lay that depends on the resistance R of the conductor and capacitance C. The velocity of propagation v in the conductor cannot be affected to any great degree. It can be shown that where c = the speed of light and ε r = the dielectric constant of the material. As emerges from the above formulas, the effect of t RC is lower the thicker the insulating layer is and the thicker the conductor which is used. The dashed line in FIG 3 shows how the curve t RC is affected when the insulating layer and the conductor are made thicker. By using the area in which the effect from t RC is low, the oscillator can have good stability and its frequency of oscillation is completely controlled essentially by the length of the conductor. Insulation layer 14 is embodied preferably with significantly greater thickness b than the occurring oxide layer. A suitable thickness exceeds 10 μm and is preferably in the range of 10-100 μm if the oscillator is to be used at frequencies around 1 GHz. A thickness b suitable for many applications is 20 μm. With increasing values of b other dimensions can also increase. External dimensions naturally also affect the final dimensions. The thickness of conductor 13 is in commonly occurring processes around 1 μm, and an increase in thickness, but not width, improves the oscillator's characteristics and performance. Inductive characteristics also need special attention as regards the length of the conductor. It is thus not suitable to embody the conductor in a spiral form or similar. The length should also be suited to the desired wavelength of the oscillator. The length of the conductor preferably amounts to a multiple of the half wavelength or, more preferably, half the wavelength. In the embodiment according to FIG 4 the invention comprises antenna elements 16, which are connected to a cell unit 22. Cell unit 22, which is designated by dashed lines in FIG 4, comprises an installation of individually switchable delay elements 18, by means of which a signal received in antenna element 16 is directed on to a receiver 17 acting in common for a number of cell units 22. The received signal is amplified in an amplifier 23. Switching of delay elements 18 is accomplished in the embodiment according to FIG 4 by means of a demultiplexer 19 and a multiplexer 21. Delay elements 18 in the form of conductors connect demultiplexer 19 and multiplexer 21 , and an individual delay element 18 is switched by means of adjustment of demultiplexer 19 and/or multiplexer 21. Adjustment is done by a control unit 20, which is connected to a central processing unit (CPU) 24 in common for several cell units 22. In order to lessen problems with persistent capacitive and inductive coupling, among other things, whereby the incoming signal can have a frequency on the order of magnitude of 12 GHz, the incoming signal is prefera- bly merged with a signal from a local oscillator 25 and sent to a mixer 26. See also FIG 8. From mixer 26 the signal suitably has a frequency on the order of magnitude of some GHz. The different control units 20 and possibly also the local oscillators 25 are connected to a CPU 24, preferably by a buss connection 27. It can be suitable to include in CPU 24 means for synchronization of the different oscillators 25. Synchronization can also occur via control unit 20. According to an alternative embodiment (not shown) the oscillator is not included in the cell unit. A common oscillator is instead preferably located in the CPU. The embodiment of the oscillator used should be adjustable for different frequency bands. All components, which are included in cell unit 22, can be embodied to be integrated in semiconducting materials. The semiconductor process used should be selected with regard to high frequency characteristics, especially as regards amplifier 23, and to characteristics that affect conductors that can be included in the delay elements. The noise ratio should be on the order of magnitude of 0.5 dBu. Very low capacity switches should be sought. The conductor is embodied as a metal conductor in an integrated process, for example a CMOS process. Receiver 17 can be embodied in a conventional way as a satellite receiver. It can be the case that satellite receivers are provided with control instruments for motorized control of a conventional parabola antenna. Receiver 17 includes similar control instruments, and a control output 28 transfers control information to CPU 24. The control information can include instructions to sweep with the electrically controlled antenna across a certain arc in connection with finding a new transmitter. When a transmitter is found, the control information will continuously control the adjustment of the antenna so that the transmitter can be followed if the antenna is physically angled or displaced in relation to the transmitter. A signal conductor 29, preferably from each of the cell units 22, conducts a received signal from the antenna to receiver 17. The quality and certain characteristics of the signal from the antenna affect how CPU 24 will be controlled in turn in order to affect the different control units 20 in the cell units 22. The number of delay elements 18, which are required in order that the desired possibilities for fine tuning of the antenna can be achieved, varies with the current application. For normal satellite receiver application some hundreds of delay elements 18 should be sufficient. The characteristics of amplifier 23 also affect how many delay elements 18 are required. With very good amplification characteristics and signal-noise relation in the amplifier the number of directionally adjusting delay elements 18 can be held down. An antenna embodied with components according to the above can be embodied as indicated by FIG 5. Provided on a surface are a number of cell units 22. Every cell unit 22 is connected to four antenna elements 16A-16D attached in pairs. Two opposing first antenna elements 16A and 16B are dedicated to reception of horizontally polarized signals, and two opposing second antenna elements 16C and 16D are dedicated to reception of verti- cally polarized signals. Other configurations can also be used for reception of different types of signals. Every antenna element 16A-16D can be some mil- limeters long and wide, and different forms can occur. Antenna elements 16A-16D are preferably embodied of metal. The outer dimensions of the antenna with a suitable number of antenna elements can be such that the surface of the antenna is on the order of magnitude of 0.1-1.0 m 2 . Buss connection 27 preferably runs through or past each cell unit 22. The different antenna elements 16A-16D can be attached to amplifier 23 directly or via a multiplexer, which is suitably controlled by control unit 20. FIG 6 shows in principle how conductors 18 can be embodied. Each conductor 18 is embodied with width w and rests on some form of insulating layer 14. The insulating layer is normally comprised of both metal oxide and field oxide and had thickness b. A semiconducting material 15, for example silicium, surrounds the section that supports the conductor. In the semicon- ductive material amplifier 23 and further semiconductor circuits can be embodied. In one embodiment the insulating layer is made of silicon oxide. The insulating layer can also be embodied of glass. As an alternative the insulating layer can be embodied as a layer of air. In such an embodiment the conductor rests on stanchion-like formations which emerge from the semicon- ductive or base material used. An example of a double conductor with selectable delay is shown in FIG 7. Signal S that is to be delayed is directed into a buffer or line amplifier 30. The inverse of the signal S' is also directed into line amplifier 30. Two parallel conductors that run together in a long loop extend from line amplifier 30. As indicated above, the total length of the loop can be as great as 0.5 m. From each of the two parallel conductors 13, terminal conductors 31 , 31 ' extend at different distances from the line amplifier to a joint output conductor 32 and 32', respectively. The selection of conductor length and thus the terminal conductor is made in a demultiplexer (not shown) that directs the delayed signal via control lines 33 and adjustable gate circuits 34. The signal that is delayed can be analog, for example in conjunction with an antenna of the type that is shown in FIG 4 and FIG 5 or digital in conjunction with an oscillator or similar. The delay can be selected in stepwise fashion with each step corresponding to the length of the conductor between two output sites. When the device according to the invention is used with antennas and receivers, mixer 26 is normally used. An example of such a mixer 26 is shown in FIG 8. Two signals in opposite phase are output from a local oscillator 25. The signals in opposite phase are directed to a mixer 26 that comprises four MOS transistors in the embodiment shown. A preferred embodiment of conductor 13 is shown in FIG 9. Conductor 13 runs in two parallel tracks in elongated loops. The two tracks are fed with signals in opposite phase. In this way the circuit is less sensitive to interference. A portion of a ground conductor 35 is located in each loop. Ground conductor 35 suitably runs in the center between two loops and essentially to the bottom of a loop. Ground conductor 35 is connected to a ground plane. The distance between two conductors in a loop is on the same order of magnitude as the distance between two loops. These distances preferably are approximately 20 μm. The width of the conductor is on the order of magnitude of half of the given distance or about 8 μm. See also FIG 12. The total length of the conductor is determined by the current application and can be on the order of magnitude of 0.5 m or larger. A ground conductor 35 can also be used if conductor 13 is embodied as a single conductor. Amplifiers can be provided in different sections of the long conductor track. A suitable embodiment of an amplifier for use in digital applications is shown in FIG 10. Two NAND gate circuits 36 provided with two inputs are fed back via their outputs so that change in the gate circuits occurs at the same time. The signal is directed to the amplifier as an input signal and its inverse S'. A corresponding amplifier for analog applications is shown schematically in FIG 11. Both signal S and its inverse S' are also directed to the am- plifier in this case. FIG 12 shows schematically an example of how a chip with delay elements according to the invention can look in cross section. Two conductors 13 in a loop are embodied with an intermediate distance D = 20 μm. In addition, another loop conductor runs parallel with a ground conductor 35 at es- sentially the same distance M = 20 μm. Both the conductors 13 are embodied so that an insulating material 14 surrounds them. In the embodiment shown the insulating material is made of silicon oxide. Silicon oxide layer 14 has a thickness that means that the distance N to a underlying silicium layer can amount to 20 μm. The width of conductors 13 amounts to approximately 8 μm, while the width of the ground conductor amounts to approximately 2 μm. The distances mentioned are approximate and can vary depending on application and material selected for insulating layer 14. A glass layer 37 is provided on top of silicon oxide layer 14. If air is used as the insulating mate- rial, glass layer 37 can be eliminated. FIG 13 to FIG 15 are diagrams that schematically show propagation of electromagnetic fields around conductors 13 when ground conductor 35 is used. A loop surrounds the two central conductors 13, while conductor 13 to the farthest on the right belongs to an adjacent loop. A boundary line 38 indi- cates in the figures that two different dielectric constants occur. Conductors 13 and ground conductors 35 are completely recessed into the insulating layer. The solid field lines reference the magnetic field and the arrows indicate electric field strength. The electric field strength at the ends of the arrows should be proportional to the length of the arrows. Above boundary line 38 the dielectric constant is ε = 1 , for example in form of air. Under boundary line 38 the dielectric constant is ε = 35 in the example in FIG 13. The figure shows that ground conductor 35 acts to diminish the connection between the conductors in different loops. A higher dielectric constant also offers the possibility of diminishing the distance between the conductors without increased risk of interference. In the example in FIG 14 the dielectric constant ε = 3.9 in the material under boundary line 38. A suitable silicon dioxide layer has proven to have this dielectric constant. In the example in FIG 15 conductors 13 and the ground conductors are provided on stanchions or similar in the air, so that dielectric constant ε = 1 around the conductors. FIG 16 to FIG 18 are diagrams that schematically show propagation of electromagnetic fields around conductors 13 when ground conductor 35 is not present. Other parameters are in agreement with the examples in FIG 13 to FIG 15. In all of these three cases it is clearly indicated that the risk of cross talk and other interference between the conductors in different portions of the loop increases when the ground conductor is not present.

Claims: CLAIMS: 1. An electronic circuit with at least one active electronic component (10) and a delay element connected to the component, characterized in that the delay element comprises an electric conductor (13, 18), that said conductor (13, 18) is formed by a conductive material on an insulating layer (14) such that the conductor's capacitive coupling to the surrounding becomes low, that conductor (13, 18) is drawn so that its inductance is minimized, that conductor (13, 18) is drawn in elongated loops, that an elongated ground conductor (35) connected to a ground plane is provided between the loops, and that the electronic component (10) and conductor (13, 18) are integrated in an integrated circuit. 2. An electronic circuit according to Claim 1, characterized in that said conductor (13, 18) is drawn in the form of conductor sections that run parallel, the directions of current being reverse in adjacent conductor sections. 3. An electronic circuit according to Claim ^characterized in that insulating layer (14) is composed of oxide material. 4. An electronic circuit according to Claim 1, characterized in that said insulating layer (14) surrounds conductor (13, 18). 5 An electronic circuit according to Claim 1, characterized in that said insulating layer (14) is composed of air. 6. An electronic circuit according to Claim 1, characterized in that terminal conductors (31) are connected to conductor (13, 18) at certain intervals, and that terminal conductors (31) are connected via transmission gates (34) to joint output conductors (32). 7. An electronic circuit according to Claim 6, characterized in that terminal conductors (31) are connected to the joint output conductors (32) via line amplifiers (30) and that line amplifiers (30) are embodied with higher amplification the longer conductor (13, 18) has been run at the connection with terminal conductor (31). 8. An electronic circuit according to Claim 1, characterized in that the thickness of the insulating layer (14) exceeds 20 μm. 9. An electronic circuit according to Claim 1, characterized in that the thickness of the conductor (13) exceeds 1 μm 10. An electronic circuit according to Claim ^characterized in that electronic component (10) comprises an inverter connection with input (11) and output (12) and that at least one conductor (13) is provided in order to connect input (11) to output (12) during formation of an oscillator. 11. An electronic circuit according to Claim 10, characterized in that inverter connection (10) and conductor (13) are made in a semiconductor process with semiconductor / metal on glass / ceramic. 12. An electronic circuit according to Claim 10, characterized in that said inverter coupling (10) is made of semiconductor material, 13. An electronic circuit according to Claim 10, characterized in that said inverter coupling (10) is made in a process which includes silicium on the insulator. 14. An electronic circuit according to Claim 1, characterized in that the length of said conductor (13) exceeds 0.2 m. 15. An electronic circuit according to Claim 1, comprising an electronically controlled antenna, a receiver (17), and a number of antenna elements (16) connected to said receiver (17), c h a r a c t e r i z e d in that individual antenna elements (16) are connected to the receiver (17) via automatically selectable delay elements (18) such that electric signals from different antenna elements (16) are differentially delayed dependent on the position of the antenna elements in relation to the impinging electromagnetic wave fronts. 16. An electronic circuit according to Claim 15, characterized in that said delay elements (18) comprise electrical conductors of different lengths. 17. An electronic circuit according to Claim 12, characterized in that individual antenna elements (16) are connected to said delay elements (18) via a demultiplexer (19), that individual antenna elements (16) are connected to said receiver (17) via a multiplexer (21), and that said demultiplexer (19) is operatively connected to a control unit (20) such that an individual delay element (18) is switched dependent on the characteristics of the received electric signals. 18. An electronic circuit according to Claim 17, characterized in that said demultiplexer (19), multiplexer (21) and control unit (20) are provided together with said delay elements (18) in a cell unit (22) made as an integrated circuit, and that an antenna element (16) is associated with a cell unit (22). 19. An electronic circuit according to Claim 17, characterized in that said receiver (17) is connected to a Central Processing Unit (24) for transfer of control information for controlling the antenna, and that said CPU (24) is connected to the control units (20) associated with said antenna elements (16). 20. An electronic circuit according to Claim 17, characterized in that individual antenna elements (16) are connected to said demultiplexer (19) via an amplifier (23). 21. An electronic circuit according to Claim 17, characterized in that individual antenna elements (16) are connected to said demultiplexer (19) via a mixer (26), and that said mixer (26) is also connected to an oscillator (25).

Inventors: AHLM ROGER, AHLM, ROGER
Applicants: AHLM ROGER, AVANCERADE LOGIKMASKINER AB, AHLM, ROGER
Application year: 2000
Main classification: H03H7
IPC codes: H01L23, H01L27, H01Q1, H01Q3, H01Q7, H03H7