A 90 degree phase shifter

Abstract: A switched line type 90° phase shifter includes two single pole double throw switches (50,51) which receive an input signal to an input terminal (1) and outputs the signal to either of two output terminals, or which receive two input signals signal to the output terminal (2), a reference transmission line (15) having an electrical length of α, connected between ones of the two terminals of the first and the second single pole double throw switches, a phase difference producing transmission line (14) having an electrical length of (90°+α) in the usage frequency, connected between the others of the two terminals of the first and the second single pole double throw switches, a phase inverting circuit (16) provided switchablely between a state of being inserted serially to and between two parts (15a,15b) of the reference transmission line, which two parts produce the entirety of the reference transmission line, the one terminal of the first single pole double throw switch is an input terminal of the entire terminal and one terminal of the second single pole double throw switch is an output terminal.

Description: FIELD OF THE INVENTION The present invention relates to a 90° phase shifter, and more particularly, to a switched line type phase shifter. BACKGROUND OF THE INVENTION Figure 8 is a circuit diagram of a conventional switched line type phase shifter. In the figure, reference numeral 1 designates an input terminal and reference numeral 2 designates an output terminal. Four field effect transistors 3 are provided at two paths from the input terminal 1 or at the two paths to the output terminal 2 (hereinafter referred to as "FET"). Reference numeral 4 designates resonance lines connected between source and drain electrodes of the FETs 3, respectively, to constitute resonance inductances, respectively. Reference numeral 5 designate gate bias terminals of respective FETs 3. A reference line 6 having a predetermined electrical length α is provided between the other end of one of the input side two FETs 3 and the other end of one of the output side two FETs 3. A phase difference producing line 7 having an electrical length (α + β) which is longer than that of the reference line 6 by a desired electrical length β is provided between the other end of the other one of the input side two FETs 3 and the other end of the other one of the output side two FETs 3. Description is given of the operation. This switched line type phase shifter is constituted by two single pole double throw switches 50 and 51 which receive signals to the one input terminal 1, 2 and output the signals to either of the two output terminals 40a and 40b, 41a and 41b, and two transmission lines 6 and 7 connected between respective output terminals of the one or the other of the two switches, that have electrical length α, (α + β), respectively. Therefore, by switching the path for the input signal which is input to the input terminal 1 of this phase shifter between that transmitted on the reference line 6 having an electrical length α to reach the output terminal 2 of this phase shifter, or that transmitted on the transmission line 7 having an electrical length (α + β) which is longer by a desired electrical length β than the reference line 6, a phase quantity of the difference β in the electrical length is obtained. In other words, the switched line type phase shifter shown in figure 8 performs a switching operation by the resonance circuit comprising the FET 3 and the resonance line 4. When the gate bias voltage of the FET 3 is set at zero volt, the path between the source and drain electrodes can be seen of equivalently a low resistance of below several Ω, meaning an on-state. When the gate bias voltage of the FET 3 is set below the pinch-off voltage, the path between the source and drain electrodes can be seen as equivalently a parallel circuit comprising a resistance of several kΩ and a capacitance at off-state (C T ), and this occurs resonance by the off-time capacitance (C T ) and the resonance line 4 connected between the source and the drain of the FET, thereby showing an off-state. Even in this off-state, however, it is actually impossible to realize an ideal off-state. Accordingly, a leakage signal is transmitted through the line of the off-state side, and as a result, a signal which is output to the output terminal of the phase shifter becomes one that is obtained by vector summing the signal transmitted on the on-state line and the leakage signal transmitted on the off-state line. Figure 9 shows a diagram of this vector summation being performed. In figure 9, reference numeral 8 represents a signal vector of a signal transmitted on the reference signal line 6. Reference numeral 9 represents a signal vector of a leakage signal transmitted on the line 7. Reference numeral 10 designates a vector obtained by summing in vector the both vectors 8 and 9. Reference numeral 11 represents a signal vector of a signal transmitted on the line 7. Reference numeral 12 represents a signal vector of a signal transmitted on the reference line 6. Reference numeral 13 designates a vector obtained by summing in vector 11 and 12. In this example, the vector 9 is in an advanced phase with relative to the vector 8, the vector 12 is in a retarded phase with relative to the vector 11, and the summed vector 13 is in an advanced phase by about 90° with relative to the summed vector 10, presenting this phase difference as a phase shift amount output of this phase shifter. In this way, in this microwave phase shifter, while the vector 9 is in an advanced phase relative to the vector 8, the vector 12 is in a retarded phase relative to the vector 11, the electrical length β the amount to which the electrical length (α + β) of the line 7 is longer than that α of the line 6 is set to a value larger than 90° so that the summed vector 13 has an 90° phase deviation relative to the synthesized vector 10. In the switched line type phase shifter of such construction, when this phase shifter is actually provided, the amplitudes of the vectors 9 and 12 vary dependent on the variation in the amplitude of the leaked signal of the FET in off-state depending on the design and the amplitudes of the vectors 10 and 13 also vary, thereby deviating the angle produced by the both vectors from 90°. Therefore, it is necessary to acquaint previously the amplitude of the leaked signal in the off-state FET on designing, and it is necessary to carry out amendment of the phase amount to that amount. In this phase shifter, however, when the two FETs located adjacent to each other have the same amount of leakage and the values are not coincident with the design values, the phase shift amount cannot be made 90°, and when the off-capacitances FET (C T ) vary between adjacent FETs depending on the non-uniformity of the production process, the amplitude of the leaked signal also vary, thereby varying the phase shift quantity from 90°. The prior art switched line type phase shifter is constituted as described above, and when the off-time capacitance varies dependent on variations in the process, the quantity of signal leaking on the off-side line varies, thereby the sum vectors 10 and 13 shown in figure 9 vary, resulting in deviation in the phase shift amount. SUMMARY OF THE INVENTION It is an object of the present invention to provide a switched line type 90° phase shifter that requires no consideration on influences by leakages through the resonance circuit. It is another object of the present invention to provide a 90° phase shifter that can prevent deterioration of phase shift amount by canceling variations in leakages through the resonance circuit. Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter. The invention is defined in claim 1. A phase shifter according to the preamble of claim 1 is known from patent document EP-A2-0409374. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram illustrating a circuit construction of a 90° phase shifter according to a first embodiment of the present invention. Figure 2 is a vector diagram illustrating an operating state of a 90° phase shifter according to the first embodiment of the present invention. Figure 3 is a diagram illustrating a circuit construction of a 90° phase shifter according to a second, a third, and a fourth embodiment of the present invention. Figure 4 is a vector diagram illustrating an operating state of the 90° phase shifter according to the second, the third, and the fourth embodiment of the present invention. Figure 5(a) is a diagram illustrating an equivalent circuit of a branch line type 3 dB directional coupler used for constituting a 180° reflector type phase shifter 20 in the 90° phase shifter according to the third embodiment of the present invention and figure 5(b) is a diagram illustrating an equivalent circuit of the branch line type 3 dB directional coupler in a state where the load terminals 58 and 59 are grounded, figure 5(c) is a diagram illustrating an equivalent circuit of figure 5(b), figure 5(d) is a diagram illustrating an equivalent circuit of the branch line type 3 dB directional coupler in a state where the load terminals 58 and 59 are opened, and figure 5(e) is a diagram illustrating an equivalent circuit of figure 5(d). Figure 6 is a diagram illustrating an equivalent circuit of the 180° reflector type phase shifter constituted by a branch line type 3 dB directional coupler in the 90° phase shifter according to the third embodiment of the present invention. Figure 7 is a diagram illustrating an equivalent circuit of a 180° reflector type phase shifter constituted by using a 3 dB directional coupler employing a Lange coupler in a 90° phase shifter according to a fourth embodiment of the present invention. Figure 8 is a diagram illustrating a circuit construction of a prior art 90° phase shifter. Figure 9 is a vector diagram illustrating an operating state of the prior art 90° phase shifter. Figure 10 is a diagram illustrating a circuit pattern of a 90° phase shifter according to a first embodiment of the present invention. Figure 11 is a diagram illustrating a circuit pattern of a 90° phase shifter according to a second embodiment of the present invention. Figure 12 is a diagram illustrating a circuit pattern of a 180° reflector type phase shifter constituted by using a branch line type 3dB directional coupler of a 90° phase shifter according to a third embodiment of the present invention. Figure 13 is a diagram illustrating a circuit pattern of a 180° reflector type phase shifter constituted using a Lange coupler, of a 90° phase shifter according to a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Embodiment 1. Figure 1 is a diagram illustrating a 90° phase shifter according to a first embodiment of the present invention. In the figure, the same reference numerals as those shown in figure 8 designate the same or corresponding elements. Reference numeral 14 designates a 90° phase shifting transmission line having an electrical length of (α + 90°) as a sum of the electrical length α of the reference line and the electrical length 90° in the use frequency. Reference numerals 15a and 15b designate one half reference lines each having an electrical length of α/2 which is equal to one half of the electrical length α of the reference line described in the 90° phase shift lines 14, both of which constitute a reference transmission line 15 of electrical length α with together. Similarly in figure 8, reference numerals 3 and 4 respectively designate an FET switch and a resonance inductance line which are provided between the two one half reference lines 15a and 15b, connected in parallel with each other, constituting a parallel resonance circuit. A transmission line 16 of an electrical length of 180° constituting a phase inverting circuit, is provided in parallel with the parallel resonance circuit comprising the FET switch 3 and the resonance line 4. Figure 10 shows a pattern diagram of the 90° phase shifter of this first embodiment. In figure 10, the reference numerals are used to represents those described above and the circuit patterns of this 90° phase shifter are produced on a substrate 101. The fundamental operation of the 90° phase shifter of this first embodiment is approximately the same as that of the prior art phase shifter, and the gist of the present invention resides in that the signal leaked from the off-state FET in the prior art switched line type phase shifter is inverted in its phase by the phase inverting circuit 16 provided at the side of the reference line 15. Figure 2 shows a vector diagram illustrating an operation state of this 90° phase shifter. A description is given of the operation with reference to this figure 2. First of all, the side of the reference lines 15a and 15b where the phase inverting circuit 16 is provided is turned on, with the phase inverting circuit 16 turned off, thereby the phase shifter is in a state not inverting the phase, while the side of the line 14 for shifting the signed by 90° is turned off. Then, the signal passing through the reference line 15 at on side is represented by the signal vector 8 in figure 2, and the leakage signal passing through the 90° phase shifting line 14 at off side is represented by the signal vector 9. Therefore, the output signal represented by the signal vector 10 obtained by the vector summation of the vectors 8 and 9 is output. Thereafter, the FET switch 3 at the reference line 15 provided with the phase inverting circuit 16 is turned off, while the phase inverting circuit 16 is turned on, thereby the phase shifter enters a phase inverting state, while the 90° phase shifting line 14 is turned on. Then, the signal passing on the on side line 14 is represented by the signal vector 13, and the signal passing on the off side line 15 and the phase inverting circuit 16 is represented by the signal vector 17. Therefore, the output signal represented by the signal vector 18 obtained by the vector summation of the vectors 13 and 17 is output. By performing such an operation, the signal leaking on the off side line becomes 90° phase advanced signals 9 and 17 with relative to the on side line signals 8 and 13 in both cases, and because the difference in the electrical length between the lines 14 and 15 is set to an electrical length generating a phase difference of 90°, the vector 10 and the vector 18 always realize a phase difference of 90° therebetween. In addition, if the characteristics of the FETs 3 which are produced adjacent each other are the same even when the off-capacitance C T of the FET varies depending on the non-uniformity of the process, the vectors 9 and 17 vary by the same amount at the same time, and the phase difference between the vector 10 and the vector 18 to be synthesized is always kept at 90°, thereby providing a 90° phase shifter performing a stable operation. The 90° phase shifter of this first embodiment has constituted a switched line type 90° phase shifter such that a phase inverting circuit 16 with a function of enabling switching between a state where the phase inverting circuit is inserted between two parts of the reference lines in series to these and a state where it is not inserted, is added in a construction where the difference in the electrical length between the reference lines 15a and 15b and the phase difference producing line 14 is 90°, and thereby the leakage signal flowing on the reference line or the phase difference producing line when the resonance circuit is in off-state is made surely in an advanced phase by 90° with relative to the signal of the on side line. Accordingly, the influences on the phase shift amount due to the leakage signal becomes the same in both cases where the leakage signals are generated in any of the two lines, and the phase shifter can cancel the influences by this leakage signal in its operation. Therefore, so far as the leakage signal of the FETs produced adjacent each other are the same, it is neither required to acquaint the amplitude of the leakage signal nor consider the same in advance on designing, thereby easing the circuit design as well as improving the precision at the circuit design to a great extent. Furthermore, non-uniformity depending on processes can be absorbed, thereby accomplishing a high yield. Embodiment 2. Figure 3 is a diagram illustrating a 90° phase shifter according to a second embodiment of the present invention. In the first embodiment the phase inverting circuit is constituted by a 180° line 16 connected in parallel with the resonance circuit comprising the FET 3 and the resonance line 4, but in this second embodiment this is constituted by a 180° reflector type phase shifter 20. Figure 11 shows a pattern diagram of this second embodiment. In figure 11, reference numeral 19 designates a 3 dB directional coupler using a Lange coupler constituting a 180° reflector type phase shifter 20 with two switches each comprising the FET 3 and the resonance line 4. Reference numeral 70 designates a ground pad and reference numeral 101 designates a substrate. Figure 4 is a vector diagram showing an operation state of the 90° phase shifter of this second embodiment, and a description is given of the operation of 90° phase shifter of this second embodiment with reference to figure 4. First of all, the reference line 15 comprising reference line parts 15a and 15b provided with the 180° reflector type phase shifter 19 is turned on, the reflector type phase shifter 19 is turned off, i.e., it is set to a state where the phase inversion is not performed, and the 90° phase shifting line 14 is turned off. Then, the signal of the on side line 15 is represented by the signal vector 8, and the signal of the off side signal 14 is represented by the signal vector 9. Therefore, the output signal represented by the signal vector 10 obtained by the summation of the vectors 8 and 9 is output. Next, the line 15 provided with the reflector type phase shifter 19 is turned off, the reflector type phase shifter 19 is turned on, i.e., it is set to a state where the phase inversion is performed, and the 90° phase shifting line 14 is turned on. Then, the signal of the on side line 14 is represented by the signal vector 13 and the signal of the off side line 15 is represented by the signal vector 17. Therefore, the output signal represented by the signal vector 18 obtained by the summation of the vectors 13 and 17 is obtained. By performing such operation, the leaked signal on the off side line is in an advanced phase by 90° in all cases with relative to the signal of the on side line, and further, since the lines 14 and 15 are produced having electrical lengths generating a phase difference of 90°, the vector 10 and the vector 18 can always realize a phase difference of 90°. In addition, even when the off-capacitance C T of the FET is changed depending on the non-uniformity of the production process, if the FETs 3 which are produced adjacent each other have the same characteristics, the vectors 9 and 17 vary by the same amount at the same time, and the phase difference between the vector 10 and the vector 18 which are to be synthesized is always kept at 90°, thereby providing a 90° phase shifter performing a stable operation. Embodiment 3. A third embodiment of the present invention is embodied by constituting a 3 dB directional coupler constituting a 180 ° reflector type phase shifter 20 in the above described second embodiment by a branch line type 3 dB directional coupler. (1) Description of a branch line type 3 dB directional coupler: Figure 5(a) shows an equivalent circuit of a branch line type 3 dB directional coupler. The transmission lines 60 to 63 shown in figure 5 all have an electrical length of 90°. Further, the characteristics impedance of the transmission lines 60 and 62 are Z 0 , and those of the transmission lines 61 and 63 are Z 0 /2. In addition, reference numeral 51 designates an input terminal, reference numeral 52 designates an output terminal, and reference numerals 58 and 59 designate load terminals. Figure 5(b) shows an equivalent circuit of a branch line type 3 dB directional coupler in which load terminals 58 and 59 are grounded. Since the load terminals 58 and 59 are grounded, it is thought to be equivalent to a circuit where the transmission line 62 is absent. Since the electrical lengths of respective transmission lines are 90°, the load terminal 58 is grounded for the transmission line 61, and the impedance viewed from the input terminal 51 toward the transmission line 61 is infinite. Similarly, for the transmission line 63 the impedance viewed from the output terminal 52 is infinite. Accordingly, the equivalent circuit of figure 5(b) is represented by an equivalent circuit of figure 5(c). Figure 5(d) shows an equivalent circuit of a branch line type 3 dB directional coupler in which load terminals 58 and 59 are opened. Since the load terminals 58 and 59 are opened, the impedance viewed from the input terminal 51 toward the transmission line 61 is zero, i.e., meaning a short-circuited state. Similarly, the impedance viewed from the output terminal 52 toward the transmission line 63 is zero. On the contrary, for the transmission line 60 the impedances viewed from the input terminal 51 and the output terminal 52 are both infinite, and it is thought to be equivalent to a circuit where the transmission line 60 is absent. Accordingly, the equivalent circuit of figure 5(d) is represented by an equivalent circuit of figure 5(e). Since the electrical lengths of respective transmission circuits are 90°, the difference in the electrical lengths in the equivalent circuits of figure 5(c) and figure 5(e) are 180°, thereby constituting a reflector type phase shifter 20 of figure 3. (2) Description of a reflector type phase shifter 20 in the 90° phase shifter of the third embodiment: Figure 6 shows an equivalent circuit of the reflector type phase shifter which is constituted employing a branch line type one for the 3 dB directional coupler 19. In figure 6, reference numeral 64 designates FETs which are loaded in series in the signal transmission path between the signal input terminal 51 and the signal output terminal 52. FETs 65 are loaded in parallel with the signal transmission path, i.e., with FETs 64. Reference numerals 66b and 66c designate gate bias terminals of the FETs 64 and 65. An SPDT switch 67 of a wide band characteristics is constituted by these FETs 64 and 65. The terminal 51 of figure 6 is employed as the input terminal A of the 3 dB directional coupler 19 of figure 3, the terminal 52 of figure 6 as the output terminal C of figure 3, the terminal 66b of figure 6 as the terminal 5b of figure 3, and the terminal 66c of figure 6 as the terminal 5c of figure 3. Figure 12 shows a pattern diagram of the reflector type phase shifter of figure 6 as the third embodiment of the present invention. In figure 12, reference numerals are used to designate elements the same as or corresponding to those described above. Reference numeral 70 designates a ground pad and reference numeral 103 designates a substrate. (3) Description of an operation of a 90° phase shifter of this third embodiment: The 90° phase shifter of this third embodiment has the band width the same is that of the branch line type directional coupler which as a band width of ±10% from the center frequency, where the center frequency is 300 MHz to 30 GHz. The 90° phase shifter of this embodiment employing such reflector type 180° phase shifter performs a fundamentally the same operation as that of the 90° phase shifter of the first embodiment. The vector diagram showing the operation of the 90° phase shifter of this third embodiment is the same as that of figure 4. (4) Description of effects of the 90° phase shifter of the third embodiment: By carrying out such operation, the signal that leaks on the off side line becomes the signal in an advanced phase by 90° with relative to the signal on the on side line in all cases, and further because the transmission lines 14 and 15 have electrical lengths producing a phase difference of 90°, the vector 10 and the vector 18 always realize a phase difference of 90°. In addition, even if the off-capacitance (C T ) of the FET varies depending on the non-uniformity of the production process, if the characteristics of the FETs 3 which are fabricated located adjacent each other are the same, then the vectors 9 and 17 vary by the same amount at the same time, and the phase difference between the vector 10 and the vector 18 which are to be synthesized with each other is always kept at 90°, thereby providing a 90° phase shifter performing a stable operation. Embodiment 4. A 180° phase shifter that performs the same operation as that which is performed by one using the above described branch line type 3 dB directional coupler can be realized by using a Lange coupler. A 90° phase shifter of this fourth embodiment of the present invention is constituted by that the 180° reflector type phase shifter 20 in the third embodiment is constituted by a 180° reflector type phase shifter using a Lange coupler, the equivalent circuit of which is shown in figure 7. In figure 7, reference numeral 68 designates a Lange coupler which has grounded its load terminals. Reference numeral 69 designates a Lange coupler which has its load terminals opened. Reference numerals 64 to 67, 51 and 52 arethe same as those shown in figure 6. That is, reference numeral 64 designates FETs which are loaded in series with the signal transmission path between the input terminal 51 and the output terminal 52. Reference numeral 65 designates FETs which are loaded in parallel with the signal transmission path, i.e., the FETs 64. Reference numeral 66 designates a gate bias terminal of the FETs 64 and 65. Reference numeral 67 designates an SPDT switch of a wide band characteristics constituted by these FETs 64 and 65. The 90° phase shifter of this embodiment using such reflector type 180° phase shifter is constituted by employing the terminal 51 of figure 7 as the input terminal A of the 3dB directional coupler 19 of figure 3, the terminal 52 of figure 7 as the output terminal C of figure 3, the terminal 66b of figure 7 as the terminal 5b of figure 3, and the terminal 66c of figure 7 as the terminal 5c of figure 3. Figure 13 shows a circuit pattern of a 180° reflector type phase shifter of this fourth embodiment. In the figure, reference numerals are used to designate elements the same as or corresponding to the those described above, and reference numeral 104 designates a substrate. The 90° phase shifter of this fourth embodiment using the reflector type 180° phase shifter of figure 7 which circuit pattern is shown in figure 13 performs the same operation as that of the 90° phase shifter of the above described second and third embodiments. Here, the 180° reflector type phase shifter of figure 7 has a wide band characteristics which is operable for ±50% band of the center frequency because the band of the Lange coupler amounts to about ±50% of the center frequency, where the center frequency is 300 MHz to 30 GHz. As is evident from the foregoing description, according to the present invention, a switched line type 90° phase shifter is constituted such that the difference in electrical lengths between the reference line and the phase difference producing line is 90° and a phase inverting circuit is added to the reference line switchably between a state where it is inserted in the reference line between two parts of the reference line in series thereto and a state where it is not inserted between the two parts of the reference line, the leakage signals flowing on the reference line and on the phase difference producing line when the resonance circuit is in off-state are made surely in a 90° advanced phase with relative to the signal on the on side line, whereby the influences by the phase shift amount due to the leakage signal are made the same as in cases where the leakage signal is generated in either of the lines, whereby the influences by the leakage signals are canceled in the operation of the phase shifter. Therefore, as far as the leakage signal of the FET produced located adjacent each other are the same, there is no necessity to acquaint the amplitude of the leakage signal or to consider the largeness of the leakage signal in advance on designing, and the circuit design can be performed quite easily and at high precision. Further, the non-uniformity depending on the production processes can be absorbed and a high yield can be accomplished.

Claims: A 90° phase shifter, comprising: a first single pole double throw switch (50) which receives an input signal input to an input terminal (1) and outputs said signal to either of two line output terminals; a second single pole double throw switch (51) which receives two signals respectively input to two line input terminals and outputs either of said two input signals to an output terminal (2); and a reference transmission line (15a, 15b) having an electrical length of α in the usage frequency, connected between ones of said two line terminals of said first and second single pole double throw switches (50, 51); said 90° phase shifter being characterized by a phase difference producing transmission line (14) having an electrical length of (90°+ α) in the usage frequency, connected between the others of said two line terminals of said first and said second single pole double throw switches (50, 51); and a phase inverting circuit (3, 4, 16) provided switchably by a state switching switch (3) between a state of being inserted serially to and between two parts of said reference transmission line (15a, 15b), which two parts produce the entirety of said reference transmission line (15) and a state of not being inserted serially thereto and therebetween. The 90° phase shifter of claim 1, characterized in that said phase inverting circuit includes a resonance circuit comprising an FET (3) and a resonance line (4), which resonance circuit is inserted between said reference transmission line (15) at a position of one-halt of the entire electrical length of said reference transmission line (15) from its one end and constitutes said state switching switch; and a half-wavelength line (16) having an electrical length of 180° in the usage frequency, connected in parallel with said resonance circuit (3, 4). The 90° phase shifter of claim 1, characterized in that said phase inverting circuit comprises a reflector type 180° phase shifter (20). The 90° phase shifter of claim 3, characterized in that said reflector type 180° phase shifter (20) comprises a 3dB directional coupler (19) using a Lange coupler. The 90° phase shifter of claim 3, characterized in that said reflector type phase shifter comprises a 3 dB directional coupler of branch line type.

Inventors: NAKAHARA KAZUHIKO, NAKAHARA, KAZUHIKO, NAKAHARA, KAZUHIKO, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1993
Main classification: H01P1
IPC codes: H01P1, H03H11, H01P5