Patent Publication Number: US-6335666-B1

Title: High frequency circuit with variable phase shift

Description:
The present invention relates to a high frequency circuit with variable phase shift, usable mainly in the field of decimetric waves. It can be used therein in particular together with a plurality of antenna elements to provide variable squint pointing by feeding each of the antennas with the same signal for transmission, and by controlling the phase shifter circuits associated with the antennas to take up a determined phase relating to the pointing to be achieved. Nevertheless, other applications are possible. The object of the invention is to use a signal of given phase at the output from a circuit to produce a signal of phase that is offset relative to said given phase. The principle of the invention is also applicable to the field of non-decimetric waves. 
     BACKGROUND OF THE INVENTION 
     In the field of phase shifters, phase shifting circuits are known, in particular those based on so-called PIN diodes. PIN diodes are constituted by juxtaposing a P layer and an N layer of semiconductor material on either side of a thin insulating layer. Because of the presence of the insulating layer, the minority carriers of the PN junction are slow. Compared with a very high frequency signal, such a diode, when properly biased, can thus behave like a circuit exhibiting pure resistance. 
     Some publications describe PIN diodes as constituting switch elements at microwave frequencies. Such an element makes it possible to provide on/off selection of one particular transmission access selected from two possible accesses. In other applications, such diodes make it possible to connect a segment of line for reflection purposes in parallel with a given line. Such applications in the form of two-state functions are tied to the fact that with such diodes, insertion losses and standing wave ratios (SWRs) at the accesses can be controlled and defined simultaneously only in the two switching states. Specifically, that type of circuit can be guaranteed to be reproducible and suitable for industrialization only providing it is not used at settings that are intermediate between those two states. Known methods do not make it possible to provide continuous and simultaneous control over phase, standing wave ratio, and insertion loss. 
     Various types of circuit based on multiple Varactor diodes or indeed based on multiple PIN diodes have been used to make variable-shift phase shifters. The problem presented by Varactor diodes is that the capacitance of such diodes varies with bias voltage. They also have the drawback, particularly in the 3 GHz range, that the voltages required for scanning significant variation of impedance need excursions of the order of 20 volts. Such excursions are quite difficult to implement, even with voltage multipliers using charge pumps. In addition, Varactor diodes give rise to variations in reactive impedance that are difficult to compensate, unless some other reactive impedance is also used. 
     As for PIN diodes, which have the advantage of proposing variation that is of a resistive type, they nevertheless need to be manufactured with great care in order to be usable beyond 3 GHz, because of the presence of parasitic capacitance. This parasitic capacitance gives rise to a limit on frequency since the diode is connected in series to convey the radio signal. In addition, the use of circuits having numerous PIN diodes implies making microwave frequency circuits that occupy a large area on a printed circuit board, which is bulky and more difficult to develop. In particular, specialized PIN diodes are diodes in ceramic packages that are mounted manually. Under such circumstances, these ceramic packages are not surface mount components (SMC) type packages that are suitable for being put into place automatically by insertion machines in mass-produced circuits. Furthermore, the connection tabs of such packages gives rise to inductances which, in combination with the parasitic capacitance of the diode, can make such a circuit very difficult to define. 
     With multiple Varactor or PIN diodes, expensive and complex solutions requiring 90° couplers or 3 dB couplers must therefore be used in order to maintain good matching of input and output impedances. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of the invention is to remedy those problems of expense and of adjustment in particular, by proposing a solution in which the diodes used are PIN diodes (having slow minority carriers) of conventional type, i.e. PIN diodes of the kind that are available in packages suitable for surface mounting using automatic machines. 
     One idea of the invention is to provide, between an inlet and an outlet of a phase shifter, a separation into two propagation paths of different lengths. In addition, at least one PIN diode, and in practice two PIN diodes in parallel, are interconnected at intermediate positions via their terminals to nodes of each of these paths. By biasing the diode and ensuring that it presents a given resistance, a bias circuit makes it possible for each of the two paths to transfer an impedance to the input that will be seen by the signal at the input. Consequently, the input signal will take one path rather than the other. Since the paths are of different lengths, the two resulting signals at the output are phase-shifted relative to each other. When they are combined, they give rise to a signal which is the result of adding them together, and which possess a phase that depends on the respective contributions of each of these two components. The more favored signal imposes its phase the more easily. 
     In practice, such a phase shifter can produce a phase shift of about 20°. That is entirely satisfactory for pointing the aiming direction of an antenna having a plurality of radiating elements onto an off-axis or “squint” direction. If a phase shift of greater than 20° is desired, then it suffices to cascade a plurality of phase shifters of the same type as the phase shifter of the invention. 
     It is shown below that compared with the state of the art, the circuit of the invention presents the advantage that the signal to be transmitted does not pass via the PIN diodes. As a result, the parasitic capacitances of the diodes does not complicate the operation of the circuit. In practice, the imaginary impedance components of the PIN diodes are compensated by matching circuits, by metallic connections of desired length. Such matching has the advantage of being effective over a very wide range of use. For example, in a given circuit operating at around 6.6 GHz, it is very easy to use the phase shifter between 6.2 GHz and 6.9 GHz, i.e. over a range of more than 10% of the center of frequency. 
     The invention thus provides a variable phase shift high frequency circuit comprising an input for a high frequency signal, two propagation paths for said signal each connected at one end to said input, a PIN diode having its terminals connected to first and second intermediate nodes on each of the two paths respectively, an output for the phase-shifted high frequency signal connected to the other ends of the two paths, and a circuit for biasing the diode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood on reading the following description and on examining the accompanying figures. The figures are given by way of indication and do not limit the invention in any way. In the figures: 
     FIG. 1 is a functional representation of a high frequency circuit of the invention in a preferred variant having two PIN diodes; 
     FIG. 2 is an enlarged view of the architecture of the connections made on a printed circuit that serves to provide the propagation paths; and 
     FIG. 3 is a diagrammatic section of a metal oxide semiconductor (MOS) type integrated circuit suitable for manufacturing PIN diodes to enable them to be mounted in SMC type packages. 
    
    
     MORE DETAILED DESCRIPTION 
     FIG. 1 shows a variable phase-shift high-frequency circuit of the invention. The circuit has an input  1  for a high frequency signal. It also has an output  2  for said signal after it has been phase shifted. Between the input  1  and the input  2 , there are provided two paths respectively referenced  3  and  4 . The paths  3  and  4  are of different lengths. In one example, the lengths of the path  4  is equal to 3λ/4 where λ is the wavelength of the wave of the signal admitted to the input  1 . In this same preferred example, the path  4  has a length of 2λ/4. Nevertheless, these lengths are approximate, particularly since the circuit is usable over a wide frequency range. However, as explained below, the real limit on the passband of the circuit of the invention is associated with the fact that a wavelength difference itself becomes equal to the wavelength of a signal to be phase-shifted, or to a multiple thereof. 
     In the invention, the circuit essentially comprises a PIN type diode  5  connected in one example via its anode  6  to a first intermediate node  7  of the path  3  while its cathode  3  is connected to a second intermediate node  9  of the second path  4 . 
     In an example, the propagation distance between the input  1  and the first intermediate node  4  is about λ/4, as is the distance between the input  1  and the second intermediate node  9 . The cathode  6  is connected to the node  7  via a segment  24  of non-negligible length, close to λ/4 in an example. Because of the difference in length between the propagation paths  3  and  4 , and/or because of the presence of the segment  24 , the resistive impedance of the diode  8  transferred to the input  1  is different as seen on propagation path  3  from that seen on propagation path  4 . Consequently, a signal reaching the input  1 , for any given value of said impedance, will select one path rather than the other. This is the criterion actually used for adjusting the phase shifter of the invention. At the output  2 , the greater signal will either be the direct signal resulting from propagation along the path  4 , or else the delayed signal resulting from propagation along the longer path, e.g.  3 . Depending on which signal is favored, the resulting signal will be more in phase with the direct signal or with the delayed signal. The desired phase shift is obtained in this way. 
     For reasons of input and output matching, the circuit is duplicated. Another diode  10  is connected between the first path and the second path between a third intermediate node  11  on said first path and the second intermediate node  9  on the second path  4 . The intermediate node  7  is remote from the intermediate node  11 . In an example, the distance between them is likewise about λ/4. In practice, the first path  3  is thus made up of three segments of length λ/4. The example shown indicates that the cathodes  6  and  12  of the diodes  5  and  10  are connected to the intermediate nodes  7  and  11 . It is entirely possible to reverse each of the two diodes and to connect their anodes  8  and  13  to the intermediate nodes  7  and  11  instead of their cathodes  6  and  12 . In which case their cathodes  6  and  12  would be connected to the intermediate node  9 . 
     The circuit for biasing the diodes  5  and  10  comprises a generator (not shown) which applies a positive voltage, e.g.  3  volts, to a connection node  14  between two parallel resistors  15  and  16 . The other ends of the resistors  15  and  16  are respectively connected to the intermediate nodes  7  and  11 . The intermediate node  9  is also connected to ground via a resistor  17 . To be able to make the diodes operate properly, the generator constituted by the voltage source and the resistors  15  and  17  is a current generator. To this end, the resistors  15  and  17  are of high resistance, e.g. 1 kΩ or 10 kΩ. Thereafter, by varying the values of one or both resistors, and/or the voltage applied to the node  14 , it is possible to modify the more or less conductive state of the diodes  5  and  10 . For example, if a conduction current in the diodes is low, then they are highly resistive. In contrast, if the current is higher, then they are less resistive. 
     In practice, the diodes  5  and  10  do not contribute only variation of the real portion of their impedance to the input  1  and to the output  2 . They also contribute an imaginary portion of said impedance. Nevertheless, this imaginary portion presents the advantage of varying little or not at all with voltage. Consequently it is easy to compensate. Compensation can be achieved by the lengths of the connections, in particular the lengths of the connection connecting the cathode  6  to the intermediate node  7  and of the connection connecting the cathode  12  to the intermediate node  11 . In this way, and for a frequency range of at least 10% of the high frequency signal admitted on the input  1 , it is possible to estimate that the impedance transferred to the intermediate nodes  7  and  11  and also to the node  9  is an impedance that is purely resistive. 
     Although using a second diode  10  is not essential, a priori, for the purpose of obtaining the effects of the invention, it is nevertheless particularly advantageous because it produces two effects. Firstly the circuit becomes reversible. A signal can be introduced via its output. The same phase shift is then obtained at its input  1 , then being used at its output, as is obtained when the same signal is applied to said input  1 . That is why the circuit is, in addition, symmetrical in architecture. 
     Furthermore, to avoid transmitting a DC component through the circuit, it is necessary to place respective capacitors  18  to  20  in the first, second, and third segments  21  to  23  of the first path  3 . These three segments, which in practice are each of equivalent length equal to λ/4, need to have real lengths that are, in fact, different. The lengths of the segments  21  to  23  depend on the presence of the capacitors  19  to  20  which cause phase to rotate. 
     The circuit described calls for two remarks. Firstly phase-shifting effects are obtained over a very wide frequency range. The range can be considerably greater than the above-mentioned 10%. What matters is that the two paths  3  and  4  are of different lengths and/or that the segment  24  provides a contribution that is different from the impedance of the diode at the input  1  (and also at the output  2 ). The connection  24  which connects the anode  6  to the first intermediate node  7  has a length of about λ/4, so the variations in impedance to which the diode  5  is subjected are not transferred identically in the first path  3  and in the second path  4 . The segment  21  and the connection  24  together form a length of about λ/2, so there is even quadrature opposition in the transferred impedance. It is not really necessary for the path length difference to be about λ/4. Nevertheless, this difference determines the phase shift that can be obtained with this circuit. The further the path length difference from the length λ/4, the smaller the range of adjustment. Secondly, the symmetrical appearance presents numerous advantages in design and implementation. 
     The invention thus produces a phase shift with an impedance, namely the impedance of the PIN diodes transferred to the input  1 , which impedance has an imaginary component that is small. It would be possible to use schottky type diodes instead of PIN diodes but schottky diodes produce intermodulation effects because their junction possesses fast minority carriers. Such a diode has impedance that varies at the same rate as the high frequency signal, in addition to the DC value as set by the bias. 
     Finally, the high frequency signal essentially does not propagate through the diodes  5  and  10 , but only along the paths  3  and  4 . If these paths  3  and  4  are made with characteristic impedances of about 50 ohms for the major part of the adjustment, then the impedance transferred to the intermediate node  7  by the segment  24  from the anode  6  is very different from 50 ohms, and as a result the segment  24  takes less of the signal to be propagated than does the segment  22 . 
     FIG. 2 shows on a larger scale a preferred embodiment of the metallization of a printed circuit that can be used for constituting the paths  3  and  4  and the connections such as  24 . In accordance with the above, the circuit of FIG. 2 is essentially symmetrical about an axis of symmetry  27 . On one side, e.g. to the left, there can be seen the input  1  of the circuit in the form of a rectangular area of metallization. This input  1  is electrically connected to matching metallization  25  which transfers on the input  1  the impedance of an open circuit  28 : the end of the metallization  25 . Starting from the input  1 , a first metallization connection leads to the intermediate node  7  by passing in series through a capacitor  18  represented solely by a space for receiving it. A segment  24  is made from the intermediate node  7  to a package  29  containing the diodes  5  and  10 . In the layout of the circuit, the segment  21  and the segment  24  are substantially parallel to each other. The metallization of the segment  21  is wider than the metallization of the segment  24 . These widths are determined by experiment and after simulation, and they correspond to the characteristic impedances to be implemented in the segments  21  and  24  respectively in order to achieve the desired result. The segment  22  is formed from the intermediate node  7  as two symmetrical crescents connected to the nodes  7  and  11  respectively. The two crescents are connected together via a capacitor  19  connected like the capacitor  18 . The segment  23  is symmetrical to the segment  21 . It leads to the output  2 . Like the input  1 , the output  2  has a matching element  26  likewise transferring an open circuit  30  on the output  2 . For impedance-matching reasons on either side of the connection to the capacitor  19 , two impedance-matching elements  31  and  32  are disposed facing each other and extending parallel to the crescents of the segment  22 . The matching elements  25 ,  26 ,  31 , and  32  are represented in FIG. 1 by dashed lines. 
     In practice, the capacitors  18  to  20  are surface-mount type capacitors making low cost manufacture possible. They are fitted to the printed circuit at the same time as the other components are fitted thereto. 
     FIG. 3 shows a preferred embodiment of the package  29  containing the two diodes  5  and  6 . The diodes are made, for example, in a semiconductor substrate  33 , which is of the P type in this example. These diodes are constituted by N type implants  34  and  35  in the P substrate with respective insulating layers  36  and  37  to constitute the PIN type diodes at the junctions between the P and N regions. The P and N regions of the semiconductor are connected to connection tabs in the bottom of the package  29 . In one example, the package  29  is of the C 115  or SOT  323  type. Connections  38 ,  39 , and  40  which connect these regions of the semiconductor to the tabs of the package are connected respectively to one end  41  of the segment  24  and to one end  42  of a segment  43  that is symmetrical to the segment  24 . The segments  24  and  43  are connected to the intermediate nodes  11  and  9  respectively. 
     The circuit can also be fully integrated on a monolithic semiconductor substrate. Under such circumstances, links proportional to λ/4 can be replaced by inductors and capacitors having the same effects. In such an embodiment, the entire circuit is in the form of a circuit having an input, an output, a control tab  14 , and a tab for connecting the resistor  17  to ground. This tab may coincide with the package of the circuit.