Abstract:
A variable phase circuit for microwave and millimeter wave applications includes a distributed-constant transmission line, a number of phase shifters respectively responsive to control signals for introducing a desired amount of phase shift to and removing the introduced phase shift from the transmission line. In order to compensate for the frequency-dependent phase deviation of each phase shifter, one or more loading stubs are selectively connected to or disconnected from the transmission line according to operation of the phase shifters.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a variable phase shifter for use in the microwave and millimeter-wave regions. This invention is particularly useful for applications such as phased array antennas. 
     2. Description of the Related Art 
     In a phased array antenna, a variable phase shifter is used to introduce a fixed amount of phase shift to signals carried on a transmission line. Conventional phase shifters are of two types, i.e., a switched line type and a loaded line type. The switched line variable phase shifter comprises two line segments of different length selectively connected to the transmission line and the differential path length between them determines the amount of phase shift to be introduced. The transmission line is switched over from one line segment of the phase shifter to the other when a phase shift is introduced and switched back to the original line segment when the phase shift is removed. If the intended phase shift is 180 degrees, the differential path length is equal to the half wavelength of the center frequency of the signal on the transmission line. However, the phase shift value tends to deviate linearly from the intended value as the frequency of the signal deviates in either direction from the center (nominal) frequency. To maintain the phase deviation in a small range, it is necessary to control the frequency of the signal to within narrow limits. 
     The loaded line phase shifter comprises a pair of spaced-apart main stubs extending from the transmission line and a pair of extension stubs selectively coupled to the main stubs. The phase shifter of this type also suffers from the frequency-dependent phase deviation. Specifically, the characteristic of the loaded line type has a minimum phase deviation at the center frequency. If the circuit layout is optimized, there exist a nonlinear negative phase deviation from the intended value that increases as the frequency decreases in a direction away from the center frequency and a nonlinear positive phase deviation from the intended value that increases as the frequency increases in a direction away from the center frequency. If the circuits are laid down perfectly, the negative and positive phase variations follow a curve that is symmetrical with respect to the nominal frequency. However, due to inherent imperfections, the characteristic curve loses the symmetricity and the phase shift value deviates from the intended value with frequency deviation of the signal from the nominal value, in a manner similar to the switched line phase shifter. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a variable phase circuit in which the frequency-dependent phase deviation is compensated. 
     According to a broader aspect, the present invention provides a variable phase circuit comprising a transmission line, a phase shifter for introducing a phase shift to and removing the phase shift from the transmission line, an open-ended loading stub, and a switch for connecting the open-ended loading stub to and disconnecting the loading stub from the transmission line according to operation of the phase shifter. 
     The open-ended loading stub has the effect of cancelling phase deviation of the signal as its frequency deviates from nominal (center) frequency. 
     According to a second aspect of the present invention, there is provided a variable phase circuit comprising a transmission line, a plurality of phase shifters respectively responsive to control signals for introducing one of a plurality of different phase shifts to and removing the introduced phase shift from the transmission line, a plurality of loading stubs, and a plurality of switches for selectively connecting the loading stubs to and disconnecting the loading stubs from the transmission line in accordance with operation of the phase shifters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in further detail with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagram of an improved variable phase circuit of the present invention using a switched line phase shifter; 
     FIG. 2 is a graphic representation of phase deviation characteristics of open-ended loading stubs of different lengths; 
     FIG. 3 is a graphic representation of phase deviation characteristics of improved variable phase circuits of the present invention by using a 180-degree switched line phase shifter in comparison with a prior art 180-degree switched line phase shifter; 
     FIG. 4 is a diagram of an improved variable phase circuit of the present invention using a loaded line phase shifter; 
     FIG. 5 is a graphic representation of phase deviation characteristics of variable phase circuits of the present invention by using a 45-degree loaded line phase shifter in comparison with a prior art 45-degree loaded line phase shifter; 
     FIG. 6 is a diagram of a variable phase circuit according to a preferred embodiment of the present invention; and 
     FIGS. 7 and 8 are diagrams of the variable phase circuit of alternative embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The variable phase circuit of the present invention can be implemented with a phase shifter of either of two configurations, i.e., the differential path length type or the differential loading type. FIG. 1 shows the basic principle of the inventive variable phase circuit implemented with a switched line phase shifter. 
     In FIG. 1, the switched line phase shifter, as indicated by numeral  2 , is connected in a distributed constant circuit, or transmission line  1 , which may be formed by a microstrip line on a microwave integrated circuit. Over the transmission line  1 , a high-frequency signal propagates in a direction from an input end  1 A to an output end  1 B. 
     Switched line phase shifter  2  comprises a line segment  3  of shorter length and a line segment  4  of longer length. In the case of a 180-degree phase shift, for example, the difference in length between the line segments  3  and  4  equals the half wavelength (i.e., 0.5λ) of the propagating signal when the signal components are concentrated at the center frequency. To the input and output sections of transmission line  1 , the opposite ends of line segment  3  are respectively connected by a pair of switches  5  and  6  and the opposite ends of line segment  4  are respectively connected by a pair of switches  7  and  8 . 
     If the intended phase shift is 90 degrees, the line segments of a 90-degree switched line phase shifter are chosen so that their differential path length corresponds to the quarter wavelength of the signal, which is half the differential path length of the 180-degree switched line phase shifter. 
     Switches  5 ,  6 ,  7  and  8  are provided for selectively connecting the two line segments. In response to a binary signal “1” from an external control circuit, not shown, switches  5  and  6  are simultaneously operated to connect the line segment  3  to the transmission line  1  and switches  7  and  8  are simultaneously released, so that the signal on the transmission line  1  exclusively passes through the line segment  3 . In response to a binary signal “0”, the switches  5  and  6  are simultaneously released and switches  7  and  8  are simultaneously operated, so that the signal on the transmission line  1  is switched over from the line segment  3  to the line segment  4  and exclusively passes through the longer line segment  4 . 
     Such alternate switching operation may be achieved by supplying individual binary signals from the control circuit or inverting a binary signal from the control signal with an inverter  9 , as illustrated. 
     In order to compensate for the frequency-dependent phase deviation of phase shifter  2 , the present invention provides an open-ended loading line segment, or loading stub  10 . Loading stub  10  is connected to the transmission line  1  through a switch  11  in response to the output of inverter  9  when the shorter line segment  3  is connected to the transmission line by the switches  5  and  6 . 
     The reactance X of open-ended loading stub  10  is given as follows: 
     
       
           X=−Z   s ·cot (2πθ/λ) 
       
     
     where, Z s  and θ are the characteristic impedance of the loading stub  10  and its electrical length, respectively, and λ is the wavelength of the signal on the transmission line  1 . 
     FIG. 2 shows results of computer simulation in which loading stubs of various electrical lengths are tested by connecting each to the transmission line  1  (with the variable phase shifter  2  being disconnected) and phase variations of signals on the transmission line as a function of different frequencies are measured and plotted against the ratio of the varying frequency (f) of the signals to their center frequency f 0 . It is seen from FIG. 2 that the phase deviation of loading stubs of lengths λ and 0.5λ are inversely variable with the frequency ratio f/f 0  and their characteristics vary symmetrically with respect to a zero degree phase shift. 
     Since the frequency-dependent phase variation of loading stubs  10  of different lengths decreases as frequency increases, it is found that their characteristics are opposite to that of the switched line phase shifter  2  itself. This is particularly clear when the length of loading stub  10  is λ or 0.5λ. Therefore, the loading stubs of these lengths can be advantageously used in combination with the switched line phase shifter  2  as a means for compensating for its undesired frequency-dependent phase deviation. 
     Since the results obtained from the loading stubs of lengths 0.08λ, 0.13λ and 0.19λ show that their phase variations significantly deviate from the zero degree point, the connection of such loading stubs would produce an impedance matching problem. A separate impedance matching circuit would therefore be necessary to solve this problem. In some applications where strict phase deviation compensation is not required, the use of such impedance matching circuit would be justified to use loading stubs of lengths other than λ and 0.5λ. These loading stubs can also be used if the design of the phase shifter  2  has taken account of such phase deviations from the zero-degree point. 
     Accordingly, by choosing the loading stub length θ as an integral multiple of 0.5λ, the phase of the signal is always at zero degree at center frequency f 0  and there is no need to take account of such impedance matching or design consideration. 
     FIG. 3 shows results of computer simulation in which loading stubs  10  of lengths λ and 0.5λ are used in combination with a 180-degree switched line phase shifter  2 . Phase variations of signals on the transmission line  1  are measured as a function of frequency ratio f/f 0  and plotted in the same manner as described above. It is seen in FIG. 3 that when a loading stub  10  of length λ is used, its negative frequency-dependent phase deviation characteristic substantially cancels the positive frequency-dependent phase deviation characteristic of the switched line phase shifter  2  itself, thus allowing a wideband signal to be precisely phase-shifted by a predetermined amount. Note that with a loading stub length 0,5λ the overall characteristic of the variable phase shift favorably compares with that of the prior art switched line phase shifter (i.e., with no loading stub) as indicated by a dotted line in FIG.  3 . With the loading stub lengths λ and 0.5λ, there is no additional phase shift at the zero degree point since the transmission line  1  and the loading stub  10  are matched in impedance. 
     FIG. 4 shows another embodiment of the present invention in which the switched line phase shifter  2  of FIG. 2 is replaced with a loaded line phase shifter  20 . This loaded line phase shifter comprises a pair of main stubs  21  and  22  extending from the transmission line  1 . These main stubs are mutually spaced apart a distance Φ which may be equal to the quarter wavelength (λ/4) of the signal at the center frequency f 0 . Main stubs  21  and  22  are connected to extension stubs  23  and  24  by switches  25  and  26 , respectively. Switches  25  and  26  are simultaneously operated in response to a binary signal “1” from the control circuit to establish connections between the main stubs  21 ,  22  to the extension stubs  23 ,  24 . 
     Loaded line phase shifter  20  has a nonlinear frequency-dependent phase deviation characteristic and this characteristic further deviates from the center frequency of the signal and loses symmetrical characteristic. Depending upon which side of the center frequency such phase deviation characteristic further deviates, the switch  11  of the loading stub  10  is operated. If the further deviation is on the higher side of the center frequency, the control signal is supplied via an inverter  27  to the switch  11  so that it connects the open-ended loading stub  10  to the transmission line  1  when the extension stubs  23  and  24  are disconnected from the main stubs  21 ,  22 . If the further deviation is on the lower side of the center frequency, the switch  11  is operated so that the loading stub  10  is disconnected to the transmission line when the extension stubs  23 ,  24  are connected to the main stubs  21 ,  22 . 
     FIG. 5 shows results of experiments in which loading stubs  10  of lengths 1.5λ, λ and 0.5λ are used in combination with a 45-degree loaded line phase shifter  20 . Phase variations of signals on the transmission line  1  are measured as a function of different frequencies and plotted in the same manner as described above. It is seen that when a loading stub  10  of length 1.5λ is used, the overall phase deviation becomes symmetrical with respect to the nominal frequency f 0 . Note that with stub lengths λ and 0,5λ the overall characteristics favorably compare with that of the prior art phase shifter (i.e., with no loading stub) as indicated by a dotted line in FIG.  5 . 
     Experiments show that the differential path length type phase shifter  2 , FIG. 1, is suitable for implementing a large value of phase shift, while the differential loading phase shifter  20 , FIG. 4, is suitable for implementing a small value of phase shift. 
     Therefore, it is preferable that the phase shifter  2  is used for implementing 180- and 90-degree phase shifts by use of loading stubs  10  of lengths λ and 0.5λ, respectively, while the phase shifter  20  is used for implementing a 45-degree phase shift by using a loading stub  10  of length 1.5λ. 
     FIG. 6 is a block diagram of a 3-bit variable phase shifter according to a preferred embodiment of the present invention. This 3-bit variable phase shifter is comprised of a 180-degree phase shifter  30 , a 90-degree phase shifter  31  and a 45-degree phase shifter  32 , all of which are connected in series in the transmission line  1 . 
     An open-ended loading stub of variable length is formed by a series of loading stubs  33  of length 0.5λ, all of which are selectively connected by switches  34  to the transmission line  1 . Phase shifters  30 ,  31  and  32  are controlled by respective bits of a 3-bit code from a control circuit  35  so that these phase shifters are connected in a desired combination to the. transmission line. 
     Alternatively, open-ended loading stubs  40 ,  41  and  42  of different length may be coupled respectively by switches  43 ,  44  and  45  to the transmission line, as shown in FIG. 7, under control of a control circuit  43 . The loading stubs  40 ,  41  and  42  have lengths λ, 0.5λ and 1.5λ, respectively. 
     To achieve space saving and utilization, the embodiment of FIG. 7 may be modified as shown in FIG. 8, in which the open-ended loading stubs  40 ,  41  and  42  are located respectively adjacent the phase shifters  30 ,  31  and  32  so that spaces which would otherwise be left unused for other purposes may be utilized.