Abstract:
An apparatus and method provide an adjustable phase and time delay to an input signal. The apparatus includes an inverting element and first and second variable capacitors. The inverting element has a first end serially coupled with the input signal and a second end. The first variable capacitor is coupled between the first end of the inverting element and first voltage. The second variable capacitor is coupled between the second end of the inverting element and a second voltage. The first and second variable capacitors are separately adjustable to controllably vary a phase shift and a delay of a reflection of the input signal. The first and second voltages may be at the same or different potentials. 
     The input signal may be coupled to the inverting element through a directional coupler, such as a circulator. The input signal, which is reflected by the inverting element, may be coupled back through the directional coupled and output as the output signal having a desired phase shift and delay relative to the input signal.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electronic and electromagnetic circuits, and more particularly, to an apparatus for introducing an adjustable delay and phase shift onto an input signal. 
     BACKGROUND OF THE INVENTION 
     In many electronic systems, it is necessary to adjust a phase and delay associated with a signal. Conventionally, the addition of delay to a signal is done using delay lines. For example, a series of delay lines is conventionally provided where each delay line has a fixed amount of delay. The delay lines may be connected together to provide discreet increases in delay. 
     Delay lines may be undesirable in applications that require a precise amount of delay because delay lines only add delay in discrete increments. Moreover, conventionally adjusting both delay and phase requires both delay lines and additional components for varying the phase. The additional components may need to be selected individually, for each manufactured circuit or system, depending on the particular manufacturing idiosyncrasies of the delay lines and the circuit or system itself. 
     In view of the shortcomings of conventional techniques, there is a need for an electrically adjustable circuit to precisely control delay and phase shift imparted to an input signal over a continuous, rather than a discrete, range of values. The need is particularly acute when the input signal is in the microwave range and precise delays in picosecond range are required. 
     SUMMARY OF THE INVENTION 
     According to the present invention, an apparatus provides an adjustable phase and time delay to an input signal. The apparatus includes an inverting element and first and second variable capacitors. The inverting element has a first end serially coupled with the input signal and a second end. The first variable capacitor is coupled between the first end of the inverting element and a first voltage. The second variable capacitor is coupled between the second end of the inverting element and a second voltage. The first and second variable capacitors are separately adjustable to controllably vary a phase shift and a delay of a reflection of the input signal. The reflection of the input signal is conveyed as the output signal. The first and second voltages may be at the same or a different potential, such as a ground potential or a ground or power potential of a power supply. 
     In one embodiment of the invention, the inverting element is a quarter wavelength transmission line. Either or both of the variable capacitors may be voltage variable, such as varactors, or they may be variable though manual adjustment. When the variable capacitors are voltage variable, a first adjustable voltage may be applied across the first variable capacitor and a second adjustable voltage may be applied across the second variable capacitor. The first and second adjustable voltages are separately adjustable, thus permitting separate adjustment of the capacitance values of the fist and second variable capacitors to controllably vary the phase and the delay of the reflected input signal. 
     The input signal may be coupled to the inverting element through a directional coupler, such as a circulator. The circulator has first, second and third ports and preferentially routes signals incident at one port to another port. For example, signals applied at the first port are routed to the second port and signals applied at the second port are routed to the third port. According to one embodiment of the present invention, the input signal is applied to the first port of the directional coupler and conveyed to the second port which is coupled with the first end of the inverting element. The input signal is reflected by the inverting element and variable capacitors back into the second port. The reflected signal is then output from the third port as an output signal having a desired phase shift and delay relative to the input signal. 
     A system for correcting the phase and delay of a linear amplifier according to the present invention includes a linear amplifier, a phase and delay shifting element, a coupler and an error circuit. The phase and delay shifting element is adjustable to impose a variable delay and phase shift to signals applied to its input. The coupler receives and divides an input signal into first and second signals and conveys the first signal to an input of the linear amplifier and the second signal to an input of the phase and delay shifting element. The error circuit is coupled to the outputs of the linear amplifier and the phase and delay shifting element and produces an error signal based on differences between the outputs. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The above identified objects, features and advantages will be more fully appreciated with reference to the detailed description and the appended drawing figures, in which: 
     FIG. 1 depicts an embodiment of the delay and phase shifting element according to the present invention which includes variable capacitors and a phase inverting element. 
     FIGS. 2A and 2B illustratively depict graphs of phase shift vs. frequency and delay vs. frequency for an input signal and an output signal according to the present invention. 
     FIG. 3 depicts an embodiment of the delay and phase shifting element according to the present invention which includes varactors and a circulator as the directional coupler. 
     FIGS. 4A-4F illustrate graphs of delay and phase shift for the circuit depicted in FIG. 2 under conditions where one capacitor of the shunt resonator is held constant and the other is varied. 
     FIG. 5 depicts an embodiment of the phase and delay shifting element according to the present invention which includes two resonant circuits coupled to a circulator element. 
     FIG. 6 depicts a system, including a linear amplifier, for correcting errors in the linear amplifier. 
     FIG. 7 depicts a system, including a linear amplifier and a phase and delay shifting element according to the present invention, for correcting errors in the linear amplifier. 
     FIG. 8 depicts a lumped element inverter. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 depicts an embodiment of a delay and phase shifting element according to the present invention for adjustably shifting the phase and delay of a signal applied at an input. Referring to FIG. 1, the delay and phase shifting element  10  includes a directional coupler  14  and a shunt resonator circuit  21 . The directional coupler has three ports  16 ,  18  and  20 . 
     The directional coupler preferentially routes signals incident at one port to another port. For example, the directional coupler  14  conveys signals incident at port  16  to port  18  for output. Similarly, the directional coupler  14  conveys signals incident at port  18  to port  20  for output. Devices for implementing the directional coupler  14  are well known and illustratively include hybrid circuits and directional couplers, such as microstrips, waveguides, circulators and isolators. Any of these components may be ferrite components, which are non-reciprocal in that the insertion loss for a wave travelling between two ports is not the same in one direction as it is in the other. Probably the most commonly used ferrite, directional coupler is a circulator. A circulator is a piece of ferrite which, when magnetized, becomes nonreciprocal, preferring progression of electromagnetic fields in one circular direction. An ideal circulator has the scattering matrix:          [   S   ]     =         0       0         S   13               S   21         0       0           0         S   32         0                                
     An isolator is a circulator with one of the ports terminated in a matched load. It is used in a transmission line to pass power in one direction but not in the reverse direction. A four port version is typically called a duplexer. Any of these components is suitable for implementation as the directional coupler  14 . 
     An input signal  12  is applied to the directional coupler  14  at port  16 . The input signal is a signal to which one desires to add a phase shift and delay. The input signal  12  propagates through the directional coupler to port  18 , where it is output to the shunt resonator circuit  21 . The shunt resonator circuit  21  adjustably imparts a phase shift and delay to the input signal, which is reflected back through port  18  of the directional coupler  14  to port  20 . The reflected signal is output from the directional coupler  14  at port  20  and is the output signal  32 . As will be described below, the shunt resonator circuit  21  includes adjustable circuit elements enabling the phase shift and delay imparted to the input signal to be precisely varied and controlled. Adjustments are made to the adjustable circuit elements in order to impart a desired phase shift and delay to the output signal  32  using the input signal  12  as a reference. 
     In one embodiment of the invention, the shunt resonator circuit  21  includes dc blocking capacitors  22  and  26 , variable capacitors  24  and  30 , and a phase inverting element  28 . The dc blocking capacitor  22  is coupled between port  18  of the directional coupler  14  and the variable capacitor  24 . The capacitor  22  typically has a low impedance value and therefore appears as a short circuit to frequencies of interest, but an open circuit to dc current. The capacitor  22  (and  26 ) is optional and may be implemented only if necessary to block dc currents. The variable capacitor  24  is coupled from the capacitor  22  to a voltage either directly, or through a network of capacitive, resistive or inductive components. The voltage may be a ground or other potential from a power supply or any other convenient voltage source or sink. The dc blocking capacitor  26 , which passes frequencies of interest and blocks dc currents, is coupled between the variable capacitor  30  and the phase inverting element  28 . 
     The phase inverting element  28  is coupled between the variable capacitor  24  and the dc blocking capacitor  26 . It inverts the phase of the input signal approximately 90 degrees. The phase inverting element may be implemented as a quarter wavelength transmission line or using lump circuit elements in a well known manner. For example, FIG. 8 illustrates a lump element example of a phase inverter. Assume, for example that the inductors  104  and  106  each have a 225 nH value, that the capacitor  108  has a 45 pf value and that the resistor  110  has a 100 Ohm value. Further assume that the source  100  operates at a center frequency of 50 MHz and has a source impedance  102  of 50 Ohms. The impedance of elements  104 - 110  seen from the output of resistor  102  is 50 Ohms. The impedance of the inductors  104  and  106  are j70.7 Ohms and the capacitor  108  is −j70.7 Ohms. Therefore, the phase through the LC network is −90° and it acts like a transmission line of characteristic impedance 70.7 Ohms. 
     For high frequencies, such as frequencies in the microwave range, the wavelengths of the input signal are small enough that quarter wavelength transmission line implementations are more convenient than lump circuit element implementations. The phase inverter  28  is terminated (through the dc blocking capacitor  26 ) with the variable capacitor  30 . The variable capacitor  30  may be connected directly to a voltage or indirectly to the voltage through a network of capacitive, inductive and resistive components. The voltage may be a ground potential or other potential from a power supply or any other convenient voltage source or sink. 
     The delay and phase shifting element may be implemented using discrete components or may be manufactured as a single integrated circuit or as a combination of an integrated circuit and discrete components. Hybrid microwave integrated circuits, for example, include transmission lines and conductors on the integrated circuit and discrete components bonded to the substrate. Monolithic microwave integrated circuits include all circuit elements on the integrated circuit. 
     During use of the circuit depicted in FIG. 1, an input signal  12  is applied to the port  16  of the directional coupler. The directional coupler then conveys the input signal to port  18 . The input signal then emanates from port  18  and is reflected by the variable capacitors  24  and  30  and the phase inverting element  28  back into port  18  of the directional coupler  14 . The amount of phase shift and delay imparted to the reflected signal is determined based on the values of each of the variable capacitors  24  and  30 . In one embodiment of the invention, the variable capacitors  24  and  30  are voltage variable and the shunt resonator has a reflection characteristic that changes linearly with voltage applied to the variable capacitors  24  and  30  when the voltage applied to the capacitors  24  and  30  is the same. As the capacitance values  24  and  30  are changed, the angle of the reflected signal is changed. This produces the desired phase shift. This also produces a specific delay for each voltage setting and its corresponding phase shift. Both phase and delay change when both or either voltage setting is adjusted. By using a separate voltage on each variable capacitor, the phase and delay may be precisely selected. The Phase shifts of −180° to +180° may be achieved at desired delays. For example, delays in the range of 10 picoseconds to 5 nanoseconds may be imparted for phase shifts in the range of −180° to +180° for frequencies in the 0.5 to 5 GHz range. It will be understood, however, that the principles of the present invention are not limited to any particular frequency range and indeed may be implemented at frequencies less than 500 MHz as well as at frequencies in excess of 5 GHz. The range of delay afforded by the shunt resonator circuit depends upon the frequencies of the input signal. 
     FIGS. 2A and 2B respectively depict a graph of phase shift vs. frequency and a graph of delay vs. frequency. In both graphs, line represents an output signal with one setting of the capacitors and line S 2  represents the output signal with different settings of the capacitors, according to the present invention. Referring to FIG. 2A, at approximately 1.25 GHz, the phase of the output signal S 1  is set equal to the phase of the output signal S 2 . Referring to FIG. 2B, the output signal S 1  is delayed by approximately 2 nanoseconds relative to the output signal S 2 . 
     FIG. 3 depicts another embodiment of the delay and phase shifting element  10  according to the present invention. Referring to FIG. 3, a hybrid circulator  50 , which includes three ports  56 ,  58  and  60 , is coupled to a shunt resonator circuit  52 . An input signal  54  is applied at port  56  of the circulator  50 . The input signal is conveyed to and output from port  58  to the shunt resonator circuit  52 . The shunt resonator circuit  52  includes a de blocking capacitor  68  coupled to the port  58  and node  69 . A varactor  70 , which produces a voltage variable capacitance, is coupled between node  69  and a ground potential  78 . The varactor is a diode that provides a variable capacitance cross itself in response to voltage applied to it. An adjustable voltage source or supply V 1  is applied to node  69  and therefore across the varactor  70 . For example, the voltage source V 1  may have a positive (or negative) terminal coupled to the node  69  and its other terminal coupled to the ground terminal  78 . The voltage source may be adjusted to vary the capacitance of the varactor. The voltage source may be implemented in any convenient manner, including using the output of a digital to analog converter, a voltage regulator, voltage supply or any other suitable technique. The capacitor  68  prevents current from the dc voltage applied at node  69  from entering port  58  of the circulator  50 . 
     A quarter wavelength transmission line  72  is coupled between another de blocking capacitor  74  and the node  69 . The quarter wavelength transmission line acts as an open circuit and thus produces a phase inversion on incident signals. It will be understood, however, that the transmission line need not be exactly a quarter of a wavelength to function properly. Rather, any length between approximately ⅕ and ⅓ of a wavelength as well as odd multiples of a quarter wavelength would work well. The capacitor  74  is coupled between the quarter wavelength transmission line  72  and the node. A second varactor  76  is coupled between node  77  and ground. The capacitor  74  prevents de current from the voltage source V 1 , applied at node  69  from reaching node  77 . The voltage source V 2  is applied at node  77  to bias the varactor  76 . For example, the voltage source V 2  may have a positive (or negative) terminal coupled to the node  77  and its other terminal coupled to the ground terminal  79 . The voltage source may be adjusted to vary the capacitance of the varactor. The voltage source may be implemented in any convenient manner, including using the output of a digital to analog converter, a voltage regulator, voltage supply or any other suitable technique. The capacitor  74 , coupled between node  77  and the quarter wavelength transmission line  72 , prevents dc current from flowing into the quarter wavelength transmission line  72 . 
     During operation, different voltages may be applied at V 1  and V 2  in order to change the phase shift and delay of signal reflected by the shunt resonator back into port  58  of the circulator  50  and output from port  60 . Each voltage V 1  and V 2  applied at a corresponding varactor allows the capacitance to be changed to a desired value. FIGS. 4A-4F illustrate examples of ranges of delay and phase shifts generated for the circuit depicted in FIG. 2 where the following conditions are assumed: The input  54  is coupled over a 50 Ohm transmission line to port  56  of the circulator  50 . Varactor  70  is denoted C 1  and varactor  76  is denoted C 2 . C 1  is held constant and C 2  is varied. The value of C 1  is noted as a heading and the range of C 2  is noted for each line present in the graphs illustrated in FIGS. 4A-4F. The quarter wavelength transmission line  72  is selected for a center frequency of 1 GHz and also has an impedance of 50 Ohms. The output port  60  of the circulator  50  is terminated with a 50 Ohm load. Each line on the graphs illustrated in FIGS. 4A-4F represents the difference between the signal input at port  56  of the circulator and output from port  60  of the circulator. 
     Referring to FIGS. 4A and 4B, it is apparent that C 1  is held constant at 5 pf while C 2  is varied over a range of 0.1 pf-5 pf. Phase shifts from −165°-180° and delay of between 0.5 nanoseconds-3.2 nanoseconds may be achieved. Referring to FIGS. 4C and 4D, it is apparent that C 1  is held constant at 10 pf while C 2  is varied over a range of 0.1 pf-10 pf. Phase shifts from −180°-180° and delay of between 0.2 nanoseconds-8.0 nanoseconds may be achieved. Referring to FIGS. 4E and 4F, it is apparent that C 1  is held constant at 1 pf while C 2  is varied over a range of 0.1 pf-5 pf. Phase shifts from −180°-180° and delay of between 0.5 nanoseconds-1.0 nanoseconds may be achieved. By selecting values for C 1  and C 2 , nearly any combination of delay and phase shift can be achieved within a desired range. 
     FIG. 5 depicts still another embodiment of the phase and delay shifting element according to the present invention. In this embodiment, a hybrid circulator  80  includes four ports  82 - 88 . An input signal  90  is applied to the port  82  of the circulator  80 . The circulator  80  conveys half of the input signal to port  84  where it is output to the first shunt resonator circuit  52 . The incident signal is then reflected by the first shunt resonator circuit  52  back into port  84  of the circulator  80  which is then conveyed to port  88 . The circulator  80  also conveys half of the input signal to port  86 , where it is output to the second shunt resonator circuit  52 ′. The second shunt resonator circuit  52 ′ reflects the incident signal back into port  86  of the circulator which conveys the reflected signal to port  88 . Port  88  in turn outputs the combined output signal  92 . The phase shift and delay of the output signal  92  relative to the input signal  82  are determined by the values of the individual voltages applied at V 1  and V 2  of each of the shunt resonator circuits  52  and  52 ′. 
     FIG. 6 depicts a system, including a linear amplifier for correcting phase and delay of the linear amplifier. Referring to FIG. 6, an input signal arrives at a coupler  110  which divides the input signal into two output signals, each of which is conveyed along a separate path 1 or 2. Path 1 includes a linear amplifier  112  which amplifies the incident signal and conveys it to the coupler  114 . Linear amplifiers are well known. The coupler  114  divides the output of the linear amplifier into two signals. One signal is conveyed as an output of the linear amplifier  112  and the other is conveyed to the input of an error element  122 . 
     Path 2 receives the divided input signal at the variable phase shifter  116 . The variable phase shifter  116  shifts the phase of the input signal a desired amount to correct phase error in the linear amplifier and conveys the shifted signal to an attenuator  118 . The attenuator  118  corrects amplitude error in the linear amplifier and conveys the attenuated signal to the delay line  120 . The delay line  120  generally comprises one or more delay lines which may be swapped in and/or out in order to correct the delay of the linear amplifier by a desired amount. The component delay lines, however, each have a fixed amount of delay. Therefore, only increments of delay may be realized and component delay lines must be swapped into and out of the delay line  120  for a proper configuration. 
     The error circuit  122  receives both the output of the linear amplifier  112  from path  1  and the output of path  2  and produces an error signal which may be used in subsequent signal processing to correct or offset error. 
     FIG. 7 depicts an alternate implementation of a system, including a linear amplifier, according to the present invention. The system of FIG. 7 includes the same functional blocks as the system of FIG. 6, except that a variable phase shifter and delay block  10  is used in FIG. 7 in lieu of (or optionally in addition to) the variable phase shifter  116 . The variable phase shifter and delay block  10  shifts the phase and the delay of the signal input to path 2 a desired amount as described with reference to the embodiments depicted in FIGS. 1-3. Both the phase and the delay imposed by block  10  may be adjusted mechanically or electronically according to the embodiment chosen. Moreover, because the variable phase and delay block  10  permits any delay over a continuous range of values to be used, delay lines need not be swapped into and out of the delay line block  120 . Rather, either no delay line block  120  needs to be used or a delay line block  120  may be chosen to implement a rough amount of delay with the variable phase shift and delay block  10  permitting fine adjustment of the delay. 
     While specific embodiments of the invention have been described, it will be understood that changes may be made to these embodiments without departing from the spirit and scope of the invention.