Abstract:
An AC signal is switched between two circuit branches using a transformer or at least one directional coupler as a coupling device. Electronically controlled switches shunt one of two terminals of the coupling device to AC ground. The input signal is propagated out of the non-shunted terminal to one of the two circuit branches. The electronically controlled switches may be relays, transistors, or diodes. Diodes prevent the AC signal from being shunted to AC ground when reverse biased and shunt the AC signal to AC ground when forward biased.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to devices that steer electromagnetic signals. Even more particularly, this invention relates to an electronically operated device that switches radio frequency electronic signals between two or more circuit branches. 
     BACKGROUND OF THE INVENTION 
     Radio frequency (RF) and microwave signal generators use a high power amplifier to generate large output signals. Unfortunately, non-ideal characteristics of these amplifiers can cause harmonic distortion of the output signal. Harmonic distortion is nonlinear distortion on the output of an amplifier characterized by the appearance in the output of harmonics other that the fundamental component when the input wave is sinusoidal. In a high-quality signal generator, these harmonics must be filtered before the signal leaves the source. The filter should reject signals an octave and greater above the fundamental output frequency. Since the frequency range of the output signal may be a decade or more, multiple sub-octave filter ranges are used. To enable the signal generator to sweep the output signal through a large range of output frequencies very quickly, or to allow computer control of the instrument, it is desirable that the switching between the multiple sub-octave filters be electronically controlled. 
     A p-i-n diode may be used as electronically controlled switch. (A p-i-n diode is constructed so that an intrinsic layer, the “i region,” is sandwiched between a positively doped layer (“p layer”) and a negatively doped layer (“n layer”). When reversed biased, a p-i-n diode behaves like an “open” switch. When forward biased, a p-i-n diode behaves like a “closed” switch. 
     An appropriate arrangement of p-i-n diodes is currently used to route the output signal through the appropriate sub-octave filter and then back to an output node. At the input node, multiple series p-i-n diodes are arranged in a star configuration with one common driven node and multiple intermediate nodes. A shunt diode is placed at each intermediate node. Each intermediate node is then connected to the input of one of the sub-octave filters. The reverse of this arrangement is used to route the outputs of the sub-octave filters back to a common output node. The path through the desired filter is used by forward biasing the series diodes in that path and reverse biasing the shunt diodes connected to that path. The series diodes on the unused paths are reversed biased and the shunt diodes connected to the unused paths are forward biased. The forward biased shunt diodes provide a path to AC ground, effectively stopping any AC signal from propagating down the path the diode is shunting. This isolates the unused paths. 
     This method of routing signals requires very compact assembly for frequencies above 20 GHz. This is difficult and expensive. Beam lead diodes are more expensive than other types of diode packaging. These diodes should be bonded directly to the shunt diodes. In a very compact assembly, this is a manual operation that increases cost. A series diode is also required to have low capacitance to achieve high isolation when shut off. Because relatively high series resistance is typically associated with low capacitance, the series diodes tend to contribute significant signal loss. Finally, due to their nonlinear characteristics, the series diodes introduce nonlinear distortion that degrades the quality of the output signal. 
     Accordingly, there is a need in the art for an improved way of electronically switching RF and microwave signals. It is desirable that such a system eliminate series diodes in the signal path since these devices are expensive, hard to assemble, reduce output power, and degrade the quality of the output of the device. Finally, the system should be easy to assemble. 
     SUMMARY OF THE INVENTION 
     In a preferred embodiment, the invention provides a switch for AC signals that does not have any diodes in series with the signal path. This improves the quality and power of the signal output after the AC switch and reduces the cost of the AC switch. The AC switch may be manufactured using traditional techniques that may be automated. 
     An embodiment of a single-pole double-throw AC switch according to the invention includes a four terminal coupling device or structure, such as a transformer, 3dB, or other directional coupler. The input signal is fed to one terminal of the coupling device. The input corresponds to the pole of the AC switch. A second terminal of the coupling device is connected to an AC ground. The third and fourth terminals connect to the outputs, or throws, of the AC switch. The third and fourth terminals are also each connected to one of two shunt switching devices that, when on, shunt the third or fourth terminal of the coupling device, respectively, to AC ground. Only one of these two shunts is turned on at a time. This results in the input signal being coupled to the terminal with the shunt that is not on without the signal passing through a series diode. 
     Another embodiment of a single-pole double-throw AC switch for switching selected frequencies of RF and microwave signals includes a directional coupler with a nominal coupling factor of 3 dB as the coupling device. P-i-n diodes are used as the shunting devices. When forward biased, the p-i-n diodes shunt the terminal to which they are connected to AC ground. 
     Another embodiment of a single-pole double-throw AC switch for switching selected frequencies of RF and microwave signals includes two cascaded directional couplers. Each of these couplers has a nominal coupling factor of 8.34 dB for a nominal net coupling factor of 3 dB for the entire structure. P-i-n diodes are used as the shunting devices. By cascading two directional couplers, the shunt diodes may be located farther from each other and one of the output terminals may be located closer to the input terminal. This structure also improves manufacturability by allowing greater line width and spacing to be used. Greater line widths and spacing also decreases signal loss inside the structure. 
     Other embodiments include multi-layer coupling structures, waveguide coupling structures, stripline coupling structures, or any other type of RF or microwave coupling structure as the coupling device. One example of a multi-layer coupling structure is a multi-layered broadside coupled structure. Other embodiments include a greater number of throws for the switch. For example, the first embodiment, above, can be made into a single-pole triple-throw switch by removing the AC ground connection from the second terminal of the four terminal coupling device and making that terminal the third the output, or throw, of the AC switch. A third shunt device would also be connected between the second terminal and an AC ground. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a single-pole double-throw AC switch according to the present invention using a transformer as the coupling device. 
     FIG. 2 illustrates a microstrip implementation of a frequency selective singlepole double-throw AC switch using diodes as the shunt devices. 
     FIG. 3 illustrates a microstrip implementation of a frequency selective singlepole double-throw AC switch that uses cascaded directional couplers. 
     FIG. 4 is a representational plot of signal power transmitted through a microstrip frequency selective single-pole double-throw switch. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a single-pole double-throw AC switch that uses a transformer as the coupling device. The input signal enters the AC switch via transmission line  102  and exits via transmission line  106  to node OUT 1  or transmission line  108  to node OUT 2 . Z 0  represents the impedance values of the respective transmission lines  102 ,  106 , and  108 . Transmission line  102  connects to coupling device  104  which is a transformer in FIG.  1 . Transmission line  102  feeds one terminal of the primary inputs. The other primary terminal of the coupling device is connected to ground. This may be either an AC or DC ground as long as it acts like an AC ground over the desired operating frequencies. 
     With a transformer, the primary inputs correspond to the terminals of the primary winding of the transformer and the secondary corresponds to the terminals of the secondary winding. A first terminal of the secondary of coupling device  104  is connected to transmission line  106 . The second terminal of the secondary of coupling device  104  is connected to transmission line  108 . Also connected to the first terminal of the secondary of coupling device  104  is a switch, S 1   110 . Also connected to the second terminal of the secondary of coupling device  104  is a switch, S 2   112 . S 1  and S 2 , when on, shunt AC signals in the desired operating frequencies to an AC ground. Switches S 1  and S 2  can be diodes, bipolar transistors, relays, field-effect transistors, or any other kind of switching device or structure that will shunt AC signals in the desired operating frequencies to an AC ground. 
     When an input signal is applied to the AC switch via transmission line  102 , that input signal is coupled, at least in part, to the secondary. If switch S 2   112  is on and switch S 1   110  is off, then no signal can propagate down transmission line  108  because its input is held at AC ground and all of the coupled signal propagates down transmission line  106  to OUT 1 . If switch S 2   112  is off and switch S 1   110  is on, then no signal can propagate down transmission line  106  because its input is held at AC ground and all of the coupled signal propagates down transmission line  108  to OUT 2 . Accordingly, by turning switches S 1  and S 2  appropriately, the input signal may be routed to OUT 1  or OUT 2  without having the signal pass through a diode or other switching device. 
     FIG. 2 illustrates a microstrip implementation of a frequency dependent single-pole double-throw AC switch using diodes as the shunt devices. The coupling device is shown generally inside box  202 . The coupling device is comprised of two parallel microstrip lines  210 ,  212  that are close together. The spacing of the lines is chosen to set certain desired characteristics of the coupling device including the coupling factor, even mode impedance (Z oe ), and odd mode impedance (Z oo ). In an embodiment, this coupling device nominally has a coupling factor of 3 dB. The length of microstrip lines (as shown in FIG. 2 as the distance from point A to point B for microstrip line  210  and the distance from point C to point D for microstrip line  212 ) is chosen to be a quarter wavelength of the frequency of maximum desired coupling. The input signal is fed to the coupling device  202  via transmission line  204 . This connects to one end of microstrip line  210  (at point A). The other end of microstrip line  210  (point B) is connected to transmission line  206 . Transmission line  206  connects to one output of the switch, node OUT 1 . 
     The signal on microstrip line  210  is coupled to microstrip line  212 . The end of microstrip line  212  (point C) that is next to the input of coupling device  202  (point A) is connected to AC ground. The other end of microstrip  212  (point D) is connected to transmission line  208 . Transmission line  208  connects to the other output of the switch, node OUT 2 . The cathode of p-i-n diode  214  is connected to the output of coupling device  202  at point B. The anode of p-i-n diode  214  is connected to AC ground and a first DC bias voltage (DC BIAS 1 ). The cathode of p-i-n diode  216  is connected to the other output of coupling device  202  at point D. The anode of p-i-n diode  216  is connected to AC ground and a second DC bias voltage (DC BIAS 2 ). By controlling the first and second DC bias voltages, p-i-n diodes  214  and  216  may be forward and reversed biased. When forward biased, diodes  214  and  216  provide a shunt to AC ground at points B and D, respectively. When reversed biased, diodes  214  and  216  act like an open switch to AC ground. 
     FIG. 4 illustrates the frequency dependent nature of the switch shown in FIG.  2 . FIG. 4 is a plot of output power relative to input power versus frequency for the two operating modes of the switch. The solid line shows the signal output power at node OUT 2  when point B is shorted to AC ground. The dashed line shows the signal output power at node OUT 1  when point D is shorted to AC ground. The plot is relative to the input signal power level so a 0 dB level on the plot means all of the input power is transmitted to the output port. 
     As shown in FIG. 4, the power transmitted to node OUT 2  has periodic “nulls” where little or no power is transmitted to node OUT 2  over certain frequencies. This is what gives this switch a frequency selective characteristic. Careful application of the switch, however, prevents the frequency selectivity from being a problem. For example, if the switch is used in the signal generator described above, the OUT 2  nodes of multiple switches can be used to pass the input signal to the multiple sub-octave filters. The cutoff frequency of each sub-octave filter is then placed in or near a flatband portion (i.e. one of frequency ranges labeled “operating frequency ranges” on FIG. 4) of the response of the switch feeding that sub-octave filter. This eliminates any problem with the “null” portion of the switches frequency response because the sub-octave filter would have removed any frequencies in the “null” portion of the switches frequency response if the frequency response of the switch had not already done so. 
     FIG. 3 illustrates a microstrip implementation of a frequency selective single-pole double-throw AC switch that uses cascaded directional couplers. Each of the cascaded couplers has a coupling factor that is nominally 8.34 dB which makes the coupling factor for the entire structure nominally 3 dB. The coupling device is shown generally inside box  302 . The coupling device is comprised of four parallel microstrip lines  310 ,  312 ,  314 ,  316  that are close together. The first directional coupler is comprised of microstrip lines  310  and  312 . The second directional coupler is comprised of microstrip lines  314  and  316 . They are connected back to back via jumper  322  and a short area of metal at point G. 
     The spacing of the lines is chosen to set certain desired characteristics of the coupling device including the coupling factor, Z oe , and Z oo . The length of microstrip lines (as shown in FIG. 3 as the distance from point E to point F for microstrip line  310 , the distance from point G to point H for microstrip line  312 , the distance from point G to point I for microstrip  314 , and the distance from point J to point K on microstrip line  316 ) is chosen to be a quarter wavelength of the frequency of maximum desired coupling. The input signal is fed to the coupling device  302  via transmission line  304 . This connects to one end of microstrip line  310  (at point E). The other end of microstrip line  310  (point F) is connected to microstrip line  316  (at point K) via a jumper  322 . Transmission line  306  connects to one output of the switch, node OUT 1 , at the other end of microstrip line  316  (point J). 
     The signal on microstrip line  310  is coupled to microstrip line  312 . The end of microstrip line  312  (point G) that is next to the input of coupling device  302  (point E) is connected to the end of microstrip line  314  that is near the output of the second directional coupler (point J). The other end of microstrip  312  (point H) is connected to transmission line  308 . Transmission line  308  connects to the other output of the switch, node OUT 2 . The cathode of p-i-n diode  318  is connected to the output of coupling device  302  at point J. The anode of p-i-n diode  318  is connected to AC ground and a first DC bias voltage(DC BIAS 1 ). The cathode of p-i-n diode  320  is connected to the other output of coupling device  302  at point H. The anode of p-i-n diode  320  is connected to AC ground and a second DC bias voltage(DC BIAS 2 ). By controlling the first and second DC bias voltages, p-i-n diodes  318  and  320  may be forward and reversed biased. When forward biased, diodes  318  and  320  provide a shunt to AC ground at points J and H, respectively. When reversed biased, diodes  318  and  320  act like an open switch to AC ground. 
     From the foregoing it will be appreciated that the AC switch provided by the invention offers numerous advantages. Such a switch does not have distortion producing and power robbing series diodes or other active devices in series with any of the signal paths. The switch is geometrically simple, and easy to assemble. 
     Although several specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. The invention is only limited by the claims.