EP0222910A1 - Active microwave switcher - Google Patents

Abstract

The low loss microwave switch (10) routes an incoming microwave signal to a selected output port (2) without the use of signal splitting couplers. A propagation path between the input and the selected port is established while an open circuit condition returning the signal is established elsewhere to block all unselected ports. PIN diodes (46, 56) are energized on or off to choose between propagation states and returning states, and shunt paths of a quarter wavelength are used to establish an open circuit condition .

Description

ACTIVE MICROWAVE SWITCHER

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates qenerally to microwave switching equipment and more particularly to a low loss apparatus for selectively switching a microwave signal between separate propagation paths. The invention has wide applicability in the microwave industry and may be used in microwave communication switching circuits in satellites or land-based equipment, and in microwave signal -processing equipment such as down converters, in which one of a plurality of microwave local oscillator synthesizers must be selected and its microwave output coupled to the mixer for frequency shifting.

The switching of microwave signals presents problems radically different from the switching of low-f equency signals. Transmission lines and other energy propagating devices must be carefully impedance matched to prevent signal loss due to reflections, and care must be taken to properly terminate transmission lines which are not otherwise connected to equipment. Even the act of switching a microwave signal from one destination to another encompasses the foregoing problems, as it is not considered good enqineering practice to switch a signal from one destination to another by simply mechanically connecting one trans- mission line to another, as one miqht construct a single pole, single throw switch at low frequencies, for example.. Such an approach would create substantial crosstalk between channels and appreciable signal loss and signal distortion due to reflections, signal radiation, and other electromagnetic phenomena. In the past, microwave switching has been accomplished using coupling junctions whereby the energy of an incoming signal is split or divided into output paths which are both carefully impedance matched to minimize signal reflection. In many cases, a plurality of such junctions are arranged in an interconnected matrix, whereby any one or more input terminals can communicate with any one or more output terminals. The switching matrix, thus, provides a means for coupling an incoming signal to a selected output signal path through the judicious interconnection of siqnal couplers.

A significant drawback to the coupler approach o microwave switching is -that each coupler divides the incoming microwave energy such that only a fraction of the microwave energy is available to propaσate in the desired manner. In more complex switching matrices, where many coupling junctions are employed, it is conceivable that a signal may be attenuated so severely ' that all useable energy may be effectively lost before the signal leaves the switching device. Indeed, this is a serious drawback particularly in microwave systems where the signal-to-noise ratio is critical. In many applications it is often difficult to recover the deteriorated signal throuqh amplification, since most amplification techniques amplify both signal and noise alike. Accordingly, there has been a vast need for improvement in the microwave switching art. «. 1 In response to this need, the present invention provides a microwave switcher which is elegant in its approach and which eliminates the signal loss associated with coupling junction matrices. The microwave switcher 5 comprises first, second and third ports (although greater numbers of ports are also possible). A first path for selectively supporting propagation of microwave energy is set up between the first port and the second port, while a second path for selectively supporting

10 propagation of microwave energy is set up between the second port and the third port. A first siqnal reflecting means is disposed in the first path. The first signal reflecting means has an energy propaqating state whereby energy propagates between the first and second ports

15 and has an energy impeding state whereby energy is substantially prevented or impeded from propagating between the first and second ports. A second siqnal reflectinq means is disposed in the second path and likewise has first and second states. In its energy

20 propagating state, the second signal reflecting means supports propagation between the second and third ports, . while in the energy impeding state, it substantially prevents or impedes energy from propagating between the second and third ports. The microwave switcher further

25 comprises a means for causinq the first and second signal reflecting means to selectively assume the propagating and impeding states.

For a more complete understanding of the 4- invention, its objects and advantages, reference may be

*»

30 had to the following specification and to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram illustrating one 35 embodiment of the microwave switcher of the invention; FIG. 2 is a schematic diagram illustrating an - alternate embodiment of the .microwave switcher of the invention;

FIG. 3 illustrates yet another embodiment of the microwave switcher of the invention;

FIG. 4 is a block diagram illustrating a ten element active microwave switcher using the principles of the invention;

FIG. 5 is a schematic diagram illustrating a conventional junction switching matrix for comparison; FIG. 6 is a schematic diagram illustrating the microwave switcher of the invention used in conjunction with a conventional coupling junction;

FIG. 7 is a schematic diagram illustrating the use of the microwave switcher of the invention in a switching matrix similar to that of FIG. 5; and

FIG. 8 is a detailed schematic circuit diagram illustrating the presently preferred circuit for - ^• implementing the microwave switcher of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, the microwave switcher of the invention is illustrated generally at 10. Microwave switcher 10 comprises a first microwave switch 12 and a second microwave switch 14. The construction of switches 12 and 14 are essentially the ^■ same ^"and will be discussed below in conjunction with FIG.8. Microwave switcher 10 further comprises three ports, designated port 1, port 2 and port" 3. A first microwave propagation path 16 connects port 1 with port 2, while a second propaqation path 18 connects port 1 and port 3. Propagation paths 16 and 18 may be conventional transmission lines or metal conductors or an equivalent means for supporting the progagation of microwave energy. Propagation paths 16 and 18 unite at a junction 20. As shown in FIG. 1, junction 20 occurs essentially where switch 12 joins propagation path 18. Switch 14 connects with propagation path 18 at junction 22. The distance between junctions 20 and 22 is one-quarter wavelength (or odd multiples thereof).

Switch 12 is connected in series with propagation path 16, while switch 14 is connected in shunt fashion between propagation path 18 and ground. Switches 12 and 14 are both conditioned by supplying a bias current signal as will be discussed below in connection with FIG. 8. Switches 12 and 14 are configured so that they are both open (off) or both closed (on). In the off condition, an input signal introduced through port 1 is substantially reflected by the open switch 12, hence little or no input signal from port 1 reaches the output port 2. An input signal introduced through port 3 is free to- propagate along path 18 to port 2. The open switch 14 offers very little impedance to the signal flow from port 3 to port 2. When both switches are in the on condition, a different result obtains. With switch 12 closed, signals introduced through port 1 propagate freely to port 2 via propagation path 16. None of the input signal is diverted to port 3 because the closed switch 14 one-quarter wavelength from junction 20 appears as an open circuit. Hence, even though port 3 is still physically connected to junction 20 through a portion of propagation path 18, the circuit behaves as if there is no propagation path to the port 3 side of junction 20 when switch 14 is closed. With switch 14 closed, any signal entering port 3 is reflected off the short circuit at switch 14. By causing the switches 12 and 14 to selectively assume either- the off states or the on states, it is possible to switch either the port 1 input signal or the port 3 input signal to the output port 2. In effect, the microwave switcher 10 provides the function of a single pole double throw microwave switch. Because no coupling junctions are utilized, the microwave switcher 10 avoids the signal loss associated with coupling junctions. FIG. 2 illustrates another embodiment of the invention wherein the shunt switch 14 of FIG. 1 is replaced with a series switch 24 in propagation path 18. Switches 12 and 24 are both disposed in close proximity to junction 20, and are both conditioned to be open (off) or closed (on) in response to bias voltage signals yet to be discussed. When switch 12 is open, switch 24 is closed, and vice versa. In the on condition, signals entering port 1 propagate freely^'via propagation path 16 to output port 2, while the open circuit at switch 24 reflects any signals entering port 3. Because switches 12 and 24 share the common junction 20 and are in close proximity thereto, very little or no siqnal energy entering port 1 will propagate to port 3. The port 3 side of junction 20 appears as an open circuit and does not support propagation. In the off condition, siqnals enterinq port 3 propaqate freely to port 2 through switch 24. Signals entering port 1 are reflected from the open circuit at switch 12. Due to the open circuit in close proximity to the port 1 side of junction 20, very little or no energy entering port 3 is diverted through the port 1 leg of propagation path 16. FIG. 3 depicts another embodiment which is similar to the embodiment of FIG. 1, but which includes an additional series switch 24 connected in propagation path 18 in close proximity to junction 22. Like switches 12 and 14, switch 24 also has open (off) and closed

(on) states. In the on state, signals entering port 1 propagate freely to port 2 while signals entering port 3 are reflected. The signal reflection at port 3 is the result of two contributing factors, the first being the shunt path through switch 14 to ground, and the other being the open circuit of switch 24. Switch 24 is configured to open and close with an opposite sense of polarity compared to that of switch 14. When switch 14 (and switch 12) are open, switch 24 is closed, and vice versa. The embodiment of FIG. 3, while more complex, provides be-tter signal isolation, less crosstalk, through the use of cumulative switchinq devices. This somewhat improved performance is attributable to the fact that no presently available microwave switching components exhibit ideal on/off switching behavior. Presently available electronic components support a certain amount of signal bleed through or coupling between input and output when switched in the nonpropagatinα state. Hence, the use of multiple and cumulative switches, as exemplified by switches 14 and 24 of FIG. 3, may be necessary in some cases to provide sufficient isolation of the nonselected ports. While FIG. 3 illustrates the use of multiple switches 14 and 24 (a shunt/series combination) a similar treatment may be used in propagation path 16 of port 1. Furthermore, while the shunt/series combination has been illustrated, it is also possible to achieve greater isolation using multiple shunt switches, multiple series switches, or any combination of the two. Having described the principles of the invention, reference may now be had to FIG. 4 which gives an example of how the microwave switcher of the invention may be utilized in a ten-element active microwave switcher. In FIG. 4, a plurality of microwave switchers 10 are illustrated in block diagram fashion, with port

1, port 2, and port 3 identified. These port identifications correspond to the port identifications of FIGS. 1, 2 and 3 discussed above. Thus, it will be understood that the circuit represented by blocks 10 in FIG. 4 may be implemented using any of the previously discussed microwave switchers of FIGS. 1, 2 and 3. For purposes of further identification, the individual microwave switchers 10 have also been designated as AMSE 1, AMSE 2...AMSE 10, the acronym AMSE referring to Active Microwave Switcher Element.^' Referring to FIG. 4, it will be seen that ten possible input signals - are provided to the respective port 1 terminals of -the microwave switchers 10. In addition, port 3 of AMSE 1 receives an input from port 2 of AMSE 2; port 3 of AMSE 2 receives an input from port 2 of AMSE 3; and so forth. The output of the ten-element active microwave switcher of FIG. 4 is derived from port 2 of AMSE 1. To provide proper termination for the circuit, a termination load 26 is coupled to port 3 of AMSE 10.

In operation, the ten-element active microwave switcher is conditioned through the proper setting of individual switches within each AMSE unit to select one and only one of the input signals for output through port 2 of AMSE 1. If, for example, input 3 is to be selected, then AMSE 3 is biased or switched so that the internal switches 12 and 14 thereof are closed, thereby establishing a propagation path between port 1 and port

2. In so doing, the propagation path between port 3 and port 2 is disabled, hence any signal entering port 3 of AMSE 3 is simply reflected. This being the case, it may not be necessary to consider the settings of AMSE 4, AMSE...AMSE 10. The internal switches 12 and 14 of AMSE 2 and AMSE 1 must both be switched open so that a propagation path is established from port 3 to port 2 of AMSE 2 and from port 3 to port 2 of AMSE 1. Such switch settings will select input 3 for output while blockinq all other inputs. Although the proper setting switchers AMSE 1, AMSE 2 and AMSE 3 is sufficient to select input 3 for output, it is usually preferable to also set all upstream switchers (AMSE 4, AMSE 5... MSE 10) so that their respective inputs are switched off. This precaution can provide additional isolation of the output from the undesired input. As stated above, one of the principle advantages of the microwave switcher o _*ver prior art coupling junctions, is lower losses. The improved performance of ^'the microwave switcher' df the invention may be demonstrated with reference to FIGS. 5, 6 and 7. FIG. 5 depicts a conventional microwave switching matrix coupling junction. FIG. 6 depicts the use of a conventional junction in one leg of the switching matrix and with the microwave switcher (AMSE) in the other leg. FIG. 7 illustrates the microwave switching matrix using two AMSE units in place of the conventional junctions.

In the conventional microwave switchinq matrix shown in FIG. 5, signals entering throuqh port A pass through coupler 28 where the incoming siqnal is split between two propagation paths. A first propagation path 30 couples directly to port D while a second propagation path 32 couples to termination load 34 and switch element 36. In a similar fashion, signals entering port B pass throuqh coupler 38 where the port B incoming signal is split. Coupler 38 supplies a first propagation path 40 to port C and a second propagation path 42 coupled to termination load 44 and to switch element 36. Switch element 36 is a microwave make or break switch which either couples or decouples the switch element terminals. When coupled, path 32 is coupled with path 42 and, hence, port A is coupled with port C. When switch element 36 is turned off or decoupled, path 32 is not coupled with path 42, and, hence port A is not coupled to port C. Port A is at all times coupled to port D and port B is at all times coupled to port C. In a microwave matrix, port A might be connected to receive input signals possibly fed through other junctions as well. Port B would be connected to receive outputs from other junctions or to a proper termination load if there are no other junctions in the matrix. Port C would be connected to supply an output signal, possibly- through other junctions Port D would be connected to the next junction in the matrix, or to a proper termination load if there are no other junctions in the matrix.

A principle disadvantage of the conventional microwave switching matrix junction of FIG. 5 can now be appreciated. If it is desired to route a signal from port A to port C, the incoming siqnal must pass through two couplers thereby sustaining a substantial signal loss associated with these couplers. It will be appreciated that if a large matrix is configured, using a plurality of such couplers, very large signal losses can accumulate. FIG. 6 illustrates a hybrid circuit using one coupler, coupler 28, and one AMSE unit of the invention. As before, the coupler 28 supports the propagation of signals from port A to port D, with an attendant signal loss. Propagation path 32 of coupler 28 is connected to port 1 of the AMSE device. Ports 2 and 3 of the AMSE device serve as output and input ports analogous to ports B and C in FIG. 5. However, the AMSE device does not insert a coupling signal loss as the prior art coupling junction does. Hence, a signal enterinq input port A and output throuqh port 2 would experience only the signal loss associated with a single coupler, coupler 28.

FIG. 7 illustrates the use of two AMSE devices in place of both couplers of FIG. 5. Since neither AMSE -device inserts a signal loss, the circuit of FIG. 7 exhibits substantially less signal loss than the circuit of FIG-. 5.

With the foregoinq in mind, the presently preferred electronic circuit is illustrated in FIG. 8. In FIG. 8, port 1, port 2, and port 3 are denoted and correspond to the similarly identified ports of FIG. 1. Switch 12 comprises PIN diode 46 to which biasinq current is supplied throuqh RF choke 48. An additional RF choke 50 is coupled between the cathode of diode 46 and ground, while blocking capacitor 52 prevents low frequency signals through output port 2 from altering the bias of diode 46. Switch 14 comprised PIN diode 56 coupled in shunt fashion to ground as illustrated. PIN diode 56 is also supplied its biasinq siqnals through RF choke 48. Blockinq capacitor 58 prevents low frequency signals from port 3 from passing through and affecting the bias upon PIN diodes 46 and 56. In operation, diodes 46 and 56 behave as near short circuits when forward biased and behave as near open circuits when unbiased or reverse biased. Hence, the on/off state of switches 12 and 14 can be caused to switch from one desired state to another by simply altering the bias signal through choke 48 as reguired. As explained above, it is sometimes necessary to use a plurality of switches in tandem in order to provide adequate circuit isolation in the off state. This may be accomplished by using a plurality of series connected PIN diodes in place of the single diodes shown in FIG. 8. Alternatively, where shunt diodes are employed, a plurality of parallel connected diodes may be substituted for the single shunt diode of FIG. 8. While the invention has been illustrated and described in its presently preferred embodiments, it will be understood that certain modifications may^' be made' without departing from the scope of the invention.

Claims

What is Claimed is as follows: 1. A microwave switcher comprising: first, second and third ports; first path for selectively supporting propagation of microwave energy between said first port and said second port; second path for selectively supporting propagation of microwave energy between said second port and said third port; first signal reflecting means disposed in said first path having an energy propagating state whereby energy propagates between said first and second ports and having an energy impeding state whereby energy is impeded from"propagating between said first and second ports; . , second signal reflecting means disposed in said second path having an energy propagating state whereby energy propagates between said second and third ports and having an enery impeding state whereby energy is impeded from propagating between said second and third ports; and means for causing said first and second signal reflecting means to selectively assume said propagating and impeding states.

2. The microwave switcher of Claim 1 wherein said first signal reflecting means comprises series switching means disposed in said first path.

3. The microwave switcher of Claim 1 wherein said second signal reflecting means comprises shunt switching means coupled to said second path.

4. The microwave switcher of Claim 1 wherein said first signal reflecting means comprises series switching means disposed in said first path and said second signal reflecting means comprises shunt switching means coupled to said second path.

5. The microwave switcher of Claim 1 wherein said first and second paths unite at a junction and wherein said second signal reflecting means is disposed a preselected distance away from said junction.

6. The microwave switcher of Claim 5 wherein said predetermined distance is selected to cause said second signal reflecting means to appear at said junction as a substantially open circuit when said second signal reflecting means is in said impeding state.

7. The microwave switcher of Claim 5 wherein said predetermined distance is then odd multiple of the quarter wavelength of said microwave energy.

8. A microwave switcher comprising: first, second and third ports; first path for selectively supporting propagation of microwave energy between said first port and said second port including first PIN diode means connected between said first and said second ports and having means for supplying a bias current signal for selectively switching said first PIN diode on and off; second path for selectively supporting propagation of microwave enerqy between said second port and said third port including second PIN diode means coupled in shunt fashion to said second path and having means for supplying a bias current signal for selectively switching said second PIN diode on and off; said first and second PIN diodes being coupled to said propagation paths and spaced an odd multiple of a quarter wavelength apart; and said first and second PIN diodes being responsive to said bias current signals to selectively assume open and closed states, in said open state said second and third ports being coupled for propagation and said first port being substantially impeded, and in said closed state said first and second ports being coupled for propagation and said third port being substantially impeded.

9. A cascaded microwave switcher comprising: a first and second active microwave switcher element each having first, second and third ports, each having a first path for selectively supporting propagation of microwave energy between said first port and said second port, and each_,having a second^' ath for selectively supporting propagation of microwave energy between second port and said third port; said first and second active microwave switcher elements each further having first signal reflecting means disposed in said first path having an energy propagating state whereby energy propagates between said first and second ports and having an energy impeding state whereby energy is impeded from propagating between said first and second ports; said first and second active microwave switcher elements each further having an energy propagating state whereby energy propagates between said second and third ports and having an energy impeding state whereby energy is impeded from propagating between said second and third ports; and said first and second active microwave switcher elements each having a means for causing said first and second reflecting means to selectively assume said propagating and impeding states, wherein said second port of said second active microwave switcher element is coupled to said third port of said first active microwave switcher.

10. The cascaded microwave switcher of Claim 9 further comprising a third active microwave switcher element having first, second and third ports, said second port of said third active microwave switcher element being coupled to said third port of said second active microwave switcher element.