Abstract:
A balanced line switching apparatus that provides high isolation at an expense of a marginal increase of loss. Practical implementation can give as much as 40 dB isolation in a single stage.

Description:
BACKGROUND 
   This invention relates generally to microwave and millimeter wave (mm-wave) radio frequency (RF) circuits, and more particularly to achieving broadband high isolation switch in Balanced Line Circuits. 
     FIG. 1  shows a balanced line. A balanced line  10  may be achieved by using two conductors  11  and in a symmetric environment. Such balanced lines can be achieved for example as in twisted pair cable or on insulating substrates. The input port  12  is composed of two terminal  12   a  and  12   b . Due to symmetry, terminals  12   a  and  12   b  have opposing voltage V 1  and −V 1  and support equal currents  16  and  17  in opposite direction or opposing current. In a balanced line configuration because there is no other path available for the current, the forward going current has to be equal to the reverse going current at any location, for example position  18 , due to charge conservation. Moreover, voltage at any location  18  along the transmission line is also equal and opposite. If the balanced line is terminated in a balanced manner (i.e., same impedance on each line) using the output port terminal  13   a  and  13   b , the output port  13  also has opposing voltages and currents,  14  and  15 , respectively, at the terminal  13   a  and  13   b.    
   Such balanced lines are widely used in substrates where ground is not easily accessible. Examples include silicon substrates without vias, which are widely used for both mm-wave and microwave frequencies. 
   Prior art electronic switches in balanced lines are achieved in series  20  and shunt  30  configuration, as shown in FIG.  2  and  FIG. 3 , respectively. 
   In  FIG. 2 , the input lines  22  and  23  have, in series, diodes  24  and  25 , respectively. While diodes are depicted in this figure, in actual practice other devices that switch from a high impedance state (or blocking state) to a low impedance state (or transmitting state) may be used to perform the task. For example, the diodes could be replaced by a three terminal device, whose state is switched using one of the three terminals such as the base of a Bipolar Transistor, where the Emitter and Collector are the two ends of the switching device. In another configuration, the Emitter current is switched while the Base forms the input and the Collector the output. Considering  FIG. 2 , in the low impedance state when the diode is forward biased, the diodes  24  and  25  connect the input lines  22  and  23  to the output lines  26  and  27 , respectively. The signal is thus transmitted in high strength. The S-parameter for the forward transmission gain, S 21 , is high, being close to zero decibels (dB) S-parameters, or scattering parameters, are analogous to frequency response functions, but the terms are used at high and lower frequencies, respectively. In the other state the diodes are in the non-conducting state. In that state the signal is reflected back. Now the transmitted signal to the output lines  26  and  27  is attenuated and the S 21  transmission coefficient is low (−10&#39;s of dB), and is determined by the high impedance state. Since the high impedance is finite, a small amount of signal trickles through and is represented by δ 1 . 
     FIG. 3  shows a shunt mounted diode  30  in a balanced line for switch purposes. When the diode is reversed biased or is in the high impedance state, since it appears as open circuit between the lines, the signal is transmitted through or S 21  is high, i.e., close to 0 dB. In the other state, diode  33  is forward biased and is in the low impedance state. In this state, because the input balanced lines  31  and  32  are effectively shorted by the small impedance, the voltage induced at the input of the balanced line  34  and  35  is effectively small. This then has very little signal transmitted to the out balanced lines  34  and  35 . 
   In case of the series configuration  20 , the impedance in the high impedance state determines the isolation. Since the impedance is finite but high impedance, a signal always leaks to the output. At mm-wave, the impedance in the high conducting state is mostly capacitive and could greatly reduce the isolation (or the magnitude of minus S 21 , where S 21  is in dB). Similarly in the shunt configuration case the forward biased impedance or the low impedance state determines the isolation. Since the low impedance state has finite impedance (resistive at low frequency and reactive at mm-wave), the isolation is limited by this impedance. 
   SUMMARY 
   In an embodiment, a high isolation switch for a balanced line includes a switch connected in series between the input and output sections of each the two balanced line conductors and two switches cross connected between the input and output sections of the balanced line conductors. In an on-state, the series connected switches are in a low impedance state and the cross-connected switches are in a high impedance state. In an off-state, the series connected switches are in the high impedance state and the cross-connected switches are in the high impedance state, providing high isolation. The balanced line conductors and switches may be, e.g., diodes or bipolar junction transistors (BJTs), and may be integrated into a silicon substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a balanced line. 
       FIG. 2  shows a prior art implementation of a series switch in balanced configuration. 
       FIG. 3  shows a prior art implementation of a series switch in balanced configuration. 
       FIG. 4  shows a high isolation switch in balanced lines according to an implementation. 
       FIGS. 5A-5C  show a simplified diode equivalent circuit in the forward biased state (low impedance state) and the reversed bias state (high impedance state). 
       FIG. 6  shows simulated S 21  in the on-state for the series mounted configuration, shunt mounted configuration and the high isolation switch. 
       FIG. 7  shows simulated S 21  in off-state for the series mounted configuration, shunt mounted configuration, and the high isolation switch. 
       FIG. 8  shows simulated S 21  in the off-state for a configuration according to an implementation with the cross diodes up to 10% smaller than the series diode. 
       FIG. 9  shows an alternative implementation of the high-isolation switch using bipolar junction transistors (BJTs). 
   

   DETAILED DESCRIPTION 
     FIG. 4  shows a high isolation switch according to an implementation. Diodes  43  and  44  are series mounted diodes connecting the input balanced lines  41  to the output balanced line  47  and input balanced line  42  to the output balanced lines  48  respectively. In addition, a set of diodes  45  and  46  are cross mounted and biased in the high impedance state in both of the states of the switch. The diode  45  connects input balanced line  41  to output balanced line  48  and diode  46  connects input balanced line  42  to output balanced line  47  respectively. The cross connection is important for high isolation. 
   The switch in  FIG. 4  has two states. In the on-state, the diodes  43  and  44  are in a low impedance state while diodes  45  and  46  are in a high impedance state. In this state, the signal in the input balanced line is directly coupled to the output balanced through the low impedance states of  43  and  44 . 
   In the off-state, the diodes  43  and  44  are in a high impedance state while diodes  45  and  46  are also in a high impedance state. In this state, since the balanced lines have opposing voltages on line  41  and  42  as described in connection with  FIG. 1 , the opposing voltages couple to output lines  47 ,  48  due to the two-diode cross-connections. Thus on line  47  a small signal (say −δ 3 ) couples from diode  43  from the input line  41 , while an opposing small signal (say +δ 3 ) couples through diode  46  from the input line  42 . Since  41  and  42  are in a balanced configuration, the voltage on each is negative of other provided that the diodes  43 ,  44 ,  45 ,  46  have the same high impedance in the non-conducting or reversed biased state. Since the circuit is electrically symmetric, that is, line  47  couples same amount of voltage from both of the input lines  41  and  42 , exact cancellation occurs. As a result of this cancellation, isolation is theoretically infinite. 
   In real circuits there are number of reasons why the isolation degrades from the theoretical value. First of all, diodes are not the same due to process variance, nor is the bias exactly the same. This makes the off-state impedance different for the series and cross paths, thereby making the circuit asymmetric. Also, because of parasitic couplings, the isolation is limited by pad-to-pad and other couplings. 
     FIG. 5  shows a simplified equivalent circuit of a diode in the high impedance and the low impedance state. In the low impedance, or forward biased, state the diode can simply be represented by a forward bias resistance  51 . In the high impedance state, or the reverse biased state, the diode can simply be represented by a capacitor  52 . For example M/A-Com&#39;s diode MA4P165 (see http://www.macom.com/data/datasheet/pindiodeschip.pdf) has a forward bias resistance of less than 2.5-ohms at 10 mA forward bias and a capacitance of 0.05 pF at 10V reverse bias. 
     FIG. 6  shows a simulation of the switch in the on-state implement as shown in  FIGS. 2 ,  3 , and  4 . For the simulation of the series configuration shown in  FIG. 2 , the diodes  24  and  25  are replaced by 2.5-ohms. Similarly for the simulation of the shunt configuration in  FIG. 3 , the diode  33  is replaced by capacitance of 0.05 pF. Moreover for simulation of the high isolation switch in  FIG. 4 , diodes  43  and  44  are replaced by 2.5-ohm resistor to represent the forward state and diodes  45  and  46  are replaced by 0.05 pF capacitance to represent the reversed bias states, respectively. In  FIG. 6 ,  61  represents the insertion loss for the series configuration shown in  FIG. 2 ,  62  represents the insertion loss with shunt configuration shown in  FIG. 3 , while  63  represents the insertion loss with the configuration in FIG.  4 . At high frequency, the insertion loss of the series mounted diode is the best and the high isolation switch of  FIG. 4  is the worst. 
     FIG. 7  shows a simulation of the switches in  FIGS. 2 ,  3 , and  4 , respectively, in the off-state. For the simulation of the series configuration shown in  FIG. 2 , the diodes  24  and  25  are replaced by 0.05 pF, while for the simulation of the shunt configuration in  FIG. 3 , the diode  33  is replaced by resistance of 2.5-ohm, and finally for simulation of the switch in  FIG. 4 , the diodes  43  and  44  are replaced by 0.05 pF capacitors to represent the reverse bias state and the diodes  45  and  46  are replaced by 0.05 pF capacitance to represent the reversed bias states, respectively. Notice that diodes  45  and  46  are not switched between the on-state and the off-state. In  FIG. 7 , curve  71  represents the isolation with series mounted diode, curve  72  represents the isolation with shunt mounted diode, while curve  73  represents the isolation loss with the switch in FIG.  4 . At high frequency the isolation of the series mounted diode is the worst and the switch in  FIG. 4  is the best. Theoretically, if the diodes are exactly matched and the circuit is symmetric, the cancellation of the coupled signal to the output is infinite as shown in FIG.  7 . 
   This tremendous increase of isolation is the desired feature of this invention. Because of the increased isolation the switch can include a larger size diode, thereby reducing the insertion loss in the on-state of the switch. Often in a circuit the loss of the switch is not important. Through this new technique, extremely high isolation is possible in a very small space, is broadband and in a single stage. 
     FIG. 8  provides a tolerance analysis of the isolation when the cross diodes are up to 10% lower than the series diode in capacitance. Even with 10% variance, substantial improvement in isolation is achieved. To reduce the effect of variance, the diode can be batch (or single wafer) processes and made in quad pair. Since the diodes would be close to each other and have similar variance, this diode-to-diode variance would not effect the isolation and one can expect substantial improvement in isolation. 
     FIG. 9  shows an implementation of a high isolation switch circuit using a three terminal device. While bipolar junction transistor (BJT) is shown here, any other three or multi-terminal device is also usable. In the figure,  91  and  92  are the input balanced line,  93  and  94  are the series mounted transistors, and  96  and  95  are the cross-coupled transistors. The transistors  95  and  96  are biased through  99   b  and are always switched off, i.e., current through their collector is zero. The transistors  93  and  94  are biased through  99   a . In the off-state  93  and  94  are biased in the off-state similar to  95  and  96 , thereby the output signal at  97  and  98  are cancelled. 
   A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, blocks in the flowcharts may be skipped or performed out of order and still produce desirable results. Accordingly, other embodiments are within the scope of the following claims.