Radio with duplexer bypass capability

Abstract

Systems and techniques are disclosed relating to a radio with duplexer bypass capability. The radio includes an antenna, a duplexer, a receiver; and a switching circuit configured to switch the antenna between the duplexer and the receiver. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or the meaning of the claims.

Description

BACKGROUND

[0001] 1. Field

[0002] The present invention relates generally to communications systems, and more specifically, to a radio with duplexer bypass capability.

[0003] 2. Background

[0004] Radio refers to a system of communications employing electromagnetic waves propagated through free space. Radio is commonly used as a public medium to send commercial broadcasts from a radio transmitter to anyone with a radio receiver. However, radio can also be used as a private medium. Cellular and cordless telephones are common examples of radio transceivers, which are devices that can both transmit and receive signals.

[0005] Many radio transceivers are designed for full duplex operation. Full duplex refers to the transmission of signals in two directions simultaneously. By way of example, cellular and cordless telephones are typically a full duplex device because both users can speak at once. In contrast, a walkie-talkie is a half duplex device because only one user can transmit at a time. Full duplex operation is generally supported with a duplexer connecting both the receiver and transmitter to a single antenna. The problem with this approach is that the duplexer introduces loss into the signal directed to the receiver. This loss tends to reduce the noise figure performance of the receiver, and as a result, limits the data rate of the signal that can be detected by the receiver. Accordingly, there is a need in the art for a methodology which allows full duplex operation without significantly degrading the noise figure of the receiver.

SUMMARY

[0006] In one aspect of the present invention, an apparatus includes an antenna, a duplexer, a receiver, and a switching circuit configured to switch the antenna between the duplexer and the receiver.

[0007] In another aspect of the present invention, an apparatus includes an antenna, a duplexer, a receiver, and means for switching the antenna between the duplexer and the receiver.

[0008] In yet another aspect of the present invention, a method of communications includes coupling an antenna to a transmitter and receiver through a duplexer in a first mode, and bypassing the duplexer by coupling the antenna to the receiver in a second mode.

[0009] In a further aspect of the present invention, an apparatus includes a processor, and a radio having an antenna, a duplexer, a receiver, and a switching circuit configured to switch the antenna between the duplexer and the receiver under control of the processor.

[0010] It is understood that other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only exemplary embodiments of the invention by way of illustration. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Aspects of the present invention are illustrated by way of example, and not by way of limitation, in the accompanying drawings, wherein:

[0012] FIG. 1 is a functional block diagram of an exemplary radio transceiver with duplexer bypass capability;

[0013] FIG. 2 is a functional block diagram of an alternative exemplary radio transceiver with duplexer bypass capability; and

[0014] FIG. 3 is a functional block diagram of an exemplary communications device employing a radio transceiver with duplexer bypass capability in combination with a processor.

DETAILED DESCRIPTION

[0015] The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of the present invention and is not intended to represent the only embodiments in which the present invention can be practiced. The term “exemplary” used throughout this description means “serving as an example, instance, or illustration,” and should not necessarily be construed as preferred or advantageous over other embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form in order to avoid obscuring the concepts of the present invention.

[0016] An exemplary full duplex radio transceiver includes a transmitter and receiver coupled to a single antenna through a duplexer. The duplexer prevents transmitter leakage from desensitizing or damaging the receiver while at the same time ensures weak signals received by the antenna are directed to the receiver. A switching circuit can be used to bypass the duplexer by connecting the antenna directly to the receiver when the transmitter is inactive. This approach tends to improve the noise figure performance of the receiver by avoiding losses that might otherwise be introduced into the signal by the duplexer.

[0017] A full duplex radio transceiver with duplexer bypass capability can be implemented in a variety of fashions depending on the overall design constraints of any particular application. FIG. 1 is a functional block diagram of one exemplary embodiment. A switching circuit comprising a pair of SPDT (single-pole-double-throw) switches 102a and 102b can be used to switch between a full duplex mode and a receive only mode. In the full duplex mode, an antenna 104 can be coupled to an antenna port at the front end of a duplexer 106 through a first switch 102a. The back end of the duplexer 106 has a transmit port which can be coupled directly to a transmitter 108, and a receive port which can be coupled to a receiver 110 through a second switch 102b. The switches 102a and 102b can be any switches known in the art. By way of example, high intercept point microwave switches with good linearity can be used to reduce out of band emissions during high power transmissions. In the receive only mode, the switches 102a and 102b can be used to bypass the duplexer 106 by switching the antenna 104 around the duplexer 106 to the input of the receiver 110.

[0018] FIG. 2 is a functional block diagram of an alternative embodiment of the full duplex transceiver with duplexer bypass capability. In this example, a switching circuit is implemented with three SPST (single-pole-single-throw) switches 202a-c in conjunction with four one-quarter wavelength transmission lines 204a-d. With this approach, the switches are removed from the signal path. As a result, the signal is not impacted by the loss of each switch. In addition, any non-linearity in the switches does not impact performance. Referring to FIG. 2, a first transmission line 204a can be disposed between the antenna 104 and the antenna port of the duplexer 106. A second transmission line 204b can be disposed between the receive port of the duplexer 106 and the receiver 110. A first switch 202a can be disposed between the antenna port of the duplexer 106 and ground, and a second switch 202b can be disposed between the receive port of the duplexer 106 and ground. A bypass path between the antenna 104 and the receiver 110 can be created with the remaining two transmission lines 204c and 204d. A third switch 202c to ground can be connected between the two transmission lines 204c and 204d.

[0019] In the full duplex mode, the first and second switches 202a and 202b are opened and the third switch 202c is closed. With the first and second switches 202a and 202b opened, both the transmitter 108 and receiver 110 are coupled to the antenna 104 through the duplexer 106. By closing the third switch 202c, the bypass path is disabled.

[0020] In the receive only mode, the first and second switches 102a and 102b are closed and the third switch is open. As a result of the first switch 202a being closed, the antenna port is shorted to ground disconnecting the antenna 104 from the duplexer 106. By closing the second switch 202b, the receiver port is shorted to ground disconnecting the receiver 110 from the duplexer 106. With the third switch 202c opened, the antenna 104 is coupled directly to the input of the receiver 110 through the transmission lines 204c and 204d. The transmission lines can be any transmission lines known in the art. By way of example, guided mode transmission lines can be used to minimize signal loss. The losses associated with the transmission lines 204c and 204d in the bypass path should be less than the loss introduced by the duplexer 106 between the antenna and receive ports to obtain any improvement in the noise figure performance of the receiver.

[0021] The concept of a full duplex transceiver with duplexer bypass capability has unlimited applications. The transceiver can be combined with a processor to support virtually any radio application requiring full duplex capability. By way of example, in wireless telephone applications supporting voice communications, the full duplex mode can be enabled only when the user's voice is being transmitted. This approach ensures that the wireless telephone can always receive voice communications whether the user is speaking or not. The same principle can be applied to full duplex data communications such as modem or fax applications. Specifically, the full duplex mode can be enabled when data is being transmitted, and the receive only mode can be enabled when no data is being transmitted. This concept can be extended to applications supporting both voice and data. The transceiver can be configured to operate in the full duplex mode for both voice and data calls, or can be adaptively switched to the receive only mode when no voice or data is be transmitted.

[0022] An exemplary communications device employing this technology is shown in the functional block diagram of FIG. 3. The communications device includes a radio transceiver 302 coupled to a processor 304. The transceiver 302 includes a switching circuit to switch the antenna 104 between the duplexer 106 and the receiver 110. The processor includes a bypass controller 306 which can be used to cause the switching circuit to switch the transceiver 302 between the full duplex mode and the receive only mode. For the purposes of explanation, the transceiver will be described with a switching circuit utilizing a pair of SPDT switches as shown in FIG. 1. However, as those skilled in the art will readily appreciate, the switching circuit can be implemented with any arrangement of switches, transmission lines, or other components.

[0023] In the full duplex mode, the bypass controller 306 connects the antenna 104 to the antenna port of the duplexer 106 through the first switch 102a, and the receiver 110 to the receive port of the duplexer 106 through the second switch 102b. In the receive only mode, the bypass controller 306 switches the antenna 104 around the duplexer 106 to the input of the receiver through the switches 102a and 102b.

[0024] In wireless telephone applications, a voice activity detector (VAD) 308 can be used to determine whether the user is speaking. If the VAD 308 detects speech, a flag 308a can be set causing the bypass controller 306 to set the transceiver 302 into the full duplex mode when the transmitter 108 is transmitting voice. In this mode, the speech from the user is digitized and provided to a transmitter processing element 310. The transmitter processing element 310 can be configured to support one or more voice compression algorithms known in the art. The compressed voice samples can then be encoded to provide forward error correction capability and modulated using QPSK (Quadrature Phase Shift Keying), 8-PSK, 16-QAM (Quadrature Amplitude Modulation), or any other modulation scheme known in the art. The modulated voice samples can then be provided to the transmitter 108 in the transceiver 302 for upconversion, filtering and amplification before being provided to the antenna 104 through the duplexer 106.

[0025] In the full duplex mode, it is possible that both users may be speaking at once. In that case, the signal received by the antenna 104 can be coupled to the receiver 110 through the duplexer 106. The receiver 110 amplifies, filters and downconverts the signal to baseband. The baseband signal can then be provided to a receiver processing element 312 where it can be digitized, demodulated, decoded and decompressed.

[0026] Conversely, when the VAD 308 detects silent periods, the flag 308a can be cleared causing the bypass controller 306 to set the transceiver 302 into the receive only mode. In the receive only mode, the signal from the antenna 104 will be directed around the duplexer 106 to the input of the receiver 110. Normally, the transmitter 108 will not be delivering power from its output in the receive only mode. The transmitter 108 may be disabled during the receive only mode with a control signal 306a generated by the bypass controller 306, or alternatively, may rely on an isolator (not shown) to protect its circuitry.

[0027] When the duplexer 106 is bypassed, the receiver loses any signal rejection capability provided by the duplexer 106. This loss could impact the overall performance of the receiver 110 operating in the presence of one or more jammers. A filter (not shown) can be inserted in the bypass path to increase the signal rejection capability of the receiver 110 in the receive only mode. However, a filter will introduce loss thereby reducing the benefits of the duplexer bypass. If the receiver 110 has a received signal strength indicator (RSSI), a RSSI signal 110a can be used by the bypass controller 306 to determine if the performance of the transceiver is better with the duplexer bypassed. The antenna 104 can then be adaptively switched between the duplexer 106b and the receiver 110 to optimize performance.

[0028] The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

[0029] The methods or algorithms described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal.

[0030] The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An apparatus, comprising: an antenna; a duplexer; a receiver; and a switching circuit configured to switch the antenna between the duplexer and the receiver.

2. The apparatus of claim 1 further comprising a transmitter coupled to the duplexer.

3. The apparatus of claim 2 wherein the transmitter is disabled when the when the duplexer is switched to the receiver.

4. The apparatus of claim 1 wherein the switching circuit is further configured to couple the receiver to the duplexer when the antenna is switched to the duplexer, and to disconnect the receiver from the duplexer when the antenna is switched to the receiver.

5. The apparatus of claim 4 wherein the switching circuit comprises a first switch configured to switch the antenna between the duplexer and the receiver, and a second switch configured to couple the receiver to the duplexer when the antenna is switched to the duplexer, and to disconnect the receiver from the duplexer when the antenna is switched to the receiver.

6. The apparatus of claim 4 wherein the switching circuit comprises a first switch having a pole coupled to the antenna, a first contact coupled to the duplexer, and a second contact, and a second switch having a pole coupled to the receiver, a first contact coupled to the duplexer and a second contact coupled to the second contact of the switch.

7. The apparatus of claim 4 wherein the duplexer comprises an antenna port coupled to the antenna, and wherein the switching circuit comprises an antenna switch between the antenna port and ground.

8. The apparatus of claim 7 wherein the antenna switch is open to switch the antenna to the duplexer.

9. The apparatus of claim 4 wherein the duplexer comprises a receiver port coupled to the receiver, and wherein the switching circuit comprises a receiver switch between the receiver port and ground.

10. The apparatus of claim 9 wherein the receiver switch is open to couple the receiver to the duplexer, and closed to disconnect the receiver from the duplexer.

11. The apparatus of claim 4 further comprising a bypass path between the receiver and the antenna, and wherein the switching circuit comprises a bypass switch between the bypass path and ground.

12. The apparatus of claim 11 wherein the bypass switch is open to switch the antenna to the receiver.

13. The apparatus of claim 4 further comprising a bypass path between the antenna and the receiver, wherein the duplexer comprises an antenna port coupled to the antenna and a receiver port coupled to the receiver, and wherein the switching circuit comprises a bypass switch between the bypass path and ground, an antenna switch between the antenna port and the ground, and a receiver switch between the receiver port and the ground.

14. The apparatus of claim 13 wherein the antenna and receiver switches are opened and the bypass switch is closed to switch the antenna to the duplexer, and the antenna and receiver switches are closed and the bypass switch is opened to switch the antenna to the receiver.

15. The apparatus of claim 13 wherein the switching circuit further comprises a first transmission line between the antenna and the antenna port, a second transmission line between the receiver and the receiver port, a third transmission in the bypass path having a first end coupled to the antenna and a second end, and a fourth transmission line in the bypass path having a first end coupled to the receiver and a second end coupled to the second end of the third transmission line, and wherein the bypass switch is connected between the second ends of the third and fourth transmission lines and the ground.

16. An apparatus, comprising: an antenna; a duplexer; a receiver; and means for switching the antenna between the duplexer and the receiver.

17. The apparatus of claim 16 further comprising a transmitter coupled to the duplexer.

18. The apparatus of claim 17 further comprising means for disabling the transmitter when the when the duplexer is switched to the receiver.

19. The apparatus of claim 16 wherein the means for switching the antenna between the duplexer and the receiver includes coupling the receiver to the duplexer when the antenna is switched to the duplexer, and disconnecting the receiver from the duplexer when the antenna is switched to the receiver.

20. The apparatus of claim 19 wherein the means for switching the antenna between the duplexer and the receiver comprises a first switch configured to switch the antenna between the duplexer and the receiver, and a second switch configured to couple the receiver to the duplexer when the antenna is switched to the duplexer and to disconnect the receiver from the duplexer when the antenna is switched to the receiver.

21. The apparatus of claim 19 wherein the means for switching the antenna between the duplexer and the receiver comprises a first switch having a pole coupled to the antenna, a first contact coupled to the duplexer, and a second contact, and a second switch having a pole coupled to the receiver, a first contact coupled to the duplexer and a second contact coupled to the second contact of the switch.

22. The apparatus of claim 19 wherein the duplexer comprises an antenna port coupled to the antenna, and wherein the means for switching the antenna between the duplexer and the receiver comprises means to ground the antenna port to switch the antenna to the receiver.

23. The apparatus of claim 22 wherein the means to ground the antenna port comprises a switch.

24. The apparatus of claim 19 wherein the duplexer comprises a receiver port coupled to the receiver, and wherein the means for switching the antenna between the duplexer and the receiver comprises means for grounding the receiver port to disconnect the receiver from the duplexer.

25. The apparatus of claim 24 wherein the means for grounding the receiver port comprises a switch.

26. The apparatus of claim 19 further comprising a bypass path between the receiver and the antenna, and wherein the means for switching the antenna between the duplexer and the receiver comprises means for grounding the bypass path to ground to switch the antenna to the duplexer.

27. The apparatus of claim 26 wherein the means for grounding the bypass path comprises a switch.

28. The apparatus of claim 19 further comprising a bypass path between the antenna and the receiver, wherein the duplexer comprises an antenna port coupled to the antenna and a receiver port coupled to the receiver, and wherein the means for switching the antenna between the duplexer and the receiver comprises a bypass switch between the bypass path and ground, an antenna switch between the antenna port and the ground, and a receiver switch between the receiver port and the ground.

29. The apparatus of claim 28 wherein the antenna and receiver switches are opened and the bypass switch is closed to switch the antenna to the duplexer, and the antenna and receiver switches are closed and the bypass switch is opened to switch the antenna to the receiver.

30. The apparatus of claim 28 wherein the means for switching the antenna between the duplexer and the receiver comprises a first transmission line between the antenna and the antenna port, a second transmission line between the receiver and the receiver port, a third transmission in the bypass path having a first end coupled to the antenna and a second end, and a fourth transmission line in the bypass path having a first end coupled to the receiver and a second end coupled to the second end of the third transmission line, and wherein the bypass switch is connected between the second ends of the third and fourth transmission lines and the ground.

31. The apparatus of claim 19 further comprising a bypass path between the antenna and the receiver, wherein the duplexer comprises an antenna port coupled to the antenna and a receiver port coupled to the receiver, and wherein the means for switching the antenna between the duplexer and the receiver comprises means for grounding the bypass path to switch the antenna to the duplexer and means for grounding the antenna and receiver ports to switch the antenna to the receiver.

32. A method of communications, comprising: coupling an antenna to a transmitter and receiver through a duplexer in a first mode; and bypassing the duplexer by coupling the antenna to the receiver in a second mode.

33. The method of claim 32 wherein the first mode comprises a full duplex mode and the second mode comprises a receive only mode.

34. The method of claim 32 further comprising coupling the transmitter to the duplexer in both the first and the second modes.

35. The method of claim 34 further comprising disabling the transmitter in the second mode.

36. The method of claim 32 wherein the bypassing of the duplexer further comprises disconnecting the receiver from the duplexer.

37. The method of claim 36 wherein the coupling of the antenna to the transmitter and the receiver through the duplexer comprises switching a first switch to couple the antenna to the duplexer, and switching a second switch to couple the duplexer to the receiver, and wherein the bypassing of the duplexer further comprises switching the first switch to disconnect the antenna from the duplexer, and switching the second switch to disconnect the duplexer from the receiver, the antenna being coupled to the receiver through the first and second switches when the duplexer is bypassed.

38. The method of claim 36 wherein the duplexer comprises an antenna port coupled to the antenna, and wherein the bypassing of the duplexer further comprises grounding the antenna port.

39. The method of claim 36 wherein the duplexer comprises a receiver port coupled to the receiver, and wherein the bypassing of duplexer further comprises grounding the receiver port.

40. The method of claim 36 wherein the coupling of the antenna to the transmitter and the receiver through the duplexer comprises grounding a bypass path between the receiver and the antenna.

41. The method of claim 36 wherein the duplexer comprises an antenna port coupled to the antenna and a receiver port coupled to the receiver, and wherein the coupling of the antenna to the transmitter and the receiver through the duplexer comprises grounding a bypass path between the receiver and the antenna, and wherein the bypassing of the duplexer further comprises grounding the antenna and receiver ports.

42. The method of claim 41 wherein the bypassing of the duplexer further comprises isolating the grounded antenna port from the antenna through a first transmission line and isolating the grounded receiver port from the receiver through a second transmission line, and wherein the coupling of the antenna to the transmitter and the receiver through the duplexer further comprises isolating the grounded bypass path from the antenna through a third transmission line and isolating the grounded bypass path from the receiver through a fourth transmission line

43. An apparatus, comprising: a processor; and a radio having an antenna, a duplexer; a receiver, and a switching circuit configured to switch the antenna between the duplexer and the receiver under control of the processor.

44. The apparatus of claim 43 wherein the radio further comprises a transmitter coupled to the duplexer.

45. The apparatus of claim 44 wherein the processor is further configured to disable the transmitter when the duplexer is switched to the receiver.

46. The apparatus of claim 44 configured to detect whether a signal is being provided to the transmitter, the switching circuit further being configured to switch the antenna to the duplexer when the processor detects the signal, and to switch the antenna to the receiver when the processor does not detect the signal.

47. The apparatus of claim 43 wherein the switching circuit, under the control of the processor, is further configured to couple the receiver to the duplexer when the antenna is switched to the duplexer, and to disconnect the receiver from the duplexer when the antenna is switched to the receiver.

48. The apparatus of claim 47 wherein the receiver is configured to provide a signal to the processor when the antenna is switched to the receiver, the processor further being configured to control the switching circuit in response to the signal.

49. The apparatus of claim 48 wherein the signal comprises a received signal strength indication.

50. The apparatus of claim 47 wherein the switching circuit comprises a first switch responsive to the processor to switch the antenna between the duplexer and the receiver under the control of the processor, and a second switch responsive to the processor to couple the receiver to the duplexer when the antenna is switched to the duplexer, and to disconnect the receiver from the duplexer when the antenna is switched to the receiver.

51. The apparatus of claim 47 wherein the switching circuit comprises a first switch having a pole coupled to the antenna, a first contact coupled to the duplexer, and a second contact, and a second switch having a pole coupled to the receiver, a first contact coupled to the duplexer and a second contact coupled to the second contact of the switch.

52. The apparatus of claim 47 wherein the duplexer comprises an antenna port coupled to the antenna, and wherein the switching circuit comprises an antenna switch between the antenna port and ground.

53. The apparatus of claim 52 wherein the antenna switch is opened by the processor to switch the antenna to the duplexer.

54. The apparatus of claim 47 wherein the duplexer comprises a receiver port coupled to the receiver, and wherein the switching circuit comprises a receiver switch between the receiver port and ground.

55. The apparatus of claim 54 wherein the receiver switch is opened by the processor to couple the receiver to the duplexer, and closed by the processor to disconnect the receiver from the duplexer.

56. The apparatus of claim 47 wherein the radio further comprises a bypass path between the receiver and the antenna, and wherein the switching circuit comprises a bypass switch between the bypass path and ground.

57. The apparatus of claim 56 wherein the bypass switch is opened by the processor to switch the antenna to the receiver.

58. The apparatus of claim 47 wherein the radio further comprises a bypass path between the antenna and the receiver, and wherein the duplexer comprises an antenna port coupled to the antenna and a receiver port coupled to the receiver, and wherein the switching circuit comprises a bypass switch between the bypass path and ground, an antenna switch between the antenna port and the ground, and a receiver switch between the receiver port and the ground.

59. The apparatus of claim 58 wherein the antenna and receiver switches are opened and the bypass switch is closed by the processor to switch the antenna to the duplexer, and the antenna and receiver switches are closed and the bypass switch is opened by the processor to switch the antenna to the receiver.

60. The apparatus of claim 58 wherein the switching circuit further comprises a first transmission line between the antenna and the antenna port, a second transmission line between the receiver and the receiver port, a third transmission in the bypass path having a first end coupled to the antenna and a second end, and a fourth transmission line in the bypass path having a first end coupled to the receiver and a second end coupled to the second end of the third transmission line, and wherein the bypass switch is connected between the second ends of the third and fourth transmission lines and the ground.