Transmission line switch

Abstract: The present invention is intended to prevent an unnecessary standing wave from being generated by simplifying a circuit in a transmission line switch to be used in a block-down converter for receiving, for example, satellite broadcasting. First and second amplifiers 1a and 2a which are adapted to be selectively switchable to an ON state to/from an OFF state are arranged in first and second input lines 1 and 2 and respectively provided with a connection line for connecting the first and second input lines 1 and 2 and a common output line 4. The connection line comprises a first transmission line la connected to an end part of the first input line 1, a second transmission line 1b connected to an end part of the second input line and a third transmission line 1c connected to an end part of the common output line. First to third transmission lines 3a to 3c are arranged in parallel to one another to transmit signals and form a DC block circuit adapted to shut down one another a DC bias to drive the amplifiers 1a and 2a.

Description: The present invention relates to a transmission line switch preferably adapted for use in a low-noise block-down converter for receiving, for example, satellite broadcasting waves. In some cases of current satellite broadcasting, different programs are transmitted with vertically polarized carriers and horizontally polarized carriers through a satellite. To enable selective reception of two orthogonal polarized signals, a transmission line switch is required for selecting one of two orthogonal polarized signals from a receiving horn of a microwave antenna. Accordingly, the low-noise block-down converter (LNB) is provided with the transmission line switch which controls the transmission lines so that only one of input signals of two input lines is amplified and transmitted to an output line and the rest of the input signals is not transmitted to the output line. In the accompanying drawings, Fig. 2 shows an example of a transmission line switch disclosed, for example, in Japanese Laid Open Patent H 2-63210 or EP-A-0 350 323 and to be used to fill the above-described requirement. In other words, the transmission line switch disclosed in this reference is adapted so that two orthogonal polarized signals from the receiving horn of the microwave antenna are supplied respectively through input lines 1 and 2 and one of these signals is supplied to a common output line 4 at a connection point 10. An amplifier 1a provided on the input line 1 preferably has an amplifier device Q1 formed with a high electron mobility transistor (HEMT) and matching circuits M1 and M1' which are respectively arranged in the front and rear stages of the amplifier device Q1. Similarly, an amplifier 2a provided on the input line 2 has an amplifier device Q2 formed with a high electron mobility transistor (HEMT) and matching circuits M2 and M2' which are respectively arranged in the front and rear stages of the amplifier device Q2. Bias lines 1b and 2b for respectively supplying a bias to the amplifiers 1a and 2a are connected to the output terminals of the amplifiers 1a and 2a. A DC block circuit 11a for making a DC potential (bias) stay in a transfer-blocked state is arranged between a connection point of the input line 1 with a bias line 1b and a connection point 10 of the input line 1 with the common output line 4. Similarly, a DC block circuit 11b for making a DC potential (bias) stay in a transfer-blocked state is arranged between a connection point of the input line 2 with a bias line 2b and a connection point 10 of the input line 2 with the common output line 4. Each of these DC block circuits 11a and 11b is generally formed with a capacity element or a connection line. In a configuration shown in Fig. 2, the amplifier 1a or 1b is selectively turned on by the DC bias supplied from the bias line 1b or 2b. This amplifier 1a or 1b transfers a signal with a gain exceeding 1 in an ON state. On the other hand, in a case that the amplifier 1a or 1b is set to OFF, a length L of the line between the amplifier 1a and the connection point 10 and a length L of the line between the amplifier 2a and the connection point 10 are selected so that the impedance as the amplifier in the OFF state is viewed from the connection point 10 is high. Accordingly, one of signals is transferred to the common output line 4 only through the amplifier remaining ON and the input signal in the amplifier remaining OFF is prevented from being transferred to the output line 4. However, in case of the above-described transmission line switch, DC block circuits 11a and 11b should be respectively inserted between bias lines 1b and 2b and the connection point 10 with the common output line 4 to make the bias to be supplied to the amplifier 1a or 2a independent. These DC block circuits 11a and 11b are formed with a capacity element or a connection line and therefore provision of independent DC block circuits 11a and 11b may cause the circuits to be complicated and an unexpected standing wave to occur and this is one of hindering factors to improvement of the block-down converter. SU-A-231 643 describes a transistorised three-way switch for switching an incoming UHF signal between two output lines. The centre incoming coaxial conductor is arranged parallel in a sector of quarter wavelength to two output lines each connected to a diode. Switchover is effected by changing the potential on the diodes. US-A-4 800 343 discloses a DC cutting circuit in which DC-free signal is transferred from an input line to an output line via two quarter wavelength resonators arranged parallel to one another. One object of the invention is to provide a transmission line switch which eliminates DC block circuits independently supplying a bias to respective amplifiers. A transmission line switch comprising: first and second input lines on which first and second amplifiers are being selectively switchable to ON and OFF states are respectively located and connection means for connecting said first and second input lines with a common output line, characterised in that said connection means comprises a connection line comprising a first transmission line having one end connected to an end of said first input line and the other end open, a second transmission line having one end connected to an end of said second input line and the other end open and a third transmission line having one end connected to an end of said common output line and the other end open, said first to third transmission lines being arranged spaced parallel to one another to block DC transfer therebetween, the length of each of the first to third transmission lines which are arranged parallel to one another to form said connection line being set to 1/4 of the wavelength of the desired transmission frequency, and wherein said first input line between said first amplifier and said connection line is connected to a first bias line for supplying a bias to said first amplifier, and a first band stop filter and a first termination circuit are connected to said first bias line, and said second input line between said second amplifier and said connection line is connected to a second bias line for supplying a bias to said second amplifier, and a second band stop filter and a second termination circuit are connected to said second bias line, the first and second band stop filters and termination circuits reduce the reflection coefficient of the switch caused by the effect of a standing wave between whichever amplifier is in. the OFF state and the corresponding open end of the transmission line. Accordingly, it is possible to provide the transmission line switch in which the circuit configuration is simplified and an unnecessary standing wave will not be caused. Further, connection and non-connection between the transmission lines is controllable based on ON/OFF operation of the amplifiers provided in the first input line and the second input line and therefore a gain loss on the transmission line can be reduced. Because the length of the first to third transmission lines which are arranged in parallel to one another and form the connection line is set to 1/4 wavelength a satisfactory 1/4 wavelength coupler is formed between the transmission lines at the amplifier held ON. In this case, the connection line has the characteristics of a band pass filter, thereby, signals except the desired band go to the termination circuit. Also because a band stop filter and a termination circuit are inserted into the bias lines for supplying the bias to the respective amplifiers, the load condition of the output side of the amplifier held ON is controllable by appropriately selecting the filtering characteristics of the band stop filter and consequently the band of the amplifier can be widened. The invention will be further described by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a block diagram showing an embodiment of a transmission line switch of the present invention. Fig. 2 is a block diagram showing an example of a conventional transmission line switch. A transmission line switch according to the present invention is described in detail below referring to an embodiment shown in the drawings. Fig. 1 is a block diagram showing a first embodiment to which the present invention applies to a transmission line switch to be used in a block-down converter for receiving satellite broadcasting. An output side of an amplifier 1a of a first input line 1 is connected with a first transmission line 3a forming a connection line 3 at a connection point A. Similarly, an output side of an amplifier 2a of a second input line 2 is connected with a first transmission line 3b forming a connection line 3 at a connection point E. In addition, a common output line 4 is connected with a third transmission line 3c forming a connection line 3 at a connection point D. The connection line 3 is arranged at the center of the third transmission line 3c and the first transmission line 3a and the second transmission line 3b are arranged in parallel at both external sides of the connection line 3 with the DC potentials thereof held in a transfer-blocked state (DC blocked state) to one another. The length of the first to third transmission lines 3a to 3c is determined to be a 1/4 wavelength, and the other end part opposing to an end part to which the input line 1 is connected in the first transmission line 3a is formed as an open end B, the other end part opposing to an end part to which the input line 2 is connected in the second transmission line 3b is formed as an open end F, and the other end part opposing to an end part to which a common output line 4 is connected in the third transmission line 3c is formed as an open end C. On the other hand, a bias line 1b is connected between the output side of the amplifier 1a of the first input line 1 and a connection line 3, and a band stop filter 1c and a termination circuit 1d are inserted into the bias line 1b and the other end part thereof is shorted to a reference potential point. Similarly, a bias line 2b is connected between the output side of the amplifier 2a of the second input line 2 and the connection line 3, and the band stop filter 2c and the termination circuit 2d are inserted into the bias line 2b and the other end part thereof is shorted to a reference potential point. The band stop filters 1c and 2c are intended to prevent a signal of a desired frequency from being transmitted to the bias line while the termination circuits 1d and 2d respectively comprise a resistance element and a capacity element for blocking a DC voltage. In a configuration shown in Fig. 1, a DC bias is selectively applied to the input line 1 or 2 through the bias lines 1b and 2b to make the transmission line switch perform a switching operation. For example, if the bias is applied to the input line 1 through the bias line 1b, the bias is applied to the first amplifier 1a and the amplifier 1a is turned on. The amplifier 1a transfers the signal with a carrier of a 10 GHz band arriving through a waveguide to the connection line 3 with a gain exceeding 1. On the other hand, the first to third transmission lines 3a to 3c are arranged in parallel to maintain the DC block state in the connection line 3 and therefore the supply of the bias to the other amplifier 2a is shut off and the second amplifier 2a is held OFF. In this case, an amplitude of a reflection coefficient when the amplifier 2a held in the OFF state is viewed from the output side is approximate to 1. Accordingly, an impedance as the amplifier 2a in the OFF state is viewed from the connection point E can be high by appropriately selecting the length L of the input line 2 between the amplifier 2a and the connection line 3. The length of the second transmission line 3b which forms the connection line 3 is approximately 1/4 of the wavelength of a desired frequency and the other end part is formed as the open end F and therefore the transmission line 3b is equivalent to a 1/4 wavelength line with both open ends in terms of the desired frequency. The other end part of the third transmission line 3c is formed as the open end C and therefore the second transmission line 3b and the third transmission line 3c are not connected with the desired frequency. In other words, the input line 2 held in the OFF state cannot be viewed with the desired frequency from the common output line 4. Accordingly, the signal on the input line 2 held OFF is not transmitted to the common output line 4. On the other hand, since the amplitude of the reflection coefficient when the amplifier 1a held ON is viewed from the output side is generally small, the first transmission line 3a and the third transmission line 3c operate with the desired frequency as a satisfactory 1/4 wavelength coupler. Accordingly, the signal on the input line 1 held ON is transmitted to the common output line 4. Though the transmission line switch of this configuration satisfactorily performs the switching operation with the desired frequency as described above, there is a frequency with which the amplitude of the reflection coefficient is increased by the effect of the standing wave between the amplifier 2a held off and the open end F of the second transmission line 3b with a frequency other than the desired frequency when the connection line 3 is viewed from the ON side amplifier 1a. Since a stability of an amplifying element deteriorates at such frequency with which the reflection coefficient is increased, it is difficult to form a wide band amplifier. A band of the band stop filter 1c on the bias line for preventing the signal of the desired frequency from being transmitted to the bias line 1b is selected so that the band is set to be wider than required. The impedance as the bias line 1b is viewed from the amplifier 1a held in the ON state is arranged so that the signal from the input line 1 goes through the bias line 1b to the termination circuit 1d except the desired band. Thereby, a load condition of the output side of the amplifier 1a held ON is improved and the band of the amplifier 1a is widened. The above describes the operation in a case that the first amplifier 1a is held ON and the second amplifier is held OFF. However, it is easily understood that, if the second amplifier 2a is held ON and the first amplifier 1a is held OFF, the input port is changed over and the same effect as the above is obtained.

Claims: A transmission line switch comprising: first and second input lines (1, 2) on which first and second amplifiers (1a, 2a) are being selectively switchable to ON and OFF states are respectively located and connection means (3) for connecting said first and second input lines with a common output line (4), characterised in that said connection means (3) comprises a connection line (3) comprising a first transmission line (3a) having one end connected to an end of said first input line (1) and the other end open, a second transmission line (3b) having one end connected to an end of said second input line (2) and the other end open and a third transmission line (3c) having one end connected to an end of said common output line (4) and the other end open, said first to third transmission lines (3a, 3b, 3c) being arranged spaced parallel to one another to block DC transfer therebetween, the length of each of the first to third transmission lines (3a, 3b, 3c) which are arranged parallel to one another to form said connection line (3) being set to 1/4 of the wavelength of the desired transmission frequency, and wherein said first input line (1) between said first amplifier (1a) and said connection line (3) is connected to a first bias line (1b) for supplying a bias to said first amplifier (1a), and a first band stop filter (1c) and a first termination circuit (1d) are connected to said first bias line (1b), and said second input line (2) between said second amplifier (2a) and said connection line (3) is connected to a second bias line (2b) for supplying a bias to said second amplifier (2a), and a second band stop filter (2c) and a second termination circuit (2d) are connected to said second bias line (2b), the first and second band stop filters (1c, 2c) and termination circuits (1d, 2d) reduce the reflection coefficient of the switch caused by the effect of a standing wave between whichever amplifier (1a, 2a) is in the OFF state and the corresponding open end of the transmission line (3a, 3b).

Inventors: KAWASAKI KENICHI, KAWASAKI, KENICHI, Kawasaki, Kenichi, c/o Sony Corp.
Applicants: SONY CORP, SONY CORPORATION
Application year: 1995
Main classification: H01P1
IPC codes: H01P5, H03F3, H01P1