Transmission line switch

Abstract: A transmission line switch allows a single output line (3)to be switched to one of two or more input lines (1,2) to which it is permanently connected at a common junction (4). Each input line (1,2) has an associated amplifier stage (10) which can be biased in a normal high gain ('on') state, or in an isolation ('off') state. Suitable biasing in the 'off' state ensures that the amplifier stage output presents a low impedance to its own input line, the length (L) of which is chosen to reflect a high impedance at the junction (4) with the other lines. Correct design enables the return loss and insertion loss of the 'on' path to be kept to low values whilst simultaneously offering a high insertion loss to the 'off' path signals.

Description: This invention relates to a transmission line switch and, in particular, to a switch for the transmission with gain of a signal from one of a plurality of input lines connected to a common output line. For many years the PIN diode has dominated as the control element in switches for microwave circuits for example, as shown in US Patent No.4595890. More recently attention has focussed on the use of single- and dual-gate FETs in the design of fast switches. Configurations using FETs include series and shunt mounted arrangements, both relying on the drain-source resistance of the device in the 'on' state as a low impedance path, either for transmission of the signal (series configuration), or as a shunt across the line (shunt configuration). An example of series FET switches is shown in US Patent No.3681699. Combinations of series and shunt mounted devices are also known, which further improve isolation in the 'off' state of the switch. All these arrangements provide a broadband (untuned) response. The insertion loss in the 'on' state can be reduced somewhat by the addition of appropriate tuning components. However, a diode or FET used as a switch in this way causes a degree of signal attenuation, a loss which adds to the noise figure of the overall system in which it is a part. Also, none of these configurations makes use of the amplifying capabilities of the FET device. In applications where a transmission line switch is at the front end of a microwave receiving system, where, for instance, the switch may be required to select one of two DBS broadcast signals having different polarisations, the performance of the switch in terms of noise figure, frequency response and isolation will have a profound effect on the quality of the signal available to the rest of the system. In such a case, the use of a FET device as a switch providing gain has the significant advantage that the noise figure of the switch, which, being at the front end of the receiving system, is the most significant stage in terms of noise performance, is substantially that of the amplifying circuit. One transmission line switch arrangement is described in "A Broadband Hybrid Active Switchable Combiner" (15th European Microwave Conference Proceedings, 9-13th September 1985, Paris, Pages 629-634, Levy et al). This article discloses a transmission line switch arrangement in which a plurality of input lines are connected to a common output line at a junction, each input line comprising respective FET amplifying means operable in an 'on' state to transmit with gain exceeding unity, and in an 'off' state to transmit substantially zero signal. It is an object of the present invention to provide a transmission line switch arrangement of this kind but having an improved noise performance and reduced cost. According to the invention, in a transmission line switch arrangement of the above kind, each amplifying means consists of one single-gate FET device connected in an amplifying configuration, each input line being connected to the gate electrode of a respective FET device and each drain electrode being connected to the junction by a length of transmission line, the length being such, in conjunction with the output impedance of the FET device in its 'off' state, that a high impedance is presented at the junction by the 'off' FET device. The Levy approach to reducing the effect of the noise contributed by the junction is to increase the signal gain ahead of the junction by providing additional amplifying stages in each input line. Levy employs two amplifying stages connected in cascade in each input line. Providing such additional gain ahead of the junction also mitigates somewhat the junction loss, since, if the 'on' path gain is high enough, loss of the wanted signal into the 'off' path can be tolerated. However, Levy's use of additional amplifying stages ahead of the junction clearly imposes a substantial cost penalty, with duplication of components in each input line. It should also be noted that Levy makes the most direct connection between the amplifiers and junction that the printed circuit will allow - see Figure 5. She does not therefore employ any transmission line between amplifier and junction. A further feature of the Levy design is the exclusive use of dual-gate FETS as the amplifying devices. While the use of dual-gate devices was indicated by the requirement for dual functions - amplification and switching - it did not achieve the simplicity and noise performance that is characteristic of the present invention employing one single-gate FET as the combined amplifying and switching means. The approach to the junction loss problem provided by the present invention is very different from that of Levy. Rather than accepting junction loss and making up for it by the costly provision of additional input line amplification, the invention seeks to minimise the junction loss and so reduce the contribution of the junction noise to the overall switch noise figure. The junction loss is minimised by controlling the impedance presented at the junction by the 'off' path. This is done by incorporating a length of transmission line between the amplifier output and the junction which is such as to transform the 'off' amplifier output impedance to a high impedance at the junction. In this way, the 'on' input line and the output line effectively form a continuous low-loss transmission line path for the wanted signal. Added amplifying stages are not therefore required to overcome junction losses, so providing significant cost savings. The invention thus combines minimum essential amplification and minimum junction loss. An alternative approach to the basic problem of switching microwave input lines to a common output line was proposed by T.M. Percival in a paper entitled "A Matched Active Microwave Switch" in the Journal of Electrical and Electronics Engineering of Australia, I.E. Aust. and IREE Aust. Volume 4 No.3 September 1984. This paper described an improved design for selecting polarisations from a radiotelescope feed. The known arrangement at that time was to select the polarisation immediately following the feed and then use a single parametric amplifier in the common output. Percival proposed using a (cheaper) GASFET amplifier in each input to a matched switching arrangement. There is no realisation in this article that the GASFET amplifiers could be incorporated with the switching arrangement in any way. Having stated that GASFET amplifier's are used, the article is then devoted almost exclusively to the design of the switching arrangement employing PIN-diodes and Schottky diodes. No information is given about the amplifier other than that it is based on GASFETs. There is no disclosure of the amplifier consisting of a single-gate FET and no suggestion that the amplifier could itself be employed as the switching means. The resulting complexity of the Percival design inherently detracts from the noise performance and increases the cost of components and assembly very considerably. In one design according to the present invention, the output impedance of the amplifying means in its 'off' state may be a low impedance relative to the characteristic impedance of the input lines. Preferably the arrangement includes matching networks to match the FET device to the associated input and transmission lines, and biasing means to determine the state of the FET device. The FET device is preferably a high electron mobility transistor (HEMT). Each transmission line may incorporate a d.c. break between the associated FET device and the junction. The input lines, transmission lines, d.c. breaks, output line and junction are preferably formed in microstrip. A transmission line switch in accordance with the invention will now be described, by way of example with reference to the accompanying drawing, Figure 1, which is a schematic block diagram of a transmission line switch having two input lines. Referring to Figure 1, two input transmission lines 1 and 2 are shown, permanently connected to a common output line 3 at a junction 4. Each of the input lines 1 and 2 includes in its path an amplifying stage 10, comprising a FET device 9, having biasing networks 6 and 7. The biasing networks 6 and 7 enable the FET device 9 to be operable in one of the two states a high gain 'on' state, in which the amplifying stage 10 amplifies a signal applied to it by the input line; and an isolation or 'off' state, in which a signal applied to the input line is substantially attenuated at the output of the amplifier stage 10, and in which the device 9 has a low output impedance. In addition to the biasing function the network 6 is designed to present the optimum noise source impedance to the device 9, while the network 7 matches the output impedance of the device 9 to the characteristic impedance of the input line. In operation, two different signals are applied separately to the input lines 1 and 2, one of which signals it is required to transmit or switch to the output line 3, the other signal being essentially isolated from the output line 3 and the other input line. Assume, for example, that the signal applied to input line 1 is the wanted signal. In this case, the device 9' is biased in its 'on' state by control of its biasing networks 6' and 7', so that the signal emerging from the network 7' is an amplified version of the wanted signal applied to the device 9' via the network 6'. The output impedance of the device 9' in its 'on' state is transformed by the network 7' into the characteristic impedance of the input line 1; this ensures maximum signal transfer from the output of the amplifier stage 10' to the input line 1. At the same time, the device 9'' in the amplifier stage 10'' of input line 2 is biased in the 'off' state by means of its biasing networks 6'' and 7''. Thus, the device 9'' provides no gain for the signal applied to input line 2, and the signal is further attenuated by the low output impedance which the amplifier stage 10'' presents at its output to the input line 2. At the junction 4, the wanted (amplified) signal on input line 1 has a choice of two paths: the output transmission line 3, which presents the characteristic impedance of the line at the junction 4, and the other input line 2. Ideally, the wanted signal from the input line 1 is transmitted solely to the output line 3, with no transmission of the wanted signal to the input line 2. Optimum transfer of the wanted signal to the output line 3, with maximum isolation between the input lines 1 and 2, is achieved by arranging that the input line 2 presents a very high impedance path to the wanted signal at the junction 4. The impedance presented by the input line 2 should be high relative to the characteristic impedance presented by the output line 3, since it is the ratio of these two impedances which determines the insertion loss at the junction 4. The low output impedance presented by the device 9'' in its 'off' state can be transformed into a high impedance at the junction 4 by choosing a suitable length L for the input line 2 between the output of the amplifier stage 10'' and the junction 4. When the length L of the line is chosen appropriately the wanted signal at the junction 4 preferentially follows the low impedance path, that is the output line 3, and signal 'loss' to the input line 2 is minimised. In a practical embodiment, the input lines 1 and 2, and the amplifier stages 10' and 10'' will generally have the same characteristics, so that the lengths L of the two input lines at the output of the amplifier stages 10 will be identical. Thus, the wanted signal can be selected from either input line by appropriate control of the biasing networks 6 and 7. For efficient transformation, the output impedance of the device 9 should be either very high or very low in the 'off' state. With a FET device or a high electron mobility transistor (HEMT) the biasing is most easily arranged to provide a low output impedance in the 'off' state. But other devices and other biasing methods can be used which give a high output impedance in the 'off' state. For such devices, the low output impedance is typically about 5 ohms, but generally would not be more than about 10 ohms. The device was found to provide a greater attenuation of the unwanted signal when operated with a low output impedance than when operated with a high output impedance. The low output impedance is transformed at the junction 4 to an impedance which is high relative to the characteristic impedance of the transmission lines (commonly 50 ohms). A minimum of 500 ohms may be regarded as high, but, in other applications, much lower impedances may be used, depending on the gain of the amplifier stage and what is regarded as an acceptable loss of the wanted signal to the other input line. In a practical embodiment the transmission lines may be printed on a common substrate. The networks 6 and 7 may then be similarly printed as 'stubs' added to the printed track of the input lines at an appropriate distance from the FET device 9. Impedance matching is achieved by determination of this distance and the length of the stub. Some of the biasing components of networks 6 and 7, which may include a low-pass filter to isolate the transmitted signal from the power source for the device 9, can also be printed on the substrate. Each of the input lines 1 and 2 necessarily includes a d.c. break 5 between the output of the amplifier stage 10 and the junction 4. The d.c. breaks 5 serve to prevent the bias voltage applied to one of the devices 9 from reaching the other device. In the printed microstrip transmission line the d.c. break 5 can be made by interrupting a section of the line with a capacitive coupling. This coupling may comprise a number of thin, closely-spaced parallel strips of track 'interwoven' between the two isolated sections of the input line. The length of these strips constitutes part of the input line and has an effective path length for the signal, which is included in the overall line length L. The input lines may be any convenient length L (as shown) which provides the required impedance transformation in the 'off' state of the device 9. The output impedance of the device 9 in the 'off' state inevitably includes a capacitive component additional to the low resistance. This is due largely to the drain-source capacitance of the device 9. In order to obtain a high impedance at the junction 4 the line length L must be increased to take account of this capacitance. The switch is inherently narrow-band, relying on fixed electrical lengths of transmission line. Therefore, the length L of the input lines should be kept as short as is practically possible to provide the greatest bandwidth and to minimise losses. Gain of the amplifier stage in the 'on' state depends on the device used, but may be typically 10dB at frequencies around 11GHz using a HEMT device. Greater than 20dB isolation between the two signals at the output transmission line 3 has been achieved. When the switch is used at the front end of a receiving system to select, for example, one of two signals, the amplifier stage becomes part of the receiving system, and the noise figure of the switch is substantially determined by that of the amplifier stage. The advantage of using the switch arrangement described in this type of application is either an improved overall noise figure compared to that of a system employing a lossy switch at the front end, which would introduce its own noise to the signal before amplification, or a saving in space and components over using a separate switch after the two input amplifiers. One area of application for the switch is in a DBS satellite receiving system, where two separate programmes may share a common frequency, the signals having different (mutually orthogonal) polarisations. If the receiving antenna is arranged to simultaneously extract the two signals and apply them separately to transmission lines feeding the switch arrangement described, then programme selection can be conveniently made by electronic control remote from the receiving antenna. Although the embodiment described has only two input lines, the principle of operation of the switch is equally applicable to an arrangement having a plurality of input lines, the selected input having its amplifier operate in the high gain 'on' state, while the other input amplifiers are biased in the 'off' state. However, as the number of inputs increases, so too does the opportunity for loss of the wanted signal into the 'off' input lines. Thus, the requirement that the 'off' input lines present a high impedance at the junction becomes more stringent if a poor insertion loss figure for the wanted signal is to be avoided.

Claims: A transmission line switch arrangement for selecting one of two orthogonally polarised signals from the receiving horn of a microwave antenna, in which input lines (1,2) associated with the respective polarised signals are connected to a common output line (3) at a junction (4), each input line (1,2) being connected to said junction (4) by an FET amplifying device (9) operable in an 'on' state to transmit a signal with gain exceeding unity, and in an 'off' state to transmit substantially zero signal, the arrangement being characterised in that each FET amplifying device (9) consists of one single-gate FET device (9) connected in an amplifying configuration, each said input line (1,2) being connected to the gate electrode of a respective said FET device (9) and each drain electrode being connected to said junction (4) by a length (L) of transmission line, said length (L) being such, in conjunction with the output impedance of the FET device in its 'off' state, that a high impedance is presented at said junction (4) by the 'off' FET device (9). A transmission line switch arrangement according to Claim 1, wherein said output impedance is a low impedance relative to the characteristic impedance of said input lines (1,2). A transmission line switch arrrangement according to Claim 1 or Claim 2, including matching networks (6,7) to match said FET device (9) to the associated input and transmission lines, and biasing means (6,7) to determine the state of said FET device (9). A transmission line switch arrangement according to any preceding claim, wherein said FET device (9) is a high electron mobility transistor (HEMT). A transmission line switch arrangement according to any preceding claim, wherein each transmission line incorporates a d.c. break (5) between the associated FET device and said junction (4). A transmission line switch arrangement according to any preceding claim, wherein said input lines (1,2), said transmission lines, said d.c. breaks (5), said output line (3) and said junction (4) are formed in microstrip.

Inventors: FLYNN STEPHEN JOHN, KING GERARD, FLYNN, STEPHEN JOHN, KING, GERARD
Applicants: MARCONI GEC LTD, GEC-MARCONI LIMITED
Application year: 1989
Main classification: H01P1
IPC codes: H03H11, H01P1