Wave transmission lines and networks – Plural channel systems – Having branched circuits
Patent
1994-05-02
1996-05-28
Gensler, Paul
Wave transmission lines and networks
Plural channel systems
Having branched circuits
330295, H01P 115
Patent
active
055215629
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a high isolation switch and, particularly, but not exclusively, to a FET switch for transmitting a signal with gain from one of a plurality of input transmission lines to a common output line.
In a switch in which a single output line is switched to one or more input lines, it is desirable that the switch performs such that cross channel isolation complies with the highest possible standards or projected standards. At the present in the United States a cross channel isolation standard of 27 dB. has been proposed and this is over the frequency range 11.7-12.2 GHz. This means that when one polarization is on and the other polarization is off, the isolation between polarizations is at least 27 dB.
A transmission line switch is disclosed in published UK Patent Application No. GB2220538A to Marconi which allows a single output line to be switched to one or more input lines to which it is permanently connected at a common junction. In the switch disclosed in this application each input line has an associated amplifier stage which can be biased in a normal high gain (on) state, or in an isolation (off) state. The biasing is selected so that when the amplifier is biased in the off state it ensures that the amplifier stage output presents a low impedance to its own input line, the length of which is selected to achieve a high impedance at the junction with the other lines. In the transmission line switch disclosed in the aforementioned application the low output impedance presented by an amplifying device which is a field effect transistor, in its off state can be transformed into a high impedance at the junction by choosing a suitable length of transmission line. It is stated in this document that when the length L of the line is chosen appropriately the wanted signal at the junction preferentially follows a low impedance path which is the output line and signal loss to the off input line or lines is minimized. It is also stated that this device was found to provide a greater attenuation of unwanted signal when operated with a low output impedance than when operated with a high output impedance. In any event, it is critical to this invention that the high impedance achieved at the junction is only obtained by combining the output impedance of the amplifying means together with an appropriate length of transmission line.
The disclosed transmission line switch provides isolation of about 20 dB over 10.95-11.70 GHz. between polarizations and does not meet the proposed U.S. standards which limits the application of such a device. In addition, the requirement of a portion of transmission line between the amplifying means and the switch increases the switch size and adds to costs. This is significant when board area is limited.
An object of the present invention is to provide an high isolation switch which obviates or mitigates at least one of the aforementioned disadvantages.
This is achieved, in one aspect, by omitting the transmission line between the output matching section and junction of the input and the output.
The output matching network is coupled directly to the common junction and the transmission line length between the output matching network and the junction is omitted for each input line.
In another aspect of the invention, the biasing of the amplifiers is changed so that when the field effect transistor is normally off it is biased off in the reverse pinch-off direction, so that the output impedance of the off amplifying means is high and, consequently, the length of the transmission line to the common junction avoids the use of a nominal quarter wavelength line so that the high impedance appears in the off leg of the junction providing an effective open circuit to the on-path. The cross channel isolation achieved with the inventive embodiments herein above described is at least 30 dBs. (and in fact 40 dBs has been demonstrated across the frequency range 10.95-12.2 GHz. in comparison to the circa 20 dB. obtainable with the Marconi device.
According to a further aspect of
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Levy et al., A Broadband Hybrid Active Switchable Combiner, 15th European Microwave Conference Proceedings, 9-13 Sep. 1985 pp. 629-634.
Cambridge Computer Limited
Gensler Paul
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