Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
1998-05-29
2001-05-01
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S109000, C333S112000, C333S116000
Reexamination Certificate
active
06225874
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to devices that steer electromagnetic signals. Even more particularly, this invention relates to an electronically operated device that switches radio frequency electronic signals between two or more circuit branches.
BACKGROUND OF THE INVENTION
Radio frequency (RF) and microwave signal generators use a high power amplifier to generate large output signals. Unfortunately, non-ideal characteristics of these amplifiers can cause harmonic distortion of the output signal. Harmonic distortion is nonlinear distortion on the output of an amplifier characterized by the appearance in the output of harmonics other that the fundamental component when the input wave is sinusoidal. In a high-quality signal generator, these harmonics must be filtered before the signal leaves the source. The filter should reject signals an octave and greater above the fundamental output frequency. Since the frequency range of the output signal may be a decade or more, multiple sub-octave filter ranges are used. To enable the signal generator to sweep the output signal through a large range of output frequencies very quickly, or to allow computer control of the instrument, it is desirable that the switching between the multiple sub-octave filters be electronically controlled.
A p-i-n diode may be used as electronically controlled switch. (A p-i-n diode is constructed so that an intrinsic layer, the “i region,” is sandwiched between a positively doped layer (“p layer”) and a negatively doped layer (“n layer”). When reversed biased, a p-i-n diode behaves like an “open” switch. When forward biased, a p-i-n diode behaves like a “closed” switch.
An appropriate arrangement of p-i-n diodes is currently used to route the output signal through the appropriate sub-octave filter and then back to an output node. At the input node, multiple series p-i-n diodes are arranged in a star configuration with one common driven node and multiple intermediate nodes. A shunt diode is placed at each intermediate node. Each intermediate node is then connected to the input of one of the sub-octave filters. The reverse of this arrangement is used to route the outputs of the sub-octave filters back to a common output node. The path through the desired filter is used by forward biasing the series diodes in that path and reverse biasing the shunt diodes connected to that path. The series diodes on the unused paths are reversed biased and the shunt diodes connected to the unused paths are forward biased. The forward biased shunt diodes provide a path to AC ground, effectively stopping any AC signal from propagating down the path the diode is shunting. This isolates the unused paths.
This method of routing signals requires very compact assembly for frequencies above 20 GHz. This is difficult and expensive. Beam lead diodes are more expensive than other types of diode packaging. These diodes should be bonded directly to the shunt diodes. In a very compact assembly, this is a manual operation that increases cost. A series diode is also required to have low capacitance to achieve high isolation when shut off. Because relatively high series resistance is typically associated with low capacitance, the series diodes tend to contribute significant signal loss. Finally, due to their nonlinear characteristics, the series diodes introduce nonlinear distortion that degrades the quality of the output signal.
Accordingly, there is a need in the art for an improved way of electronically switching RF and microwave signals. It is desirable that such a system eliminate series diodes in the signal path since these devices are expensive, hard to assemble, reduce output power, and degrade the quality of the output of the device. Finally, the system should be easy to assemble.
SUMMARY OF THE INVENTION
In a preferred embodiment, the invention provides a switch for AC signals that does not have any diodes in series with the signal path. This improves the quality and power of the signal output after the AC switch and reduces the cost of the AC switch. The AC switch may be manufactured using traditional techniques that may be automated.
An embodiment of a single-pole double-throw AC switch according to the invention includes a four terminal coupling device or structure, such as a transformer, 3dB, or other directional coupler. The input signal is fed to one terminal of the coupling device. The input corresponds to the pole of the AC switch. A second terminal of the coupling device is connected to an AC ground. The third and fourth terminals connect to the outputs, or throws, of the AC switch. The third and fourth terminals are also each connected to one of two shunt switching devices that, when on, shunt the third or fourth terminal of the coupling device, respectively, to AC ground. Only one of these two shunts is turned on at a time. This results in the input signal being coupled to the terminal with the shunt that is not on without the signal passing through a series diode.
Another embodiment of a single-pole double-throw AC switch for switching selected frequencies of RF and microwave signals includes a directional coupler with a nominal coupling factor of 3 dB as the coupling device. P-i-n diodes are used as the shunting devices. When forward biased, the p-i-n diodes shunt the terminal to which they are connected to AC ground.
Another embodiment of a single-pole double-throw AC switch for switching selected frequencies of RF and microwave signals includes two cascaded directional couplers. Each of these couplers has a nominal coupling factor of 8.34 dB for a nominal net coupling factor of 3 dB for the entire structure. P-i-n diodes are used as the shunting devices. By cascading two directional couplers, the shunt diodes may be located farther from each other and one of the output terminals may be located closer to the input terminal. This structure also improves manufacturability by allowing greater line width and spacing to be used. Greater line widths and spacing also decreases signal loss inside the structure.
Other embodiments include multi-layer coupling structures, waveguide coupling structures, stripline coupling structures, or any other type of RF or microwave coupling structure as the coupling device. One example of a multi-layer coupling structure is a multi-layered broadside coupled structure. Other embodiments include a greater number of throws for the switch. For example, the first embodiment, above, can be made into a single-pole triple-throw switch by removing the AC ground connection from the second terminal of the four terminal coupling device and making that terminal the third the output, or throw, of the AC switch. A third shunt device would also be connected between the second terminal and an AC ground.
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patent: 233901 (1985-11-01), None
Agilent Technologie,s Inc.
Lee Benny
Neudeck Alexander J.
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