Low-Loss, Wide Bandwidth Limiter

This invention relates to the field of power limiters and more particularly to microwave power limiters using superconducting material.

Power limiters are commonly used in a variety of military and commercial microwave systems. The basic function of a power limiter is to protect sensitive electronics from RF powers above a predetermined critical value because some devices can be permanently damaged when subjected to an electrical field of a sufficient magnitude. An ideal RF limiter should have the following characteristics:

wherein P.sub.in and P.sub.out are, respectively, the input signal power to the limiter and the output signal power from the limiter, and P.sub.max is the specific limiting power level of the power limiter. This implies that an ideal limiter would have zero "off" resistance when P.sub.in .ltoreq.P.sub.max, and would provide a proportional resistance as the device turns "on" such that the output signal is limited to P.sub.max when P.sub.in >P.sub.max.

Standard power limiters are typically composed of single or multiple diodes (usually PIN diodes) either in a series or a shunt configuration to reflect and attenuate strong incident RF signals. When used in the series configuration, the diodes are biased off to reflect and attenuate the RF signal and when used in the shunt configuration the diodes are biased on to reflect the RF signal. In either of these configurations, the finite conductance characteristics of the PIN diode causes an undesired reflection and attenuation of the RF signal, that is, when the power limiter need not limit power. Therefore, these standard diode limiters always cause some insertion loss. Further, the inherent reactance of the conventional limiters limits the bandwidth or frequency range of operation. Examples of commercially available diode limiters can be found in various product catalogs, such as Triangle Microwave, 1986.

Accordingly, there is a need in the electronics industry to provide a power limiter with an increased bandwidth and a lower insertion loss. The present invention addresses such a need.

Accordingly, one object of the present invention is to provide for a power limiter which has an extremely low loss in the non-attenuating state.

Another object of the present invention is to provide for such a limiter which can operate at a wide frequency bandwidth.

These and other objects of the present invention are achieved by providing a high-temperature superconductor (HT.sub.c S) in a microstrip configuration which is electrically related to a superconducting coupler and which is controlled by a heater disposed over the superconducting microstrip line. This limiter configuration is used in a series configuration with other circuitry and utilizes the low loss characteristics of the HT.sub.c S material and the wide bandwidth characteristics of microstrip. The structure itself is a five layer microstrip geometry which includes a normal metal ground plane; a dielectric substrate disposed on the metal ground plane; a superconducting microstrip transmission line disposed on the substrate; a dielectric film layer disposed on the superconducting microstrip; and a heating element disposed on the dielectric film layer and over the superconducting microstrip.

These and other features of the present invention will be better understood in light of the ensuing Detailed Description of the invention and the related figures wherein:

FIG. 1 is an exploded perspective view of one embodiment of the invention;

FIG. 2 is a cross-sectional view of the embodiment shown in FIG. 1;

FIG. 3 is a generalized block diagram of the present invention;

FIG. 4 is a top view of a second embodiment of the present invention;

FIG. 5 is a cross-sectional view of the embodiment shown in FIG. 4; and

FIG. 6 is a graph of the invention's return and insertion loss as compared to frequency.

It should be noted that for purposes of illustration some of the elements shown in the figures have not been drawn to scale.

Now referring to FIGS. 1 and 2, there is shown one preferred embodiment of the present invention in a microstrip configuration. As shown, the device is a five layer structure with a conductive ground plane 1, a dielectric substrate 2, a superconducting transmission line 3, with an input and an output a second dielectric substrate 4, and a heating element 5 which is placed on top of the second dielectric substrate directly over the microstrip line. The signal is fed into the input of the limiter as shown in FIG. 1. A loosely coupled coupler 6 samples and directs RF energy into a circuit that detects the energy level and drives a variable current through the heating element 5. When the input signal reaches a predetermined level this circuitry drives sufficient current through the heating element 5 to locally heat the superconducting microstrip line 3 to put it in a nonsuperconducting state or resistive state. The resistance will attenuate the signal and thus limit the incoming RF signal. By varying the control current, one can control the resistance of the superconducting line. Because the "on" and "off" states are dependent upon the superconducting properties of the microstrip line, the limiter will have practically zero "on" resistance. Further, the limiter will have very low losses because 1) the microstrip line is superconducting, 2) a low loss dielectric substrate can be selected and 3) the coupler only couples a minimum amount of signal energy.

A general configuration of the present invention is shown in FIG. 3. It includes a transmission line, with an input and an output, a coupler, and a control circuit to control the conductivity of the transmission line or the resistance of a variable resistor. When the RF signal reaches a predetermined level, the control circuit decreases the conductivity of the transmission line or increases the resistance of a variable resistor, and therefore limits the RF energy, which flows through the limiter. An example of a control circuit, which would be connected to the coupler in series to the element controlling the resistivity or conductivity of the transmission line, would include a diode connected to a ground and to a transistor. Of course, those skilled in the art could devise other circuits to control the conductivity/resistivity of the transmission line.

Now referring to FIGS. 4 and 5, there is shown a second embodiment of the present invention. FIG. 4 is a diagram of the superconducting power limiter in a top view of the full device showing the superconducting transmission line 13 with an input and an output and with a heating element 15, in the form of a mendering heater line, above one section of the transmission line 13 and is preferably made from a thin film of YBa.sub.2 Cu.sub.3 O.sub.7-x, a high-T.sub.c superconductor with a transition temperature near 90.degree. K., and the heating element 15 is a thin layer of gold. There is also a superconducting coupler 16 near the input of the limiter. This coupler is used to sample the input RF signal power and drive a feedback circuit (shown as a diode connected to a transistor which, in turn, is connected, as shown, to the meander line) that, in turn, provides current to the heating element 15 if the input power exceeds a critical value.

FIG. 5 shows that the power limiter, in its preferred embodiment, uses a five layer microstrip geometry. This five layer structure includes a normal metal ground plane 11, a dielectric substrate 12, a superconducting transmission line 13, a dielectric film layer 14 and a heat generating element 15. The dielectric substrate 12 must be of low loss and a proper substrate for growth of the superconducting film in order to allow fabrication of a low loss device. In a preferred embodiment of the present invention, the device employs single crystal LaAlO.sub.3 as the substrate 12. The thin dielectric layer 14 between the transmission line 13 and the heater 15 is necessary in order to prevent electrical connection between the microstrip conductor and the heater. In a preferred embodiment, this thin dielectric layer 14 is MgO.

In order to maximize the isolation in the "limiting" state while minimizing the insertion loss in the "non-limiting" state, the transmission line is preferred to be as narrow as possible, i.e. maximize its resistance when in the limiting state. A wet etch lithography may be used to pattern the transmission line because the minimum feature size that is reproducible with wet etch lithography is approximately 10 .mu.m. Although for purposes of this invention a transmission line width of 25 .mu.m would be sufficient to function.

In the "non-limiting" state, the high-T.sub.c line is superconducting and, therefore, will have essentially zero resistive losses. However, the reflective losses resulting from impedance mismatches must be minimized. For most common systems, this would mean that the input and output lines would impedance matched to 50 ohms. For example, if 0.25 mm thick substrate of LaAlO.sub.3 is used, a 75 .mu.m wide transmission line would be required to match the impedance to a 50 ohm system.

An example of a transition between input/output and the heated portion of the HT.sub.c S line would be a 25 .mu.m HT.sub.c S line directly connected to the 75 .mu.m wide transmission lines for the input and output sections. However, this type of transmission can lead to unwanted reflections. To eliminate this impedance discontinuity caused by this change in width, the transmission line should be made a multiple of a half wavelength. With the configuration shown in FIG. 4, zero reflections will occur from a change in characteristic impedance at any frequency in which the transmission line is a multiple of a half of a wavelength. If it is not made a multiple of a half wavelength, then the reflections are still very minimal. The RF line can be tapered from 75 .mu.m to 25 .mu.m to further reduce any reflections caused by the change in width, also as shown in FIG. 4.

FIG. 6 shows the present invention's return and insertion losses compared to signal frequency. Note that the signal's periodicity is caused by the half wavelength effect mentioned above. With this effect removed/alleviated the graph of the return and insertion losses would provide a flat response.

The directional coupler should be designed to couple approximately 1% (.apprxeq.0.04 dB of insertion loss) of the power from the main transmission line and to introduce zero reflections. The coupled power from the coupler to the detection circuitry will be detected by the diode mentioned previously. When the detected signal reaches a certain level, the current will be driven through the meander line heating element.

The multilayered structure shown in FIG. 5 is fabricated in several different steps. The initial step is fabricating a superconducting film of YBa.sub.2 Cu.sub.3 O.sub.7-x on the dielectric substrate of LaAlO.sub.3 by a pulsed laser deposition process. Such a process is described by D. B. Chrisey et al., MRS Bulletin, Vol. 27, no. 2, 37 (1992) which is incorporated herein by reference. For this deposition, the substrate is heated to approximately 750.degree. C. in an oxygen atmosphere of 150 mTorr. The substrate is positioned 7 cm from a ceramic target of the YBa.sub.2 Cu.sub.3. A 248 nm light source formed from a pulsed KrF excimer laser is focused onto the target for an energy density of about 1.5J/cm.sup.2. The laser pulses induce a plasma or plume of the ceramic material, some of which is transported onto the nearby LaAlO.sub.3 substrate to form the superconducting film. Once the deposition is complete, the sample is cooled in 1/2 atm of oxygen to reach the proper oxygen stoichiometry in the film.

The next step in the fabrication process is defining the transmission line and coupler. The pattern is defined by the wet etch photolithography, mentioned above, and the etch is accomplished using a saturated aqueous solution of ethylenediaminetetraacetic acid, commonly referred to as EDTA. Once the HT.sub.c S circuit has been defined, the MgO dielectric layer is deposited over that part of the transmission line to be covered by the meandering heater line. This may be accomplished by depositing the MgO by pulsed laser deposition in a fashion similar to that for the superconductor, except that the MgO may be deposited at room temperature resulting in an amorphous layer of the MgO.

Photolithography is used again to define the heater pattern and to define gold contact pads on the transmission line and coupler line. Gold is used for the heating element so that the contact and the heater metallization can be accomplished in one deposition. Of course, more resistive materials may be used for the heater, such as titanium. The final step is to metallize the ground plane with gold.

From this general description, those skilled in the art can form other transmission line configurations such as coplanar waveguides, suspended substrate, stripline, etc. Further, the present invention has been described using a heating element to control resistance of the HT.sub.c S transmission line, but the heating element and the HT.sub.c S transmission line can be replaced by any controllable variable resistor that has a very high conductance in the "non limiting" state. Furthermore, the limiter can be built with the controlled variable resistance connected in a shunt configuration. Additionally, those skilled in the art will recognize that the configuration of the meandering heating line may vary considerably in both length, width, composition and the number of meanders. For example, if a highly resistive material is used, such as titanium, then less electrical coupling will occur and accordingly, the meander line could be fabricated to be very short in length or simply be fabricated entirely over the transmission line.

Accordingly and although the present invention has been described with respect to two embodiments, those skilled in the art would be readily able to fabricate any number of devices given this disclosure. Therefore, this invention should be limited by the above description, but only by the following claims.