Tunable waveguide filter

A tunable filter includes a waveguide with at least one resonant cavity and a tunable impedance structure coupled to each resonant cavity. Each resonant cavity has a resonant frequency and its corresponding impedance structure can be tuned to adjust the resonant frequency. The filter transmits the signal in a pass-band that includes the resonant frequency and reflects signals outside the pass-band.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to waveguides and, more particularly, to tunable waveguide filters.

2. Description of the Related Art

Electromagnetic signals with wavelengths in the millimeter range are typically guided to a destination by a waveguide because of insertion loss considerations. An example of one such waveguide can be found in U.S. Pat. Nos. 6,603,357 and 6,628,242 which disclose waveguides with electromagnetic crystal (EMXT) surfaces. The EMXT surfaces allow for the transmission of high frequency signals with near uniform power density across the waveguide cross-section. More information on EMXT surfaces can be found in U.S. Pat. Nos. 6,262,495 and 6,483,480.

In some waveguide systems, filters are used to control the flow of signals during transmission and reception. The filters are chosen to provide low insertion loss in the selected bands and high power transmission with little or no distortion. A typical millimeter wave system includes separate waveguide and filter combinations, with each combination being sensitive to a different resonant frequency. The filters include a resonant cavity that can be tuned to a particular resonant frequency using mechanical adjustments such as tuning screws as disclosed in U.S. Pat. No. 5,691,677 or movable dielectric inserts as disclosed in U.S. Pat. Nos. 4,459,564 and 6,392,508. In both of these cases, tuning is accomplished by mechanically adjusting the screw or insert to change the length of the resonant cavity and the resonant frequency.

If the mechanical adjustment cannot tune the resonant frequency quickly enough, then more waveguide and filter combinations will be needed, with each one tuned for a different resonant frequency. For example, a single antenna can be coupled to separate filters and their corresponding waveguides. In this setup, one filter-waveguide combination can be tuned to transmit and receive communication signals in one frequency band and another can be tuned to transmit and receive radar signals in a different frequency band. It is desired, however, to reduce the number of waveguide-filter combinations needed to transmit signals over the different frequency bands.

SUMMARY OF THE INVENTION

The present invention provides a tunable filter which includes a waveguide with one or more resonant cavities. Each resonant cavity has a resonant frequency that is tunable in response to tunable impedance structures coupled to each of the resonant cavities. The filter transmits the signal in a pass-band which includes the resonant frequency and reflects the signal outside the pass-band. The tuning can be done by adjusting the impedance and/or resonant frequency of the impedance structures to change a propagation constant of the signal and provide the filter with a desired frequency response.

The tunable filter can be used in a communication system which includes multiple communication platforms. The waveguide filter can be connected to the communication platforms to provide frequency selective communications between them and an external system, such as an antenna.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1a,1b, and1cshow top, side, and front elevation views, respectively, of a waveguide filter10which includes tunable impedance structures24on opposed sidewalls12and14. The other waveguide sidewalls11and13are spaced apart by a height b (SeeFIG. 1b) and sidewalls12and14are spaced apart by a width a (SeeFIG. 1c) so that filter10has a rectangular cross-section. The cross-sectional shape of filter10typically depends on the polarization of the signal propagated through the filter, so it can have a cross-section other than rectangular. For example, the cross-section can be circular for a coaxial waveguide structure which guides circularly polarized signals. The impedance structures in this case can be positioned 180° from one another.

Cavity forming boundary structures16, which are conductive posts with diameters D, are positioned within the waveguide and are electrically spaced apart by a distance Lcavto form cavities26. Structures16extend vertically between sidewalls11and13and the spacing of structures16extends longitudinally along filter10between ends17and19. Lcavrefers to the electrical length of each resonant cavity26. This is equal to the physical length of the cavity multiplied by the ratio of the propagation time of a signal through the cavity to the propagation time of a signal in free space over a distance equal to the physical length of the cavity.

The number and arrangement of structures16can be chosen to provide filter10with a desired quality factor Q. For example, optional cavity forming boundary structures18can be positioned adjacent to structures16and between sidewalls12and14so that multiple conductive posts define each end of resonant cavity26. This has the effect of changing the total inductance and Q of cavity26because the posts are electrically connected in parallel.

Impedance structures24, each with a width w, are spaced apart by a distance23so that there is one pair on opposed sidewalls12and14within each cavity26. Structures24include electromagnetic crystals (EMXT) surfaces which can be used to obtain a desired surface impedance in a band of frequencies around the resonant frequency, Fres, of structure24with one such band being the Ka-Band.

Cavities26are one half of a wavelength long at the cavity resonant frequency Fcav, so the surface impedance of structure24can be changed to tune Fresrelative to Fcav. This has the effect of allowing some signals with a desired propagation constant β and operating frequency F to be outputted through end19as signal Sout, while reflecting signals with different β values and frequencies. For example, Soutwill equal S(β1) or S(β2) if the impedance of structures24is chosen so that Fresresonates with signals S(β1) or S(β2), respectively. Because the impedance of structure24determines which β values will resonate with Fcav, filter10can selectively transmit some frequencies in a pass-band while reflecting others outside the pass-band. The signals are represented by an electromagnetic wave with an electric field E, a magnetic field H, and a velocity U (SeeFIG. 1b). β is related to the waveguide wavelength λgthrough the well-known equation β=2π/λg. Wavelength λgis related to F by the equation λg=λo/√{square root over ((1−(λo/2a)2)} in which λo=c/F where λois the free space wavelength and c is the speed of light.

FIG. 2shows a more detailed view of impedance structures24which include a dielectric substrate28with conductive strips30which extend parallel to the waveguide's longitudinal axis and face its interior. A conductive sheet27, which is used as a ground plane, is positioned over the exterior of dielectric substrate28and can form a portion of sidewalls12and14. Adjacent conductive strips30are spaced apart by gaps32and variable capacitance devices40are coupled between them to allow their capacitance to be varied to tune Fresand, consequently, Fcav.

Conductive vias31extend from strips30, through substrate28to conductive layer27. Vias31and gaps32reduce substrate wave modes and surface current flow, respectively, through substrate28and between adjacent strips30. The width of strips30present an inductive reactance L to the transverse E field and gaps32present an approximately equal capacitive reactance C. Although structures24are shown inFIG. 2as having width W, they can extend down the lengths of sidewalls12and14as shown inFIG. 4.

Numerous materials can be used to construct impedance structure24. Dielectric substrate28can be made of many dielectric materials including plastics, insulators, poly-vinyl carbonate (PVC), ceramics, or semiconductor material such as indium phosphide (InP) or gallium arsenide (GaAs) Highly conductive material, such as gold (Au), silver (Ag), or platinum (Pt), can be used for conductive strips30, conductive layer27, and vias31to reduce any series resistance.

With impedance structures24on sidewalls12and14, waveguide10is particularly applicable to passing vertically polarized signals that have an E field transverse to strips30. At a particular resonant frequency, strips30present an inductive reactance L to the transverse E field, and gaps32between strips30present an approximately equal capacitive reactance. Hence, structure24presents parallel resonant L-C circuits to the signal's transverse E field component (i.e. a high impedance). By controlling and varying the impedance of structures24with a bias across capacitors40, β can be varied and Lcavcan be changed.

Structures24provide a high surface impedance at Fresand over a band of frequencies around Fres. Hence, an incident wave at Freswill have a reflection coefficient of one and a phase of zero degrees. For a passive EMXT, without a tuning mechanism such as capacitors40, the thickness of substrate28, the area of strips30, the permittivity ε and permeability μ=0of substrate28, and the width of gap32determine Fresand the bandwidth of the pass-band. With capacitors40, however, Fresand β can be varied with a bias voltage by changing the impedance of structures24. At Fres, structure24is in its highest impedance state so that little or no surface currents can flow normal to strips30and, consequently, tangential H fields along strips30are zero and the E field is uniform across width a. At frequencies below or above Fres, structures24behave as a non-zero inductive or capacitive surface impedance, respectively.

The capacitance of each capacitor40is inversely proportional to the bias across it. Since capacitors40between adjacent conductive strips30are in parallel, if the reverse bias applied across capacitors40increases, then the total capacitance decreases. In this case, structure24resonates at a higher frequency. If the reverse bias across capacitors40decreases, then the total capacitance increases. In this case, structure24resonates at a lower frequency.

Variable capacitors40can include varactors, MOSFETS, or micro-electromechanical (MEMS) devices, among other devices with variable capacitances. The varactors can include InP heterobarrier varactors or another type of varactor embedded in impedance structure24so that its resonant frequency is electronically tunable. A MOSFET can also be used as an alternative by connecting its source and drain together so that it behaves as a two terminal device. In any of these examples, the capacitance of capacitors40can be controlled by devices and/or circuitry embedded in waveguide10or positioned externally.

FIGS. 3aand3bare simplified side and top views, respectively, of impedance structure24with variable capacitors40which include micro-electromechanical (MEMS) devices81. Devices81can include magnetic materials, such as nickel (Ni), iron (Fe), and cobalt (Co). The magnetic properties of devices81are chosen so that the distance between an end83and strip30can be changed by applying a magnetic field. Each device has multiple fingers82extending between adjacent strips30. The magnetic field then controls the capacitance between adjacent conductive strips30. As the distance between them decreases, the capacitance increases. Also, the number of fingers82that bend increases as the magnitude of the magnetic field increases, so that the capacitance of devices81is more linear as a function of magnetic field. The capacitance also increases as the overlap between end83and conductive strip30increases. These relationships are given by the well-known equation C=εA/d, in which ε is the permittivity, A is the overlap area, and d is the distance, all between end83and strip30.

FIG. 5is a graph of the propagation constant β (rad/cm) of a signal that will resonate with Fcavverses Fres(GHz). In this graph, a range of operating frequencies F between 28 GHz to 40 GHz is plotted where width a is equal to 4 mm. The center of the pass-band is tuned from 31.6 GHz to 33.2 GHz by varying the bias of variable capacitors40from 0 V to 10 V. Curve56is the β value in the absence of impedance structures24(i.e. sidewalls11–14are all conductive). Curve58is the β value for free space, which corresponds to the signal propagating outside waveguide10.

For resonance to occur, Lcavshould be one-half of the signal wavelength which, in this case, is equal to 5 mm so that a signal with β=6.28 rad/cm will resonate with Fcav. If it is desired to have signals at F=30 GHz, 36 GHz, or 40 GHz resonate with cavity26, then Fresshould be equal to about 30 GHz (point61), 34 GHz (point62), or 49 GHz (point63), respectively. Hence, filter10is tuned by changing the impedance of structures24which changes Fres.

FIG. 6is another graph of the propagation constant β (rad/cm) of a signal that will resonate with Fcavverses Fres(GHz). The variation of β is shown for three cases in each of which the cavity length Lcavis 5 mm (i.e. β=6.28 rad/cm), the width w of the impedance structures is 2 mm, and the diameter D of boundary structures16is 0.8 mm. In curves50,52, and54, the signal frequency F is 30 GHz, 30 GHz, and 34.3 GHz, respectively, while the respective waveguide widths a are 7 mm, 4 mm, and 7 mm. In each case, the waveguide height b is equal to the corresponding width a.

At a constant F, β decreases when Fresincreases, so Frescan be chosen so that a desired F resonates with Fcav. For example, curves50,52, and56intersect at about Fres=30 GHz so that β is equal to 6.28 rad/cm (point51in the graph). In this case, a signal at F=30 GHz will be transmitted through filter10. Curve54is asymptotic to Lcav=λg/2 at higher values of Fresindicating that its β value will not fall below 6.28 rad/cm. Since curve54does not intersect curve56, a signal at F=34.3 GHz will not be transmitted through filter10. Hence, if F is too large, filter10will not propagate signals effectively.

FIG. 6shows that as width a is reduced, the values of F in which Lcav=λg/2 increases. For example, curve50intersects curve56at point51, but curve54with a larger value of width a is asymptotic to curve56and does not intersect it. This means that cavity26will not resonate with a signal with F=34.3 GHz if a=7 mm. This result can be compared to the curves inFIG. 5in which width a is equal to 4 mm. Here, curve60at 40 GHz intersects curve56at point63indicating that the upper limit of frequencies capable of being propagated through filter10has increased. Thus, width a can be used to control the frequency tuning range of filter10.

FIG. 7shows the frequency response in dB of filter10for various bias voltages as a function of F (GHz). Shown are the responses at bias voltages of 0 V (curve71), 1 V (curve72), and 10 V (curve73) for filter10. Curve70is the β value in the absence of impedance structures24(i.e. sidewalls11–14are all conductive). The cavity frequency Fcavmoved from 31.6 GHz (Point74) to 33.2 GHz (Point75) when the reverse bias on capacitors40increased from 0 V to 10 V. The center of the pass-band for the waveguide with conductive sidewalls is measured to be about 34.3 GHz (Point76), which is consistent with the expected value for Lcavequal to 5 mm in a waveguide with width a equal to 7 mm.

At 0 V bias, cavity26is ‘electrically long’ and Fcavis about 31.6 GHz. As the reverse bias across capacitors40increases, Fresincreases towards 35 GHz. Fcav, which is slightly higher than Fres, rises ahead of Fresbut at a slower rate. Fcavwill be equal to Fresat a frequency in the range between 31.6 GHz to 33.2 GHz. Above this ‘coincident frequency’, Fcavwill be lower than Fres, but it will still increase as Fresincreases.

FIGS. 8a,8b, and8cshow top, side, and front elevation views, respectively, of a waveguide filter100with an iris structure25. Filter100includes similar numbering to filter10with the understanding that the discussion above applies equally well here. Structure25includes cavity26which is formed from cavity forming boundary structures41extending from surfaces11and13towards the interior of filter100so that a distance44separates them. Impedance structures24are positioned on surfaces91between structures41and within cavity26to adjust the resonant frequency of cavity26as discussed above. The operation of filter100is similar to the operation of filter10in that the capacitance of impedance structure24can be adjusted to change Lcav.

FIG. 9shows curves of the frequency response of filter100when Lcavis 5 mm, width a is 2.4 mm, height b is 7 mm, distance44is 4 mm, and operating frequency F is varied between 32 GHz and 42 GHz. Without structure24, i.e. with metal surfaces91only, the transmission pass-band peaks at 44 GHz. With impedance structures24, however, the half-wavelength pass-band moves from about 34.4 GHz (Point85) to about 41.5 GHz (Point86). Hence, filter100can be tuned like filter10to obtain a desired frequency response.

In all of the above embodiments, sidewalls11-14can have impedance structures. The waveguide can then be used to filter a vertically and/or a horizontally polarized signal. For vertically polarized signal, impedance structures on sidewalls12and14filter the signal. For horizontally polarized signals, impedance structures on sidewalls11and13filter the signal. Only one of sidewalls11–14can have an impedance structure to make the bandwidth of the pass-band narrower than the case with two impedance structures positioned on opposed sidwalls. The bandwidth can also be controlled by making the impedance of one impedance structure high while making the impedance of the opposed impedance structure low so that the structure with low impedance behaves like a metallic surface.

In the filters, the cavity forming structures can also include tunable impedance structures so that their impedance can be adjusted to change Lcav. In filter10, for example, surfaces of cavity-forming structures16can include EMXT structures similar to structures24to adjust the impedance of cavity26. In waveguide100surfaces92,93,94, and95can include EMXT structures to adjust the impedance of iris structure25.

FIG. 10shows how filter10can be used as a notch or band-stop filter. InFIG. 10, a waveguide filter110includes two filters10positioned side by side. The impedances of structures24can be chosen to be different so that the electromagnetic wave flowing through both of them experiences two different β values. When the waves recombine near end19, they will be out of phase. The phase difference can be used to provide a desired constructive and destructive interference pattern so that certain frequencies are not included in the output signal. In this way, filter110behaves as a band-stop or “nulling” filter. Filter110can be independently used to rapidly adjust the frequency that is nulled by adjusting the impedance of each structure24. In one application, this is useful to attenuate an undesired signal from being received by a communication system connected to filter110. If the undesired signal changes frequency as a function of time, then filter110can provide signal tracking by rapidly retuning from one frequency to another.

Hence, a tunable waveguide filter is disclosed. It can be used in systems which typically require multiple filters to provide different resonant frequencies. The filter can provide different resonant frequencies because it can be tuned which decreases the complexity and component count of the communication system. For example, using the waveguide filter, one antenna can provide radar, communications, and other communication functions over many different frequencies.