Drop-in Magnetically Tunable Microstrip Bandpass Filter

This invention relates to microwave bandpass filters and more specifically relates to a tunable microstrip bandpass filter.

Microwave bandpass filters are two port devices which allow the reception of a predetermined band of frequencies and suppress all other frequencies. An ideal filter will accept frequencies within a frequency range which precisely matches the band width and center frequency of the communication spectrum desired. With that range, it will pass signals with almost no dissipation or distortion.

Current parallel coupled microstrip bandpass filters have a fixed center frequency. They employ spaced parallel coupled microstrip lines printed on a planar dielectric. Thus their structure is simple and inexpensive. However, these and other bandpass filters operating above 1GHz have only a fixed center frequency and bandwidth.

A tunable filter has an electronically adjustable center frequency which, when used correctly, will operate within a linear frequency range. Tunability allows the filter to be adjusted to the center frequency of the signal to be received and also allows reception of multiple signals in a multiplexing scheme and is usable in a number of microwave applications.

The invention provides a means to adjust (e.g. selectively modify) the magnetic permeability of the ferrite of a microstrip band pass filter to permit the filter to be tunable over a limited linear range of center frequencies. Preferably, the adjustment means comprises a winding which encircles the parallel coupled microstrip lines and the underlying ferrite. A d/c current applied to the winding produces a biasing magnetic filed along the along axis of the filter which changes the magnetic permeability of the ferrite, and thus the center frequency of the filter. The d/c current can be controlled in any desired manner, either manually or electronically, in response to the behavior of some other control circuit for the purpose of adjusting the center frequency of the filter.

More specifically, the permeability (u') of the ferrite changes when a magnetic biasing field is applied. This change in permeability results in a change in the velocity of standing waves (Vp) between coupled microstrip pairs, according to the relationship Vp=c.sqroot.u'.sub.r er. This change in standing wave velocity results in a change in the frequency of the standing wave, f=Vp/2.lambda..

Magnetic biasing is preferably produced by winding a copper coil around the ferrite microstrip and applying a d-c current to the coil. The induced magnetic field within the coil and ferrite changes the permeability (u') of the ferrite. By varying the coil current, one can either increase or decrease the permeability of the ferrite, thus changing the standing wave velocity (Vp) and hence the frequency, (v=Vp/2.lambda.). This makes it possible to tune the center frequency of a bandpass filter.

Such a tunable microstrip filter is low in cost and can easily be fabricated using existing technology.

These and other objects, features, and details of the invention will become apparent in light of the ensuing detailed disclosure, and particularly in light of the drawings wherein:

FIG. 1 is a perspective schematic view of a microstrip bandpass filter.

FIG. 2 is a drawing similar to FIG. 1 but shows the addition of a ferrite substrate and a d/c bias for varying the permeability of the ferrite to enable tuning of the filter.

FIG. 1 shows a fixed frequency microstrip filter. This filter is fabricated by printing continuous parallel coupled microstrip lines 12 and 13 upon a dielectric substrate 10. This dielectric substrate 10 is most commonly aluminum (E.sub.T =9.9) or Duroid (E.sub.T .apprxeq.2.2) which is the name of a trademarked material. As noted previously, an ideal filter of this type only accepts frequencies within a specific frequency range which precisely matches the desired band width and a fixed center frequency of the communication spectrum.

In accordance with the present invention, and as shown in FIG. 2, a thin tapered ferrite substrate 11 is secured to the dielectric substrate 10. The ferrite substrate may be attached to the dielectric substrate by a conductive epoxy. Continuous parallel coupled microstrip lines 12 and 13 are printed upon both ferrite substrate 10 and the dielectric substrate 11. A multiturn copper winding or coil 20 extends around the ferrite substrate 11 and microstrip lines 12 and 13 and extends through slots 30 and 31 in support 10. The terminals 40 and 41 of winding 20 are connected to a source of variable d/c current, as labeled. The production of a d/c current in coil 20 produces a magnetic biasing field 50 within the ferrite. As described above, the induced magnetic biasing field 50 changes the magnetic permeability of the ferrite, and thus the center frequency of the filter may be manipulated due to the resultant change in the velocity of the standing waves between the coupled microstrip lines 12 and 13.

Any desired structure can be employed to generate the magnetic biasing field, for example, permanent magnets or electromagnets which are separate from or integrated with the ferrite 11 can be used. Furthermore, the invention is applicable to microwave devices having different orientations of microstrip lines and ferrite than that shown in FIGS. 1 and 2.

Although the present invention has been described in relation to a particular embodiment thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.