1. Statement of the Technical Field
The present invention relates to the field of phase delays, and more particularly to variable phase delays.
2. Description of the Related Art
Delay lines such as phase delays are used for a wide variety of signal processing applications. For example, broadband time delay circuits are used in beam-forming applications in phased array antennas. Typical fixed geometry, true time delay circuits used in phased array antennas are comprised of switched lengths of transmission line. Despite the importance of broadband delay lines in such systems, the conventional approach to designing and implementing these components suffer from a number of drawbacks. For example, conventional delay line devices often require a relatively large number of RF switches that can result in signal losses. Also, conventional time delay circuits can be limited with regard to the delay resolution that can be achieved.
RF delay lines are often formed as ordinary transmission lines coupled to a dielectric. Depending upon the structure of the transmission line, the dielectric can be arranged in different ways. For example, microstrip and stripline circuits commonly are formed on a dielectric substrate. Two important characteristics of dielectric materials are permittivity (sometimes called the relative permittivity or ∈r) and permeability (sometimes referred to as relative permeability or μr). The relative permittivity and permeability determine the propagation velocity of a signal, which is approximately inversely proportional to √{square root over (μ∈)}. The propagation velocity directly effects the electrical length of a transmission line and therefore the amount of delay introduced to signals that traverse the line.
Further, ignoring loss, the characteristic impedance of a transmission line, such as stripline or microstrip, is equal to √{square root over (L1/C1)} where L1 is the inductance per unit length and C1 is the capacitance per unit length. The values of L1 and C1 are generally determined by the permittivity and the permeability of the dielectric material(s) used to separate the transmission line structures as well as the physical geometry and spacing of the line structures. For a given geometry, an increase in dielectric permittivity or permeability necessary for providing increased time delay will generally cause the characteristic impedance of the line to change. However, this is not a problem where only a fixed delay is needed, since the geometry of the transmission line can be readily designed and fabricated to achieve the proper characteristic impedance.
When a variable time delay or phase delay is needed, however, such techniques have traditionally been viewed as impractical because of the obvious difficulties in dynamically varying the permittivity and/or permeability of a dielectric board substrate material and/or dynamically varying transmission line geometries. Variable length lines have been implemented using mechanical means to vary the length of a line. These generally have involved an arrangement of telescoping tubes to produce a variable length coaxial line. These devices were at one time commonly used in laboratories for tuning circuits. However, these arrangements suffered from certain drawbacks. For example, they were subject to wear, difficult to control electronically, and are not easily scalable to microwave frequencies. Accordingly, the only practical solution has been to design variable delay lines using conventional fixed length RF transmission lines with delay variability achieved using a series of electronically controlled switches.
The possibility of having an electronically steerable antenna system is of significant current importance for many communication technologies. Applications such as airport traffic control, satellite tracking for mobile communication system, and radar systems emphasize the importance of electronically steerable antennas. One design using ferroelectric material which dramatically reduces the size and the cost of the phase shifter used to achieve an electronically steerable antenna system can obtain a total beam scan of 36° for a two element microstrip antenna system while the side lobe level is kept below 10 dB, and the reflection coefficient at resonance frequency is maintained below 20 dB. Using thin ceramic ferroelectric materials for the design of phase shifters used in the realization of the electronically scanned antenna system employs two ferroelectric phase shifters in conjunction with two microstrip antennas operating at 2.1 GHz. The design goal was to obtain as much as possible beam scan, while minimizing the amplitude of the sides lobes and the reflection coefficient. Because for this prototype example only two microstrip elements are used, a maximum scan of 36° (18° on each side) can be achieved, if a side lobe level below 10 dB is desired. Increasing the total number of antenna elements, allows for larger scan for the same side lobe level. The use of ferroelectric material in the microwave frequency range has been limited in the past due to high losses of these materials and due to the high electric field necessary to bias the structure in order to obtain substantial dielectric constant change. Barium modified Strontium Titanium Oxide (Ba1-xSrxTiO3) is used in this example which has ferroelectric properties at room temperature. Use of thin ceramics also reduces the required bias voltage, with almost no power consumption required to induce a change in the dielectric constant. Still, ferroelectric materials requires a biasing voltage and associated circuitry to vary the dielectric constant.
In yet another alternative, a microwave phase shifter that can be tuned by varying both electric and magnetic fields combines all the advantages of prior electrically and magnetically tunable microwave devices. Devices like this one are suitable for use in monolithic microwave integrated circuits.
This microwave phase shifter is a thin-film ferroelectric/ferrite device. One can alter the propagation of electromagnetic waves in such a device by (1) varying an applied electric field and thereby varying the permittivity of the ferroelectric layer and/or (2) varying an applied magnetic field and thereby varying the permeability of the ferrite layer.
This microwave phase shifter has a layered structure as the main component of a phase-shifting circuit. The substrate is a polycrystalline yttrium iron garnet (YIG) ferrite material. In the fabrication of the device, buffer layers of Si3N4 and MgO were deposited on the substrate, then the ferroelectric layer was formed by ion-beam-assisted deposition of Ba0.6Sr0.4TiO3 on the MgO. Then a transmission line comprising a central strip and two lateral ground-plane strips was patterned on an electron-beam-evaporated gold film.
In tests of this device, significant phase shift was observed at frequencies up to 18 GHz when an electric bias or a magnetic field was applied. For example, at a bias of 250 V, phase shifts of 20° and 34° were observed at 7 and 9 GHz, respectively. When an externally generated magnetic field of 800 gauss was applied in tests at 5 and 6 GHz, phase shifts of about 230° were observed. As the magnetic field was increased beyond 800 gauss, the phase shift gradually saturated at about 300°. The ferroelectric film and ferrite substrate of this device have electrically variable permittivity and magnetically variable permeability, respectively. These characteristics can be exploited to control the phase shift between the input and output terminals. Again, this arrangement requires a biasing voltage and associated circuitry.
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