Patent Application
20070030100
The present invention generally relates to tunable filters, tunable dielectric capacitors, and, more particularly, this invention relates to a voltage-controlled LTCC based tunable filter.
Electronically tunable microwave filters have found wide applications in microwave systems. Compared to mechanically and magnetically tunable filters, electronically tunable filters have the most important advantage of fast tuning capability over a wide band application. Because of this advantage, they can be used in applications such as cellular, PCS (personal communication system), Point to Point, Point to multipoint, LMDS (local multipoint distribution service), frequency hopping, satellite communication, and radar systems. Electronically tunable filters can be divided into two types: one is a dielectric capacitor based tunable filter and the other is semiconductor varactor based tunable filter. Compared to the semiconductor varactor based tunable filters, tunable dielectric capacitor based tunable filters have the merits of lower loss, higher power-handling, and higher IP3, specifically at higher frequencies.
Tunable filters have been developed for radio frequency (RF) applications. They are tuned electronically by using either dielectric varactors or Micro-electro-mechanical systems (MEMS) based varactors. Tunable filters offer service providers flexibility and scalability, which were never possible before. A single tunable filter solution enables radio manufacturers to replace several fixed filters covering adjacent frequencies. This versatility provides front-end RF tunability in real time applications and decreases deployment and maintenance costs through software controls and reduced component count. Also, fixed filters need to be wide band so that the total number of filters to cover a desired frequency range does not exceed reasonable numbers. Tunable filters, however, are narrow band and maybe tuned in the field by remote command. Additionally, narrowband filters at the front end are superior from the systems point of view, because they provide better selectivity and help reduce interference from nearby transmitters. Two of such filters can be combined in diplexer or duplexer configurations.
Inherent in every tunable filter is the ability to rapidly tune the response using high-impedance control lines. The assignee of the present invention has developed and patented tunable filter technology such as the tunable filter set forth in U.S. Pat. No. 6,525,630 entitled, “Microstrip tunable filters tuned by dielectric varactors”, issued Feb. 25, 2003 by Zhu et al. This patent is incorporated in by reference. Also, patent application Ser. No. 09/457,943, entitled, “ELECTRICALLY TUNABLE FILTERS WITH DIELECTRIC VARACTORS” filed Dec. 9, 1999, by Louise C. Sengupta et al. This application is incorporated in by reference.
The assignee of the present invention and in the patent and patent application incorporated by reference has developed the materials technology that enables these tuning properties, as well as, high Q values resulting low losses and extremely high IP3 characteristics, even at high frequencies. The elaboration of the novel tunable material technology is elaborated on in the patent and patent application incorporated in by reference.
Also, tunable filters based on MEMS technology can be used for these applications. They use different bias voltages to vary the electrostatic force between two parallel plates of the varactor and hence change its capacitance value. They show lower Q than dielectric varactors, but can be used successfully for low frequency applications.
Thus, there is a strong need in the communications industry to provide several layers of dielectric material or low-temperature-co fired-ceramic (LTCC) tape based electronically tunable multilayer microstrip-stripline combline filter operable over a wide frequency band and that is small in size.
An embodiment of the present invention provides an apparatus, comprising a multilayer filter including a first resonator on a first layer of dielectric material or low-temperature-co fired-ceramic, a second resonator coupled to the first resonator on a second layer of dielectric material or low-temperature-co fired-ceramic, a third resonator coupled to the second resonator and cross coupled to the first resonator, and wherein a voltage tunable dielectric capacitor is connected to at least one of the resonators to electrically tune the multilayer filter.
It is an object of the present invention to provide a voltage-tuned filter having a very small size, low insertion loss, fast tuning speed, high power-handling capability, high IP3 and low cost in the RF and microwave frequency range. Compared to voltage-controlled semiconductor varactors, voltage-controlled tunable dielectric capacitors have higher Q factors, higher power-handling capability and higher third order intercept point (IP3). Voltage-controlled tunable diode varactors or voltage controlled MEMS varactors can also be employed in the filter structure to achieve the goal of this object, although with decreased performance. Yet another object of the present invention is to have a compact filter capable of being tuned over the three transmit bands of a wireless handset application.
A first embodiment of the present invention provides for a tunable filter in a low-temperature-co fired-ceramic (LTCC) package. The tuning elements are preferably voltage-controlled tunable dielectric capacitors or, in a less preferred alternate embodiment, MEMS varactors placed on the resonator lines of each filter. Since the tunable dielectric capacitors show high Q, high IP3 (low inter-modulation distortion) and low cost, the tunable filter in the present invention has the advantage of low insertion loss, fast tuning speed, and high power handling. It is also low-cost and provides fast tuning. The present technology makes tunable filters very promising in the contemporary communication system applications.
The tunable dielectric capacitor in the present invention is made from a low loss tunable dielectric film. The range of Q-factor of the tunable dielectric capacitor is between 50, for very high tuning material, and 300, for low tuning materials. It decreases with the increase of the frequency, but even at higher frequencies, say 30 GHz, can have values as high as 100. A wide range of capacitance of the tunable dielectric capacitors is available; for example, 0.1 pF to several pF. The tunable dielectric capacitor is a packaged two-port component, in which the tunable dielectric can be voltage-controlled. The tunable film is deposited on a substrate, such as MgO, LaAIO3, sapphire, Aha3 and other dielectric substrates. An applied voltage produces an electric field across the tunable dielectric, which produces an overall change in the capacitance of the tunable dielectric capacitor.
The tunable capacitors based on MEMS technology can also be used in the tunable filter and are part of this invention. At least two varactor topologies can be used, parallel plate and interdigital. In the parallel plate structure, one of the plates is suspended at a distance from the other plate by suspension springs. This distance can vary in response to electrostatic force between two parallel plates induced by applied bias voltage. In the interdigital configuration, the effective area of the capacitor is varied by moving the fingers comprising the capacitor in and out and changing its capacitance value. MEMS varactors have lower Q than their dielectric counterpart, especially at higher frequencies, but can be used in low frequency applications.
The tunable filter with asymmetric response consists of combline resonators implemented in microstrip-stripline form. In a preferred embodiment it can be a 3-pole tunable combline filter as described below. Variations of the capacitance of the tunable capacitor affect the distribution of the electric field in the filter, which in turn varies the resonant frequency.
The combline resonators are implemented in stripline as well as microstrip line form. The filter needs several layers of dielectric material or low-temperature-co fired-ceramic (LTCC) tape. In one preferred embodiment, a three-pole filter is realized using LTCC tapes, although it is understood that design choice would dictate the number of poles and number of layers provided and it is understood that any number of poles or layers are included in the scope of the present invention. The present preferred embodiment of the present invention and the one described below is a filter that uses a total of nine tape layers.
Turning now to
At 179 is an isolation in the bottom layer 110 for RF/IP port 165 and at 180 is a via connecting an inner stripline to the bottom ground plane 110. At 181 are thruholes for the left-side DC bias via and at 183 is an inner stripline portion of the microstrip-stripline resonator. Thruholes for center DC bias via is provided at 185. At 190 is a via connecting upper microstrip to upper internal ground plane 110 and at 199 is a via connecting inner stripline to bottom ground plane 110.
Turning now to
Referring now to
Referring now to
The regular combline resonator is roughly one eighth of a wavelength. If the combline resonator is implemented in one layer, the filter size is generally large. Therefore, the comb line resonators in the present invention are implemented in multilayer topology to miniaturize the filter. To achieve better Q from the resonator structure, the good portion of the resonator has been implemented in the stripline form. The stripline portions of the resonators are shown in
The striplines go though apertures in the top ground plane (layer 6) to the top layer of the board. The microstrip portions of the resonators are folded back as shown in
Turning now to
While the present invention has been described in terms of what are at present believed to be its preferred embodiments, those skilled in the art will recognize that various modifications to the disclose embodiments can be made without departing from the scope of the invention as defined by the following claims.
| Number | Date | Country | |
|---|---|---|---|
| 60445351 | Feb 2003 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10757314 | Jan 2004 | US |
| Child | 11546808 | Oct 2006 | US |
