This invention relates to electronic filters, and more particularly, to variable bandwidth bandpass filters.
Electrically tunable filters have many uses in microwave and radio frequency systems. Compared to mechanically and magnetically tunable filters, electronically tunable filters have the important advantage of fast tuning capability over wide band application. Because of this advantage, they can be used in the applications such as, by way of example and not by way of limitation, LMDS (local multipoint distribution service), PCS (personal communication system), frequency hopping, satellite communication, and radar systems.
Filters for use in radio link communications systems have been required to provide better performance with smaller size and lower cost. Significant efforts have been made to develop new types of resonators, new coupling structures and new configurations for the filters. In some applications where the same radio is used to provide different capacities in terms of Mbits/sec, the intermediate frequency (IF) filter""s bandwidth has to change accordingly. In other words, to optimize the performance of radio link for low capacity radios, a narrow band IF filter is used while for higher capacities wider band IF filters are needed. This requires using different radios for different capacities, because they have to use different IF filters. However, if the bandwidth of the IF filter could be varied electronically, the same configuration of radio could be used for different capacities which will help to simplify the architecture of the radio significantly, as well as reduce cost.
Traditional electronically tunable filters use semiconductor diode varactors to change the coupling factor between resonators. Since a diode varactor is basically a semiconductor diode, diode varactor-tuned filters can be used in various devices such as monolithic microwave integrated circuits (MMIC), microwave integrated circuits or other devices. The performance of varactors is defined by the capacitance ratio, Cmax/Cmin, frequency range, and figure of merit, or Q factor at the specified frequency range. The Q factors for semiconductor varactors for frequencies up to 2 GHz are usually very good. However, at frequencies above 2 GHz, the Q factors of these varactors degrade rapidly.
Since the Q factor of semiconductor diode varactors is low at high frequencies (for example,  less than 20 at 20 GHz), the insertion loss of diode varactor-tuned filters is very high, especially at high frequencies ( greater than 5 GHz). Another problem associated with diode varactor-tuned filters is their low power handling capability. Further, since diode varactors are nonlinear devices, their handling of signals may generate harmonics and subharmonics.
Commonly owned U.S. patent application Ser. No. 09/419,219, filed Oct. 15, 1999, and titled xe2x80x9cVoltage Tunable Varactors And Tunable Devices Including Such Varactorsxe2x80x9d, discloses voltage tunable dielectric varactors that operate at room temperature and various devices that include such varactors, and is hereby incorporated by reference. Compared with the traditional semiconductor diode varactors, dielectric varactors have the merits of lower loss, higher power-handling, higher IP3, and faster tuning speed.
High power amplifiers are also an important part of any radio link. They are required to output maximum possible power with minimum distortion. One way to achieve this is to use feed forward amplifier technology. A typical feed forward amplifier includes two amplifiers (the main and error amplifiers), directional couplers, delay lines, gain and phase adjustment devices, and loop control networks. The main amplifier generates a high power output signal with some distortion while the error amplifier produces a low power distortion-cancellation signal.
In a typical feed forward amplifier, a radio frequency (RF) signal is input into a power splitter. One part of the RF signal goes to the main amplifier via a gain and phase adjustment device. The output of the main amplifier is a higher level, distorted carrier signal. A portion of this amplified and distorted carrier signal is extracted using a directional coupler, and after going through an attenuator, reaches a carrier cancellation device at a level comparable to the other part of the signal that reaches carrier cancellation device after passing through a delay line. The delay line is used to match the timing of both paths before the carrier cancellation device. The output of carrier cancellation device is a low level error or distortion signal. This signal, after passing through another gain and phase adjustment device, gets amplified by the low power amplifier. This signal is then subtracted from the main distorted signal with an appropriate delay to give the desired non-distorted output carrier.
Traditionally, delay lines have been used to give the desired delay and provide the above-described functionality. However, delay filters have become increasingly popular for this application because they are smaller, easily integrated with other components, and have lower insertion loss, as compared to their delay line counterpart. A fixed delay filter can be set to give the best performance over the useable bandwidth. This makes the operation of a feed forward amplifier much easier, as compared to the tuning of a delay line, which simulates adjustment of the physical length of a cable. However, fixed delay filters still have to be tuned manually.
There is a need for high performance, small size tunable bandwidth filters for wireless communications applications, as well as other applications. There is a further need for electronically tunable delay devices.
A tunable electrical filter constructed pursuant to the teachings of the present invention includes: (a) a plurality of resonator units coupled between an input locus and an output locus; and (b) a plurality of tunable dielectric varactor units; respective individual varactor units of the plurality of varactor units being coupled between respective pairs of the plurality of resonator units, coupled between the plurality of resonator units and the input locus, and coupled between the plurality of resonator units and the output locus.
A method for delaying an electrical signal includes the steps of: (a) Providing a plurality of resonator units coupled between an input locus and an output locus. (b) Providing a plurality of tunable dielectric varactor units. Respective individual varactor units of the plurality of varactor units are coupled between respective pairs of the plurality of resonator units, coupled between the plurality of resonator units and the input locus, and coupled between the plurality of resonator units and the output locus. Each respective individual varactor unit includes a substrate, a layer of voltage tunable dielectric material established in a first land on the substrate, a first electrode structure for receiving an electrical signal established in a second land on the first land, and a second electrode structure for receiving an electrical signal established in a third land on the first land. The first land and the second land are separated by a gap. (c) Applying the electrical signal to the input locus. (d) Applying a respective tuning voltage to the first electrode structure and the second electrode structure of each respective varactor unit. Each respective varactor unit exhibits a respective capacitance, the respective capacitance varying in response to the respective tuning voltage. (e) Receiving an output signal at the output locus. The output signal is delayed with respect to the electrical signal.
Tunable bandpass filters constructed in accordance with this invention include first and second resonators, an input, a first tunable dielectric varactor connecting the input to the first resonator, an output, a second tunable dielectric varactor connecting the second resonator to the output, and a third tunable dielectric varactor connecting the first and second resonators. The capacitance of the varactors can be controlled by applying a tuning voltage to each of the dielectric varactors, wherein the capacitance of each of the dielectric varactors varies substantially linearly with the tuning voltage.
Changing the capacitance of the tunable bandpass filters also changes the delay of signals passing through the filters. This makes the filters suitable for use in devices requiring a tunable delay function. Thus the invention also encompasses the use of such filters as tunable delay devices. In particular, the invention also encompasses a method of delaying an electrical signal, the method comprising the steps of: providing first and second resonators, an input, a first tunable dielectric varactor connecting the input to the first resonator, an output, a second tunable dielectric varactor connecting the second resonator to the output, and a third tunable dielectric varactor connecting the first and second resonators; coupling the electrical signal to the input; and extracting a delayed version of the electrical signal at the output.
The invention further encompasses a feed forward amplifier comprising an input port, a signal splitter coupled to the input port for producing first and second signals, a main amplifier for amplifying the first signal to produce an amplified first signal, a voltage tunable dielectric varactor delay filter for delaying the second signal to produce a delayed second signal, a first combiner for combining a portion of the amplified first signal with the delayed second signal to produce an error signal, an error signal amplifier for amplifying the error signal to produce an amplified error signal, and a second combiner for combining the amplified first signal and the amplified error signal. The voltage tunable dielectric varactor delay filter comprises an input, a first resonator, a first tunable dielectric varactor connected between the input and the first resonator, an output, a second resonator, a second tunable dielectric varactor connected between the output and the second resonator, and a third tunable dielectric varactor connected between the first resonator and the second resonator.