Electronic tunable filters with dielectric varactors

#The present invention generally relates to electronic filters and, more particularly, to such filters that include tunable dielectric capacitors (dielectric varactors).

#One of most dramatic developing areas in communications over the past decade has been mobile and portable communications. This has led to continual reductions in the size of the terminal equipment such as the handset phone. Size reduction of the electronic circuits is progressing with the development of recent semiconductor technologies. However, microwave filters occupy a large volume in communications circuits, especially in multi-band applications. Multi-band applications typically use fixed filters to cover different frequency bands, with switches to select among the filters. Therefore, compact, high performance tunable filters are extremely desirable for these applications, to reduce the number of filters and simplify the control circuits.

#Electrically tunable filters are suitable for mobile and portable communication applications, compared to other tunable filters such as mechanically and magnetically tunable filters. Both mechanically and magnetically tunable filters are relatively large in size and heavy in weight. Electronically tunable filters have the important advantages of small size, lightweight, low power consumption, simple control circuits, and fast tuning capability. Electronically tunable filters can be divided into two types: one is tuned by tunable dielectric capacitors (dielectric varactors), and the other is tuned by semiconductor diode varactors. The dielectric varactor is a voltage tunable capacitor in which the dielectric constant of a dielectric material in the capacitor can be changed by a voltage applied thereto. Compared to semiconductor diode varactors, dielectric varactors have the merits of lower loss, higher power-handling, higher IP3, and faster tuning speed. Third intermodulation distortion happens when two close frequency signals (f1 and f2) are input into a filter. The two signals generate two related signals at frequencies of 2f2-f1 (say f3), and 2f1-f2 (say f4), in addition to the two main signals f1 and f2. F3 and f4 should be as low as possible compared to f1 and f2. The relationship between f1, f2, f3 and f4 is characterized by IP3. The higher the IP3 value is, the lower the third intermodulation. Considering the additional attributes of low power consumption, low cost, variable structures, and compatibility to integrated circuit processing, dielectric varactors are suitable for tunable filters in mobile and portable communication applications.

#FIG. 1 is a schematic diagram of a lumped element LC tunable filter constructed in accordance with one embodiment of the invention;

#FIG. 2 is a schematic diagram of a DC bias circuit for varactors used in the filters of this invention;

#FIG. 3 is a schematic diagram of another lumped element LC tunable filter constructed in accordance with the invention;

#Referring to the drawings, FIG. 1 is a schematic diagram of a three pole lumped element LC tunable filter 10 constructed in accordance with one embodiment of the invention. The filter includes an input 12 and an output 14. A plurality of resonant circuits 16, 18 and 20 are electrically coupled to the input and the output. Resonant circuit 16 includes inductor L1 and capacitor C1. Resonant circuit 18 includes inductor L2 and capacitor C2. Resonant circuit 20 includes inductor L3 and capacitor C3. Capacitor C4 couples resonant circuit 16 to the input 12. Capacitor C5 couples resonant circuit 16 to resonant circuit 18. Capacitor C6 couples resonant circuit 18 to resonant circuit 20. Capacitor C7 couples resonant circuit 20 to the output 14. Capacitors C1, C2 and C3 are tunable dielectric varactors. C4 and C7 are port coupling capacitors used to provide a specific port impedance, typically 50 ohms or 75 ohms. More or fewer resonators can be used in the filter to obtain specific filter rejection. Each of the tunable varactors is connected to a voltage bias circuit not shown in FIG. 1, but shown in FIG. 2 as bias circuit 22.

#FIG. 2 shows a voltage source 24 connected to varactor C1 through an inductor 26. A blocking capacitor 28 is electrically connected in series with the varactor. By varying the voltage supplied by source 24, the capacitance of the varactor changes. This enables tuning of the filter. The DC blocking capacitor is used to prevent the DC bias voltage from entering into the other parts of the filter. Inductor 26 works as an RF choke to prevent RF signal leaking into the bias circuit.

#FIG. 3 is a schematic diagram of another lumped element LC tunable filter 30 constructed in accordance with the invention. Filter 30 is similar to filter 10 of FIG. 1, except that capacitors fixed C4 and C7 in FIG. 1 have been replaced by varactors C8 and C9 in FIG. 3.