Tunable filter using variable impedance transmission lines

Abstract

Techniques for implementing tunable lumped element filters with transmission line sections. Transmission lines sections are used to implement one or more inductive or capacitance component elements of the filter. The filter is tunable by changing the dielectric constants of the transmission lines. In particular implementations there is an individual transmission line section for each lumped element component of a filter. Different filter circuits may be combined to provide a universal tunable filter assembly.

Claims

1. A tunable filter apparatus comprising: at least one capacitive element including a first transmission line section disposed adjacent a first dielectric material section having a dielectric constant; and at least one inductive element including a second transmission line section disposed adjacent a second dielectric material section having a dielectric constant; and an impedance controller connected to effect a change in dielectric constant of the second dielectric material section, the impedance controller configured to thereby change an impedance of the at least one inductive element to adjust the frequency response of the tunable filter.

2. The apparatus of claim 1 wherein the impedance controller effects a change in dielectric constant by controlling a voltage applied to at least the second dielectric material.

3. The apparatus of claim 1 wherein the at least one capacitive element and the at least one inductive element are connected in a filter topology configured as a low pass filter.

4. The apparatus of claim 3 wherein the at least one inductive elements includes at least two inductive elements, the at least two inductive elements are disposed in series between an input terminal and an output terminal of the tunable filter apparatus.

5. The apparatus of claim 4 wherein the at least one capacitive element includes at least two capacitive elements, the at least two capacitive elements are disposed in shunt with respect to the inductive elements.

6. The apparatus of claim 1 wherein the first and the second transmission line sections and the first and the second dielectric material sections are disposed on a printed circuit board having a ground plane.

7. The apparatus of claim 6 wherein the at least one inductive element further comprises a conductive via coupling the second transmission line section to the ground plane.

8. The apparatus of claim 1 additionally comprising: an input switch having a single input terminal and multiple output terminals; an output switch having multiple input terminals and a single output terminal; a set of filter sections, each filter section coupled between a respective one of the multiple output terminals of the input switch and a respective one of the multiple input terminals of the output switch, the filter sections comprising two or more of a low pass filter section formed from a second set of one or more capacitive elements and a second set of one or more inductive elements; a band pass filter section formed from a third set of one or more capacitive elements and a third set of one or more inductive elements; a high pass filter section formed from a fourth set of one or more capacitive elements and a fourth set of one or more inductive elements; a band stop filter section formed from a fifth set of one or more capacitive elements and a fifth set of one or more inductive elements; and a notch pass filter section formed from a sixth set of one or more capacitive elements and a sixth set of one or more inductive elements; and a controller for selecting a state for the input switch and output switch.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The description below refers to the accompanying drawings, of which:

(2) FIGS. 1A and 1B are a schematic diagram of a tunable low pass filter.

(3) FIG. 2 is an example frequency response for the low pass filter of FIGS. 1A and 1B for various configurations.

(4) FIGS. 3A, 3B and 3C are a schematic diagram of a tunable bandpass filter.

(5) FIGS. 4 and 5 are the frequency response for the bandpass filter of FIG. 3.

(6) FIG. 6 is an example printed circuit board implementation of certain parts of the filters of FIGS. 1A, 1B and 3A, 3B and 3C.

(7) FIG. 7 is a high-level block diagram of a universal filter implemented using a low pass filter, bandpass filter, high pass filter, band stop filter and notch filter, each filter implemented using the techniques described herein.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

(8) As described in more detail below, a tunable microwave frequency filter may be implemented using variable impedance transmission line sections. The variable impedance transmission line sections may be used to synthesize one or more discrete elements in a lumped element filter. For example, a first variable transmission line section may be used to synthesize a variable capacitor, and a second variable transmission line section may be used to synthesize a variable inductor. The overall frequency response of the filter is then tuned by adjusting one or more of the components. In one embodiment, the impedance of the transmission line sections may be adjusted by changing the electromagnetic wave propagation constant of a dielectric material used to implement the transmission line sections. In one such implementation, the transmission line sections may be fabricated as conductive tracks on a printed circuit board, with the printed circuit board having one or more dielectric layers having a dielectric constant that can be adjusted.

(9) The filters may be any desired topology, class or type such as low pass, bandpass, band stop, high pass or the like.

(10) More particularly now, distributed element filters can be obtained by using multiple transmission line sections. When terminated by a load impedance, Z.sub.L, a transmission line with characteristic impedance Z.sub.o has an input impedance Z.sub.in defined by the following equation:

(11) $Z_{in}=Z_o\frac{\left(Z_L+j{Z}_o\tan \theta \right)}{\left(Z_L-j{Z}_o\tan \theta \right)}$
where θ is the electrical length of the transmission line section.

(12) It is observed that: For transmission line sections implemented on a printed circuit board (PCB),

(13) $\theta =\frac{2\pi *\mathrm{freq}*\sqrt{\mathrm{Er}}}{C}*L$
where Er is the dielectric constant of a material layer disposed between a conductive strip and a ground plane, freq is the operating frequency, “C” is the speed of light, and L is the physical length of the transmission line section. If Z.sub.L=0 then Z.sub.in=jZ.sub.0*tan θ; thus a shortened line shares the same input impedance characteristic as an inductor (Z.sub.in=jω*ind) If Z.sub.L∞ then

(14) $Z_{in}=\frac{z_0}{j\tan \theta };$
thus an open line behaves like a capacitor.

(15) From the above derivations, it is understood that capacitors and inductors having different impedances can be synthesized with transmission line sections having different dimensions. That is, different values of capacitance

(16) $\left(Z_{in}=\frac{z_0}{j\tan \theta }\right)$
and inductance (Z.sub.in=jZ.sub.0*tan θ) can be synthesized with respective transmission line sections of different sizes, since

(17) $\theta =\frac{2\pi *\mathrm{freq}*\sqrt{\mathrm{Er}}}{C}*L,$
where L is the length of the variable Er section and I results in a corresponding change in Z.sub.in of a capacitor and inductor. Whether a given length of transmission line operates as an inductor or capacitor depends primarily on L. If L<λ/4, then the transmission line section is primarily capacitive, if L>λ/4, then it becomes primarily inductive. The ultimate impedance presented also depends on other dimensions of the transmission line section, such as width, W.

(18) Most any filter topology can therefore be built using inductors and capacitors implemented with transmission line sections configured in this way.

(19) FIGS. 1A and 1B are a schematic of one possible implementation of such a tunable filter exhibiting a low pass response. Input and output termination impedances are provided as 50 ohm resistors 101, 190. The filter consists generally of three L-sections, each section including a series inductance and shunt capacitance. Inductors are provided by variable transmission line sections 102, 103, 104, 105, 106 and 107. Capacitors are provided by variable transmission line sections 112, 113, 114, 115, 116, and 117. In addition, transmission line junction sections 121, 122, 123 are provided between the components.

(20) The characteristics of the individual transmission line sections are written next to each respective element. For example in the case of capacitor 113, the W=W.sub.up parameter indicates its width, the L=L.sub.up indicates its length, and substr=M.sub.sub1 indicates the type of substrate.

(21) In the example shown, the transmission line section dimensions are such that W.sub.stp.sub._.sub.d=17 mil, L.sub.stp.sub._.sub.d=170 mil, W.sub.in.sub._.sub.LPF=32 mil, L.sub.in.sub._.sub.LPF=80 mil, W.sub.out1=10 mil, and W.sub.up1=17 mil.

(22) An example junction 122 is a four port transmission line section that has widths W1, W2, W3, W4 clockwise from the topmost leg section adjacent capacitor 115.

(23) Although not shown in detail in FIGS. 1A and 1B, each adjustable transmission line section 101, 103, . . . , 117 is implemented with a length of metallic conductor line disposed over a dielectric substrate layer. The dielectric layer has a variable dielectric wave propagation constant which may be controlled, for example, by applying a voltage across it. Suitable dielectric materials may include Barium Strontium Titanate (BST) although other materials are possible.

(24) FIG. 2 is an expected frequency response diagram for the circuit of FIGS. 1A and 1B. A range of low pass cutoff frequencies, from about 1.75 GHz to about 2.3 GHz is seen to be achievable for different values of E.sub.r ranging from 1.5 to 3.0.

(25) FIGS. 3A, 3B and 3C are a bandpass filter (BPF) implemented using the same techniques as the low pass filter of FIGS. 1A and 1B circuit. This band pass filter consists of a number of transmission line inductor sections 301, 302, 303, 306, 307 and 308. Transmission line capacitor sections include 311, 312, 313, 318, 319, 320, 321, and 322. Junctions 331 and 332 and 333 are also provided as well as couplers 304-1, 304-2, 304-3, 304-4 to interconnect the lumped elements. Also provided in this particular design is a discrete inductor 350 and corners 360.

(26) Components 315 and 316 are stepped impedance lines, used to match input and output impedance. In the example of component 315 and 316, at one end the trace width is 28 mil and at the other end 43 mil. Components 340 and 341 are similar stepped lines, but 60 mil wide at one end, and 10 mil wide at the other end.

(27) This BPF is a particular filter topology (e.g., Chebyshev II) although other filter topologies can be realized.

(28) FIG. 4 is a filter response for the bandpass design of FIGS. 3A, 3B and 3C, here tuned to a particular frequency of 7.015 GHz, for a substrate Er of 3.38.

(29) FIG. 5 is a course sweep of the filter design of FIG. 4 obtained by varying the dielectric constant 1.5 to 7, in steps of 0.5. The filter is expected to provide good performance between approximate 5 GHz and a 9 GHz with at least 25 dB of isolation.

(30) FIG. 6 illustrates different types of transmission line sections in more detail. Here is seen that a PCB material, laminate, or some other sort of dielectric material substrate 610 is provided with or on a ground plane 611. At least one layer 610 of the substrate is formed of a dielectric having a propagation constant which may be varied such as by impressing a DC voltage across it.

(31) Conductive sections such as formed of copper 612 are placed on the top layer.

(32) A first conductive trace 601 may be one or more straight or meandering line conductors with a via 602 through to the PCB material 610 to the ground plane 611. This straight section with via 602 acts primarily as an inductor. Along with the coupling capacitor(s) 607 between parallel traces, this forms an LCLCL network (see e.g., element 103 of FIG. 1).

(33) Another element 604 is constructed as two conductive traces with a gap between them, and thus implements a capacitive section with capacitor value being a function of the spacing between the lines (e.g., element 112 of FIG. 1A).

(34) Other transmission line sections such as section 603 implement a junction and act as a capacitor element (e.g., element 122 of FIG. 1A).

(35) An adjustable DC voltage source 650 controls a voltage applied to control the propagation constant of the dielectric layer 610.

(36) FIG. 7 illustrates a universal filter 700 that can be constructed by combining is different types of filters in a single assembly. In this example, there may be a tunable band pass filter 701, a tunable low pass filter 702, a tunable high pass filter 703, a tunable band stop filter 704, and a tunable notch filter 705, each implemented with variable transmission line sections as described above. An input switch 710 and output switch 712 may control which filter 701, 702, 703, 704, 705, etc. to which an input signal 720 and output signal 722 are applied. The resulting assembly is a bidirectional such that input and output can be interchangeable.

(37) A dielectric constant controller 750 may apply a DC voltage to the dielectric substrate to set its desired dielectric constant property.

(38) The desired filter function is then chosen by setting the single pole, multiple throw switches 710, 712, to provide input and output connections to the desired filter section. The assembly is conveniently packaged in a common housing 730 so that the device becomes a two terminal RF device i.e., needing only RF input and RF output connections, switch control input settings, and DC voltage to control the transmission line dielectric (not shown).