Wideband tunable monolithic inductor

Abstract: A wideband tunable monolithic inductor has a conical construction that presents very low parasitic capacitances. The frequency response of the inductor is determined by the diameter of the smallest coil at the tip for high frequency cutoff and by the total inductance for low frequency cutoff.

Description: A wideband inductor comprising a mandrel, having a conical section, of a dielectric material and an electrically conductive wire wrapped at least around the conical section to form contiguous coils of wire of increasing diameter. In a conventional, cylindrically constructed inductor the inter-winding capacitance in conjunction with the individual inductances of each coil winding produces many resonances throughout a frequency range for the inductor, as shown on the Smith chart of Fig. 1. The Smith chart is a circular plot of the impedance of a transmission line, such as is formed by an inductor, in terms of the resistive and reactive components over a frequency range of interest. The loops in the Smith chart occur at frequencies where the parasitic capacitances of the inductor form resonances with the coil windings. Existing inductors that operate into high frequency use the technique of circuit synthesis where a small series inductor is used to buffer out the parasitic capacitances from a large value, low frequency inductor. Sometimes several inductors are connected in series, each optimized for a narrow band of frequency, the series of inductors yielding an equivalent wideband inductor. One conventional variable inductance device is disclosed in US-A-2,005,203. This disclosed inductance device comprises a conical form of insulating material on which an inductance coil is wound. Thereby contiguous coils of wire of increasing diameter are formed. In addition, US-A-2,442,776 discloses a radio frequency choke coil comprising a continuous conductor wound in spiral and conical form, every helix thereof being longitudinally spaced from all adjacent helices, the helices thereof progressively increasing gradually and regularly in diameter from one end to the other and longitudinal spacing between successive helices thereof decreasing gradually and regularly in the same direction, whereby said coil is provided with a complete and continuous enclosing, insulating and stiffening layer. Neither of said disclosed means are free of resonances throughout a frequency range. Therefore, it is an object of the invention to provide the above-mentioned wideband inductor, having a wideband characteristic with a very small amount of resonances, showing a low parasitic capacitance and simultaneously being damped against vibrations. This problem is solved for the above-mentioned wideband inductor in that said conical section comprises means for securing the electrically conductive wire at the tip thereof and that said mandrel further comprises a cylindrical section contiguous with the base of and axially aligned with the conical section, a base section contiguous with the cylindrical section having means for securing the mandrel to a circuit board; whereby the mandrel securing means comprises a spacer; and a means for connecting the spacer between the base section and the circuit board to secure the wideband inductor fixedly to the circuit board. Accordingly the present invention provides a wideband tunable monolithic inductor of conical and cylindrical construction having low parasitic capacitances and a very small amount of resonances. The mandrel of a dielectric material has a slot in the tip to secure one end of an electrical wire to be wound upon the mandrel. The wire is wrapped around the mandrel in contiguous windings to achieve the desired inductance. Also a base on the mandrel provides means for mounting the inductor on a circuit board by inserting the tip of the inductor in a hole in the circuit board adjacent a lead and supporting the inductor with spacers between the base and the circuit board. A slug may be mounted in the core of the mandrel to tune the inductance as needed. These and further objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawings. Fig. 1 is a Smith chart for a cylindrically constructed inductor of the prior art. Fig. 2 is a side plan view of a wideband tunable monolithic inductor according to the present invention mounted on a circuit board. Fig. 3 is a bottom plan view of a mandrel for the wideband tunable monolithic inductor of Fig. 2. Fig. 4 is a Smith chart for the wideband tunable monolithic inductor of the present invention. Description of the Preferred Embodiment Referring now to Figs. 2 and 3 a wideband tunable monolithic inductor 10 has a mandrel 11 with a conical section 12, a cylindrical section 14 contiguous with the conical section 12, and a base 16 contiguous with the cylindrical section 14. The tip 18 of the conical section 12 has a hole or slot 20 for securing one end of a wire 22. The wire 22 is wrapped around the mandrel 11 in contiguous windings around the conical section 12 and a contiguous portion of the cylindrical section 14 to achieve a desired total inductance. The wire 22 is secured to the mandrel 11 by suitable means, such as gluing. An axial hole 24 in the cylindrical section 14 through the base 16 is threaded to accept an appropriate slug for tuning the inductor 10 to a desired inductance, as is well known in the art. The mandrel 11 is of a dielectric material, such as styrene. The base 16 has holes 26 that are used in mounting the inductor 10 on a circuit board 28. The inductor 10 is mounted on the circuit board 28 by inserting the tip 18 into a suitable hole 30 in the circuit board 28. Spacers 32 are placed between the base 16 and the circuit board 28 and suitable means, such as nuts and bolts or screws, are inserted through the base holes 26 and spacers 32 to secure the inductor 10 to the circuit board 28. A short end of the wire 22 from the tip 18 is electrically connected to a conductive lead 34 on the circuit board 28, and a long end of the wire 22 from the cylindrical section 14 is electrically connected to another conductive lead 34 by conventional means, such as soldering. The conical construction presents a very small inductance and, consequently, a very small inter-winding capacitance at the tip 18, providing a high impedance at a very high frequency, such as 6-10 gigahertz. As the coil windings gradually increase in diameter, the frequency presented with a high impedance decreases. At very low frequencies the inductor 10 behaves like an ideal inductor, i.e., the reactive component of the impedance decreases linearly with decreasing frequency. The result is a wideband inductor with very small parasitic shunt capacitances at high frequency, and hence capable of delivering a high impedance at very high frequency. Theoretically with a point tip and infinitely small wire, the upper frequency limit is infinite. But the theoretical limit is constrained by the amount of current needed for a particular circuit application which in turn determines the minimum wire diameter and smallest possible first winding. Fig. 4 is a Smith chart for the inductor 10 over a range of frequencies from 45 MHz to 6 GHz. At 4.124175 GHz, indicated by marker #1, the resistive component of the impedance is 230 ohms while the reactive component is -772 ohms which compares with 15 and 70 ohms for the conventional cylindrical inductor of Fig. 1 at approximately the same frequency, also indicated by marker #1. The total inductance of the coil windings determines the lower cutoff frequency, and the diameter of the first coil winding, limited by the wire size, determines the highest cutoff frequency. Examples of applications for the inductor 10 are bias-T and split-path amplifiers using an LC diplexer network. In the bias-T amplifier the inductor is used to inject a DC bias signal, but at high frequencies the inductor is required to present a high impedance so that the AC signals are not degraded by the parasitic shunt capacitances of the inductor. In the split-path amplifier the diplexer network, having an inductor and a capacitor, is used to combine/split a signal from/into high and low frequencies. If the inductor looks like a low impedance at high frequencies, then the high frequency signals, AC-coupled through the capacitor, are loaded by the conventional inductor, resulting in overall signal degradation. However the conical inductor 10 provides a high impedance at high frequencies so that the high frequency signals are not loaded by the inductor, improving signal performance. Thus the present invention provides a wideband tunable monolithic inductor having a conical construction to present low parasitic capacitances and provide high impedances over a wide frequency range without appreciable amounts of resonances and which are very insensitive to vibrations.

Claims: A wideband inductor (10) comprising a mandrel (11) of a dielectric material, having a conical section (12) and an electrically conductive wire (22) wrapped at least around the conical section (12) to form contiguous coils of wire of increasing diameter, characterised in that said conical section (12) comprises means (20) for securing the electrically conductive wire (22) at the tip (18) thereof and that said mandrel (11) further comprises a cylindrical section (14) contiguous with the base of and axially aligned with the conical section (12), a base section (16) contiguous with the cylindrical section (14) having means for securing the mandrel (11) to a circuit board (28); whereby the mandrel securing means comprises a spacer (32); and a means for connecting the spacer (32) between the base section (16) and the circuit board (28) to secure the wideband inductor (10) fixedly to the circuit board (28). A wideband inductor as recited in Claim 1 further comprising means for tuning the inductance. A wideband inductor as recited in Claim 2, wherein the tuning means comprises a slug of a magnetic material adjustable within a cavity (24) along the axis of said mandrel (11).

Inventors: VEENENDAAL CORNELIS T, VEENENDAAL, CORNELIS T.
Applicants: TEKTRONIX INC, TEKTRONIX, INC.
Application year: 1990
Main classification: H01F21
IPC codes: H01F27, H01F19, H01F17, H01F21