Tuning Coil Structure

This invention relates to high frequency signal shielding such as is used for television receiver components and, in particular, to a shield assembly having reduced size and cost.

High frequency tuning coils, such as those used in video and audio intermediate frequency amplifiers for television receivers, require shielding from external signals which alter the electromagnetic fields and therefore the tuning of the coils. It is known to shield a coil by placing a thin metallic box or can around the coil. The box or can, connected to signal ground potential, presents a much lower reluctance path for the external electromagnetic fields than air, causing the unwanted external fields to be shunted to ground.

Shield structures of the known type are typically made large relative to the coil being shielded, so that the coil does not accidentally come into contact with the grounded shield structure. As a result, such shield structures require a relatively large amount of mounting area on an electrical circuit board. Small screen television receivers have correspondingly small cabinets, which limits the size of the circuit boards that may be used. Circuit board area must be allocated wisely, and large shield structures such as those previously described are therefore undesirable.

Typically, coils used in intermediate frequency (IF) amplifier stages need to be aligned to properly define the frequency response of the IF stage. This is commonly done by adjusting the position of a magnetically permeable slug which is located within the tuning coil winding frame or bobbin. The slug and bobbin are threaded so that rotation of the slug causes its longitudinal position within the bobbin and surrounding coil to change. This may be done by automatic alignment equipment which automatically rotates the slug until a desired tuning condition is attained. When automatic alignment equipment is used, a plastic sleeve may be used to surround the coil in order to provide mechanical protection and support for the alignment tool. Typically, the top of the sleeve includes a funnel-type entrance for the alignment tool which aids in guiding the alignment tool into proper position. Use of this sleeve increases the size of the coil structure and may require an increase in the size of the surrounding metal shield structure.

The present invention is directed to an arrangement for providing electromagnetic interference shielding for tuning coils that is reduced in size and cost with respect to the commonly used metal cap.

In accordance with the present invention, a tuning coil assembly comprises a bobbin on which is wound a plurality of wire turns forming a coil. A rigid nonmetallic coil support sleeve surrounds the coil and bobbin and generally conforms to its shape. The sleeve may include a funnel-type entrance for an alignment tool. Terminal members for insertion into a circuit board are attached to the coil alignment sleeve. A thin metallic coating is formed on the outer surfaces of the sleeve to provide electromagnetic shielding for the coil, the coating also overcoating the terminal members.

In the accompanying drawing, FIG. 1 is a cross-sectional elevational view of a tuning coil assembly constructed in accordance with the present invention;

FIG. 1A is a cross-sectional elevational view of a portion of the coil assembly of FIG. 1, illustrating an alternate embodiment; and

FIG. 2 is a cross-sectional elevational view of another embodiment of the tuning coil assembly of the present invention.

Referring to FIG. 1, there is shown a tuning coil assembly 10 comprising a bobbin 11, on which is wound a coil 12 comprising a plurality of wire turns. Bobbin 11 is illustratively shown as cylindrical, but may be of other shapes, such as square. The coil terminal ends may be soldered to circuit board terminals 9 and thereby to a utilization circuit. A magnetically permeable slug 13 is shown disposed within bobbin 11. Slug 13 and the cylindrical inner wall of bobbin 11 are correspondingly threaded so that rotational movement of slug 13 longitudinally positions it with respect to coil 12 and thereby determines the inductance of coil 12. The resonant frequency of the circuit to which coil assembly 10 is connected is thereby adjusted.

A rigid plastic coil support sleeve 14 is disposed around coil 12 and bobbin 11. Coil support sleeve 14, having a shape which allows it to be closely spaced or conform to coil 12 and bobbin 11, provides mechanical protection and support for coil 12. In order to cooperate with an adjustment tool of an automatic alignment machine, sleeve 14 comprises a funnel-type entrance arrangement 15 having a central aperture to allow access to the adjustable slug 13 by an alignment machine tool, such as a screwdriver. Funnel-type arrangement 15 guides the alignment machine tool into the aperture in the event the tool is slightly displaced with respect to the slug 13.

In accordance with the present invention, a thin metallic coating 20 is deposited or formed on sleeve 14, thereby creating an electromagnetic interference shield structure for coil 12.

To provide effective electromagnetic field shielding of coil 12, the conductive coating 20 on coil support sleeve 14 must be connected to ground potential. For this purpose, circuit board 22 has a conductive surface 23 connected to ground potential. In order to provide connection between coating 20 on coil support sleeve 14 and ground potential surface 23 on circuit board 22, coil support sleeve 14 incorporates terminal members 16 which, as shown in FIG. 1, may comprise metal legs driven or embedded into the plastic of sleeve 14. In order that good electrical contact is made between terminal members 16 and conductive coating 20, coating 20 is applied continuously over sleeve 14 and terminal members 16, essentially overcoating terminal members 16.

FIG. 1A illustrates an alternate embodiment in which terminal members 16 are formed as a part of a nonconductive coil support sleeve 14 and are coated by conductive coating 20 along with the rest of sleeve 14. The use of high temperature plastics, such as polysulfone or polyetherimide, for example, permits connection to conductive surface 23 by soldering directly to the conductive coating 20 on terminal members 16 of sleeve 14.

For illustrative purposes, metallic coating 20 may comprise a first layer of copper having a thickness in the range of 0.2-0.5 mils and a second thin layer of nickel, commonly referred to as a flash layer, having a thickness in the range of 0.005-0.01 mils. These metals may be plated onto the surface of sleeve 14 by dipping sleeve 14 into solutions of appropriate concentrations of the desired metals for predetermined periods of time. It is also possible to coat sleeve 14 via vacuum deposition techniques, in which the metal is vaporized in a vacuum and deposited onto an electrically charged sleeve 14.

FIG. 2 illustrates an alternate embodiment of the coil assembly 10 in which a coil support sleeve 21 is formed of an electrically conductive plastic, comprising a conventional plastic substrate, such as polypropylene, which incorporates metallic fibers or flakes. These fibers or flakes may be aluminum, carbon or stainless steel, for example. A structure of this type would not require a metallic coating in order to provide electromagnetic radiation shielding.

It may be possible, by controlling the choice of plating materials or their thicknesses, or by controlling the type or concentration of conductive fibers in a conductive plastic structure for coil alignment sleeve 14 or 21, to vary the conductivity of the shielding structure and hence vary the effective load resistance of the tuning coil. This would in turn vary the Q or selectivity of the associated resonant circuit. This may aid in designing tuning or amplifier circuits and may eliminate the need for a separate selectivity-determining load resistor that would otherwise be required.