Preselector Cavity Arrangement and RF Selectivity Assembly

This invention relates to the field of RF shielding and particularly to a housing arrangement which will provide satisfactory shielding for the RF circuit modules to be mounted on a PC board of a miniature receiver.

More specifically the invention relates to an arrangement of interconnected housings for the prevention of spurious RF radiation.

A major problem in the design of UHF receivers is preventing spurious RF energy from bypassing the filtering circuits and entering the circuit at a point where only a narrow band of frequencies is desired. Shielding has previously been accomplished by enclosing the entire circuit in one complex housing to control spurious energy. Other prior art approaches have included separate housings with threaded interconnectors. All such solutions suffered from either high cost, difficulty of access or both.

It is, therefore, an object of the invention to provide improved shielding for the RF circuits of a receiver with a housing arrangement which is inexpensive, easy to fabricate and which allows easy access for repair. Another object is to provide an improved means of coupling from the RF amplifier coil to the preselector input. These objectives are accomplished according to the present invention by the use of a central case housing for the preselector coils in the form of a regular parallelepiped with one side removable. The interior of this housing is partially compartmentalized by partitions forming Faraday shields between coils. Interconnections with the preselector are done by means of apertures in the sides of the housing, each of which is surrounded or substantially surrounded by a boss. Each adjoining circuit module is housed in a shield can having a slot which is dimensioned to fit around one of the bosses. Because of the bosses, the tolerances for the can slots need not be excessively close though maintaining tight shielding. The preselector input wire is formed into a loop which passes through an aperture into the RF amplifier housing, slips down around the RF amplifier output coil and returns to ground at a notch in the surrounding boss. The high impedance connection of the second pole of the injection filter is coupled from a terminal supported by the filter coil form, through a second aperture to a tap on the preselector output coil. The preselector output coil and injection filter are coupled through a third aperture to a terminal on the tripler/mixer module. All interconnections are thus made without bringing leads to the PC board where spurious radiation can enter the system. After all modules have been completely assembled, a grounding strap is soldered across the tops of all the housings to complete the shielding. The plated ground connections on the PC board under or near the housing arrangements are enlarged to cover all area not needed for tuning holes, etc., thus forming a large Faraday shield under the whole housing arrangement.

FIG. 1 is an exploded view of a preferred embodiment of a housing arrangement according to the invention with a portion of a PC board for mounting the arrangement.

FIG. 2 is a detail of one of the structure interconnections of FIG. 1.

FIG. 3 is a schematic diagram of an RF selectivity circuit such as might utilize the invention.

The structure of the housing arrangement according to the invention is best understood in relation to FIGS. 1 and 2. A typical RF circuit, which might form a part of the assembly as used, is shown in FIG. 3 but, apart from its shielding requirements, does not form a part of the invention.

In FIG. 1 a portion of a PC board 10 is shown with some of the parts of the RF selectivity assembly to be mounted on it. A main preselector housing portion 11 and a mating preselector housing side 12 are shown with some components of the four pole preselector circuit 13 (FIG. 3) visible within the main portion. The RF amplifier housing 14 has been raised to show one of the mounting tabs 15, a tuning aperture 16, an output coil 17, a coupling loop 18 of the RF amplifier circuit 19 (FIG. 3) and one of six mounting holes 20 in the PC board 10. The injection filter housing 21 is elevated to show the placement of the coil 22 of the injection filter circuit 23 (FIG. 3). Supported by the coil form of coil 22 is a terminal 24 to which the capacitors 25, 26 (FIG. 3) are connected. Also elevated from its mounting position is the housing 27 for the tripler/mixer circuit 28 which in this embodiment is encapsulated. Three slots 30 in the adjoining housings (FIGS. 1 and 2) fit over three bosses 31 (one visible). The boss at the tripler/mixer end of the preselector housing portion 11 is a mirror image of the boss for the RF amplifier interconnection, except that in the latter boss a small notch 32 is positioned for receiving a ground end 33 (FIG. 3) of the coupling loop 18. In this embodiment, the loop 18 consists of three quarters of a turn. Each of the two end bosses 31 includes a slot 34 which allows easier insertion of the circuit 13 components. When subassembly of the main preselector housing 11 and the preselector circuit 13 is completed, the mating side 12 is soldered into place with two tabs 35 (one shown) fitting partially into the slots 34 to complete the bosses shielding the end interconnections.

FIG. 2 shows more clearly the details of the housing interconnection with cut away portions of main part 11 of the cast housing and the RF amplifier housing 14. Shown in this view are one of two mounting pins 36 and one of four tuning apertures 37 in the cast housing part 11. More visible in this view is one of three partitions 38 which will be discussed hereinafter. Visible here in the RF amplifier housing 14 are the slot 30 and a portion of the Teflon coating 36 which lines the three housings 14, 21 and 27 for electrical insulation. With referenece to both FIGS. 1 and 2, it is clear that when one of the housings 14, 21 or 27 is properly in place next to a side of the main preselector housing part 11, the slot 30 will be fitted around the boss 31 and any connection between the two housings will be completely shielded. Since each boss 31 protrudes into a slot 30, the tolerances on the dimensions of the slot 30 are relatively broad and each adjoining housing can, as a whole, be more inexpensively produced than if mating slots in the sides of two housings had to be carefully aligned.

A circuit for a receiver front end such as would require housing and shielding according to the invention is shown in FIG. 3, and is made up essentially of the RF amplifier circuit 19, the preselector circuit 13, the injection filter circuit 23 and the tripler/mixer circuit 28. An input terminal 40 is coupled to the antenna which may be the housing cover (not shown) of the receiver. The coupling circuitry (not shown) would include preliminary filtering of the input signal. A B+ terminal 41 is filtered to prevent signal input at that point. In the RF circuit 19 the RF signal is amplified and filtered, then coupled to a first interconnection point 42 which is the input to the preselector circuit 13. Low impedance coupling to the preselector is accomplished by the three quarter turn coil 18 which, as shown in FIG. 1, passes through the slot 34 in the main housing portion 11, passes around the supporting form of coil 17 and is grounded in the notch 32 in the adjacent boss 31.

In the housing of the four pole preselector 13, four cavities are defined by the inner surface of the housing portions 11 and 12, and the partitions 38. The coupling between the four poles is electrostatic and is determined by the dimensions of a plurality of ports 43. Each cavity is a capacity in parallel with the circuit contained therein, and together with the variable capacitor in series with the coil, provides the required filtering capability of the preselector. At a terminal 44, the output of a crystal oscillator circuit (not shown) is brought into the injection filter circuit 23. This oscillator output, which is rich in harmonics, is sharply filtered in circuit 23 to provide only the third harmonic of the oscillator frequency at a second interconnection point 45 between the circuits 23 and 13. This point, then, is one of the most critical points at which the entrance of spurious RF spray radiation must be prevented. This interconnection is made through the aperture 34 which need not be a slot, but is surrounded by the boss 31 which, in use, is enclosed by the housing 21 via one of the slots 30. Connection is made to the terminal 24. A third interconnection point 46, between the preselector circuit 13 and the tripler/mixer circuit 28, is the other most critical shielding point. This connection is made by bringing the preselector output lead through one of the apertures 34 to the circuit 28. As before, when the housing 27 is positioned, the boss 31 projects into it, the tab 35 of the housing mating side 12 completes the surround boss 31 and shielding of the interconnection point 46 is complete. The received and filtered signal and the filtered oscillator output are mixed in the tripler/mixer circuit 28 to provide the desired IF signal at the output terminal 47.

The purpose of the invention then is to provide a high level of shielding for the most critical interconnection points of an RF selectivity assembly which is built for relatively easy assembly and repair. This is done by means of apertures in a central cast housing, wherein after assembly, each aperture is completely surrounded by a boss which protrudes into an adjoining housing through a slot in that housing. All interconnections are made through the well-shielded apertures including particularly the loop from the preselection input which slips over the coil form of the RF amplifier. A ground strap 50 may be soldered across the tops of the housings. If repair or replacement of one of the adjoining circuit modules should become necessary, its housing is removable by merely unsoldering the two mounting tabs 15 from the PC board 10 and one end of the ground strap 50.