Coaxial shielded helical delay line and process
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention pertains to a substrate with a shielded delay line and to a method for making the same. 
     2. Description of the Prior Art 
     In various electronic circuits it is frequently desirable to provide a means or delaying certain signals for a predetermined length of time. For circuits operating at very high frequency, where the required delay is in the microsecond or sub-microsecond range, the delay means comprises a conductor having a preselected length. In order to reduce the overall size of the delay means the conductor is usually formed in the shape of an axially coiled spiral. 
     Usually the various electronic devices used in the electronic circuits are mounted on a printed circuit board. However it is fairly difficult to mount the above-mentioned delay coil on a circuit board because of its size and fragility. Frequently, the coil is encapsulated in a dielectric material to give it rigidity, however this process is expensive. Furthermore, at high frequency the conductor must be shielded to eliminate noise or extraneous signals. This shielding even further complicates the construction of the delay coil and the manner of mounting it to a printed circuit board. 
     OBJECTIVES AND SUMMARY OF THE INVENTION 
     In view of the above, it is a principal objective of the present invention to provide a delay means which may be easily interfaced with electronic devices mounted on a printed circuit board. 
     A further objective is to provide a delay means which can be provided as part of said printed circuit board. 
     Another objective is to provide a delay means which is shielded to eliminate noise. 
     Other objectives and advantages shall become apparent from the following description of the invention. 
     A delay line, according to this invention, comprises a coil of axially spaced turns, and formed of a plurality of conductive strips. The strips may be overlaid by using standard photomasking techniques. A dielectric material is used between the strips. The delay line may also comprise a shield co-extensive with said coil and formed simultaneously therewith. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an isometric view of the conductor formed in the shape of a spiral to form a delay line; 
     FIG. 2 is an end view of a substrate with a shielded delay line; 
     FIG. 3a shows an end cross sectional view of a delay line formed of a plurality of superimposed layers; 
     FIGS. 3b-3i show the method of constructing the delay line of FIG. 3a; 
     FIG. 4 shows a top cross-sectional view of the delay line taken along lines 4--4 in FIG. 3a; 
     FIG. 5 shows a top cross-sectional view of the delay line taken along lines 5--5 on FIG. 3a; and 
     FIG. 6 shows an electrical equivalent for the delay line constructed in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to this invention and as shown in FIG. 1 a delay line comprises a conductor 10 which is formed as a helical coil and imbedded in the substrate for mounting various electronic devices. For reasons that shall become apparent below, the helical coil is formed of a plurality of substantially straight elements such as vertical elements 12 and 14 which are interconnected by horizontal elements 16 and 18. Preferably as shown in FIG. 2 a shield 20 is formed around the conductor 10 as shown in FIG. 2 and co-extensive therewith. The spaces between shield 20 and conductor 10 and between the elements of conductor 10 are filled with a dielectric material 22. 
     Free ends 24 and 26 of the conductor are connected to a shielded conductor or are provided with a pad for connection with elements disposed outside the substrate. 
     In my copending and commonly assigned application entitled &#34;MICRO-COAXIAL SUBSTRATE&#34; Ser. No. 671,276 filed on even date herewith and incorporated herein by reference, I disclose a method of forming a substrate with an imbedded shielded conductor. The same method may be used to form a substrate with the delay unit of FIGS. 1 and 2 as described below. 
     Basically, the substrate is formed by superimposing a plurality of layers to obtain the desired conductor profile as shown in FIG. 3a. Initially a first layer 28 is made of a conductive material (see FIG. 3b). This first layer shall form a first ground plane for the conductor 10. The second layer is formed by applying two conductive strips 30, 32 on the base (FIG. 3c) and then applying a dielectric material 22 such as polyimide evenly across layer 28 (FIG. 3d). The third, fourth and fifth layers (FIGS. 3e, 3f, and 3g) also include portions 38 of conductive material which are shaped to form parts of coil 10. The parts of coil 10 formed by portions 38 include free ends 24 and 26, vertical elements 12 and 14, and horiziontal elements 16 and 18. After the fifth layer (FIG. 3g) another layer is applied which is identical to the second layer of FIG. 3d. After the sixth layer of FIG. 3h, a second conductive layer 40 is applied (FIG. 3i). This layer provides a second ground plane. It is obvious from the Figures that strips 30, 32 and layers 28 and 40 cooperate to form a rectangular shield around conductor 10, with a base 42, two side walls 44, 46 and a top 48. As can be seen in FIGS. 4 and 5 the horizontal elements 16, 18 comprise straight strips which extend diagonal from vertical elements 12 to vertical elements 14 to form the helical coil of FIG. 1. 
     After the last layer of FIG. 3i is applied, other layers may be added as required to form a printed circuit board. Furthermore, it should be understood that FIGS. 3a-i show just the portion of the substrate incorporating the delay line. Other portions may be dedicated for shielded conductor described in the above-mentioned application as well as various other imbedded circuit elements such as a Resonator as disclosed in my copending and commonly assigned application entitled &#34;RESONATOR&#34;, Ser. No. 671,369 filed on even date herewith and incorporated herein by reference. 
     For the delay line with the dimensions indicated in FIG. 2 and legs 12 (or 14) being spaced at 10 mils, a delay of 10 microseconds/ft can be obtained. 
     Obviously numerous modifications can be made to the invention without departing form its scope as defined in the appended claims. For example the substrate may be made using nine layers. Various other geometric configurations may be used to obtain a helical coil.