US 3541474 A
Description (OCR text may contain errors)
Nov. 17; 1970 H. B. HOLTON v 3,541,474
MICROWAVE TRANSMISSION LINE TERMINATION Filed July 51, 1969 FIG. I
REAL COMPONENT 5 m- 5 O 2 0 2 2 2 4 2 .6 28 3 0 3. 2 FBEQUENCY (G HZ) g-lo- REACTIVE COMPONENT lNl/E/VTOR H. B. HOLION Bl ATTORNEV United States Patent 01 hce Patented Nov. 17, 1970 US. Cl. 33322 1 Claim ABSTRACT OF THE DISCLOSURE A terminating impedance characterized by a strip of resistive material secured to a substrate, preferably by film deposition techniques. The strip may be an integral extension of the center conductor of a microwave transmission line and has the distributed parameters of such a line. By properly dimensioning the strip, its reactive component may be made substantially zero over an appreciable frequency band about the desired operating frequency. While the strip is preferably ungrounded, it may be grounded at its receiving end. It is readily applicable to either microstrip, strip transmission line or coaxial transmission line configurations.
BACKGROUND OF THE INVENTION This invention relates to power dissipating terminations for microwave transmission lines.
Terminating impedances currently in use are generally constructed to appear as lumped resistances. To make such devices appear as lumped resistance elements, it is essential that their physical lengths be made small compared to a wavelength at the operating frequency, generally less than a quarter wavelength. At the higher microwave frequencies, especially above 2 gHz., it becomes increasingly diflicult to fabricate these lumped elements in such small sizes and even more difficult to provide them with an adequate practical power dissipation capability. The practical problems of size and power dissipation become exceedingly acute at these higher frequencies where the design objective is to hold the device length to onetenth or to one-twentieth of the wavelength.
SUMMARY OF THE INVENTION The present invention is of an impedance termination fabricated as a section of transmission line with distributed parameters and may be either connected to a transmission line by conventional means or fabricated as an integral part of the line. This construction is made possible by the discovery that the termination can be made an essentially pure resistance over an appreciable frequency band by properly proportioning the dimensions of the strip, particularly its length. Aside from the obvious advantage of a very much greater power dissipating capability by reason of its greater size, this invention has at least three additional advantages. One advantage resides in the fact that the termination need not be grounded, thereby simplifying the topological layout of the circuit and its termination. Another advantage is that it is far more economical because it can be fabricated by film deposition methods. Finally, because film deposition methods can be employed, the termination can be easily fabricated integrally with the microwave circuit, thereby making separate connectors unnecessary.
BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understood by reference to the accompanying drawings in which:
FIG. 1 discloses a preferred embodiment of the invention in microstrip configuration;
FIG. 2 shows a cross section through a conventiona strip transmission line showing how the invention can be readily applied to that configuration; and
FIG. 3 are curves showing the real and reactive components of a typical terminating impedance plotted against frequency.
DETAILED DESCRIPTION A preferred embodiment of the invention as applied to microstrip is shown in FIG. 1, although the invention is also applicable to other microwave transmission lines such as the strip transmission line and the coaxial line. FIG. 1 shows a fragmentary microstrip circuit 1 secured to the top surface of a suitable dielectric substrate 2, to the underside of which is secured a conductive ground plane 3. The construction is conventional and it is to be understood that the showing is not to scale but rather quite exaggerated for clarity. It is assumed that a resistive termination 4 is to be connected to a portion of the circuit conductors 1. This connection is preferably formed by a suitable film deposition method of the same kind as was used to form the circuit conductors 1. Being so formed, no additional connecting means is needed to electrically join the terminating impedance to the, circuit. Methods for accomplishing this are Well known and may be found in Thin Film Technology, Berry, Hall and Harris (1968), pp. 525 to 529. While any suitable resistance material may be used. Nichrome, which is commonly used in film deposition processes, is satisfactory. Nichrome is a registered trademark for a Well-known nickel-chromium resistance alloy obtainable from the Driver-Harris Company of Harrison, NJ.
Terminating impedance 4 is shown in FIG. 1 to have a length of x units and a width of y units and its receiving end is shown ungrounded. The fact that it has the distributed properties of a microwave transmission line makes it possible to leave its receiving end insulated from ground, a distinct advantage facilitating the topological layout of the circuit. Because it is ungrounded, there are no restrictions on placement of the termination to permit grounding nor is it necessary to make holes through the dielectric substrate to permit connection to the ground plane 3.
The driving end impedance which termination 4 presents to circuit 1 is, in general, complex, having both real and reactive components. However, it has been discovered that if its dimensions are properly proportioned, the reactive component can be made essentially zero over a reasonably broad band centered about the operating frequency. The equations for obtaining the conventional microstrip circuit dimensions as well as the width y of impedance 4 are well known and are described in Reference Data for Radio Engineers, 4th edition, American Book-Stratford Press, pp. 595-598. The input impedances of both the open circuited and the short circuited lines are expressed as follows:
oe o l'j o) Goth 7 Where:
Z and Z are the open circuit and the short circuit impedances, respectively, of the line,
(R -H'X is the characteristic impedance of the line 7 is the propagation constant, and
.x is the length of the line.
The various parameters of expressions (1) and (2') are further defined on pp. 549-553 and pp. 560-561 of the above-cited Reference Data for Radio Engineers. Thus, on page 561 of the reference, it is stated that:
In applying the expressions given in the text to a practical circuit design using low loss dielectric material, the conductance per unit length G can become negligibly small and may be set' equal to'zero. Then expression (4) becomes:
l +j Z J' and expression (5) becomes:
v=[(* +i )(i )l* By substituting the values set forth in expressions (6) and (7) into expression (3) the following is obtained:
R i L LEV-Z coth [(R+ w (140)1 x sion (12) through substituting suitable values for R X u, [3, and x, the following can be obtained:
From this it follows that a length x can be established at which the reactive part of the input impedance of the termination is termination impedance which is resistive, having a magnitude which can be adjusted to a predetermined value by variation of the characteristic resistance R of the termination.
As previously stated, the invention is especially suited to microstrip lines but it is not limited thereto. A fragmentary section through a strip transmission line c0nfiguration is shown in FIG. 2, the section being assumed taken through the teminating impedance 4. Fabrication is by conventional processes and the design may follow the expressions given-on' pp. 598-600of the above-cited Reference Data for Radio Engineers. The second ground plane 3A of FIG. 2 is placed on the side of the dielectric centered about an operating frequency of 2.8 gHz. The
impedance at the operating frequency is, therefore, essentially a pure resistance of about 60 ohms. These measurei ments were obtained from a Nichrome thin film termination having a sheet resistivity of about 4 ohms per square deposited on an alumina substrate 24 mils thick, the ground plane being a good conductor having a thickness of several skin depths. Its length was 0.517 inch and its width was 35 mils. It is to be understood that these dimensions are exemplary only and that those skilled in this art can readily design terminations of difierent resistances to operate at different frequencies by simply following the teachings of this invention.
What is claimed is:
1. In combination, a strip transmission line designed for use at a selected microwave operating frequency and comprising a dielectric substrate having a first side and a second side,
an electrically conductive ground plane secured to said first side of said dielectric substrate,
and an electric line circuit conductor formed by an 7 appropriate film deposition method upon said second side dielectric substrate,
said strip transmission line being characterized by having a microwave transmission line termination adapted for constituting an impedance suitable for terminating said strip transmission line,
said microwave transmission line termination having the distributed properties of a transmission line and comprising a thin coating of resistive material formed integrally and contiguously with said electric line circuit conductor upon said second side of said dielectric substrate by said film deposition method,
said coating having a length and width which are so selected that said coating functions in the manner of a pure resistance with a negligible reactive component at said microwave operating frequency,
and said microwave transmission line termination being electrically insulated from said ground plane by said dielectric substrate.
References Cited UNITED STATES PATENTS Filmohm Develops Thin Stripline Resistor, Microwave Journal, October 1962, p. 206.
Microstrip Stripline Resistors, Bulletin SL, Filmohm Corporation, New York, N.Y., Acquisition Date Mar. 25, 1959, 2 pp.
HERMAN CARL SAALBACH, Primary Examiner M. NUSSBAUM, Assistant 'Examiner US. Cl. X.R. 3338 1, 84