Browse Prior Art Database

VARIABLE ATTENUATOR FOR MICROSTRIP

IP.com Disclosure Number: IPCOM000005484D
Original Publication Date: 1983-Mar-01
Included in the Prior Art Database: 2001-Oct-08
Document File: 2 page(s) / 73K

Publishing Venue

Motorola

Related People

James P. Phillips: AUTHOR

Abstract

Variable attenuators for microstrip are often implemented by placing a vane of resistive material mounted on a pivot above a section of the line. The attenuation is adjusted by rotating the vane and thus varying the amount of line covered. Although this design is simple, it does have several 5 drawbacks. First of all, appreciable area is needed on the circuit board. Secondly, and more significant- ly, the impedance seen by both the input and the output is usually a large mismatch relative to that of the microstrip line. Most attenuation is due to this mismatch which makes this device often unusable in sensitive circuits.

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[If; MOTOROLA Technical Developments Volume 3 March 1983

VARIABLE ATTENUATOR FOR MICROSTRIP

By James P. Phillips

   Variable attenuators for microstrip are often implemented by placing a vane of resistive material mounted on a pivot above a section of the line. The attenuation is adjusted by rotating the vane and thus varying the amount of line covered. Although this design is simple, it does have several 5 drawbacks. First of all, appreciable area is needed on the circuit board. Secondly, and more significant- ly, the impedance seen by both the input and the output is usually a large mismatch relative to that of the microstrip line. Most attenuation is due to this mismatch which makes this device often unusable in sensitive circuits.

L

   A new attenuator structure has been developed which eliminates all these limitations. The new structure (Figure 1) uses a bulk lossy material which is slid within a short section of line below ashield. The line curves away from the lossy material at one end to provide a gradual and therefor reflectionless interface. The attenuator element is tapered in one or two planes on the other end to provide a gradual impedance transistion. Both ends are therefor well matched to the impedance of the microstrip line. The shield over the line is attached to the ground plane to create a short unbalanced stripline sec- tion with the dielectric on one side being the substrate material and either air or the attenuator element on the other. The amount...