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TECHNIQUES FOR USE OF SPIRAL INDUCTORS ON PRINTED CIRCUIT BOARDS

IP.com Disclosure Number: IPCOM000009014D
Original Publication Date: 1999-Jan-01
Included in the Prior Art Database: 2002-Jul-31
Document File: 3 page(s) / 138K

Publishing Venue

Motorola

Related People

Adrian Nickerson: AUTHOR [+2]

Abstract

Spiral inductors of relatively low manufacturing accuracy are constructed on a PCB. An indication of the inductors' values is marked on the PCB. Parts can then be added to the PCB later in such a way as to compensate for variation between the inductors' actual and optimum values.

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Technical Developments

TECHNIQUES FOR USE OF SPIRAL INDUCTORS ON PRINTED CIRCUIT BOARDS

by Adrian Nickerson and Andrew Webster

ABSTRACT

  Spiral inductors of relatively low manufacturing accuracy are constructed on a PCB. An indication of the inductors' values is marked on the PCB. Parts can then be added to the PCB later in such a way as to compensate for variation between the inductors' actual and optimum values.

INTRODUCTION

  PCB spiral inductors are becoming common- place in electronic design. They reduce both costs and the manufacturing difficulties associated with placing conventional airwound spring inductors used previously. Unfortunately, PCB inductors have some disadvantages. Most notable, perhaps, is the sometimes quite large variation in inductance val- ues, both between and within batches of PCBs. This paper describes improvements which will allow low-tolerance printed circuit board components to be used in tolerance-critical circuitry, without requiting the use of expensive tuning circuitry.

PROBLEMS TO BE SOLVED

  Where PCB spiral inductors are used in critical circuit applications, the make-tolerance of the com- ponents is also critical to the performance of the cir- cuit. The key elements of the make-tolerance are the etching accuracy, PCB thickness control and the material permeability. If the application requires high accuracy for the PCB components, this requires tight controls on the PCB manufacture process. This in turn leads to increased cost and restriction of the PCB vendor to those that can meet the tolerance requirements.

  Figure 1 shows an example of a simple filter cir- cuit. This is one traditional approach to resolving the problems of variation in PCB spiral inductor values.

  In the arrangement of Figure 1, on next page, tuning has been used to compensate for tolerance errors in the spiral inductors. The varactors are expensive and are additional components in the cir- cuit. They also require the provision of a control- ling voltage. In modem low voltage applications, this in turn may also require a negative voltage gen- erator in order to provide sufficient tuning control range to compensate for the spiral inductor toler- ances. This is the case in the present example, and clearly adds further cost and complexity. It also requires additional board area, which, since the PCB is often the most expensive item in the device, may also increase the cost of the board.

PROPOSED SOLUTION TO THE PROBLEMS

  The proposals detailed below in connection with Figures 2 and 3, page 193, allow a relaxation of the tolerance requirements for the PCB, thereby reduc- ing cost. Another advantage is that the circuits can have lower loss, because fewer poles are required to guarantee performance. Fewer components means lower cost and increased reliability.

  Figure 2 shows a simplified version of the filter of Figure 1. For this configuration, the values for the associated capacitors are selec...