TABLE OF CONTENTS For many years the engineer requiring a precise ( 0.1% or less) resistor has used precision wirewound products. These can be made to tight tolerances, have good stability and are particularly useful in network arrays. Because of their coiled wire construction, they have poor high frequency performance particularly above 50 kc/sec. The reliability of these resistors has been quite variable in the past. They also have poor wattage and resistance range to size relationships. Nevertheless, these resistors have been the standby for most of the close tolerance resistor requirements.
Evaporated metal film resistors have been developed extensively in the last fifteen years [1-7]. They have offered somewhat improved reliability, versatility, and excellent high frequency performance. Their ac-to-dc resistance ratio remains close to unity up to the 100 megacycle/sec region. These resistors have replaced wire types in many applications because of smaller sizes, and in the premium grades (T-9) have reasonably small temperature coefficients (TC) of resistance ( 25 ppm/ C). However, the temperature coefficient changes of any two resistors will randomly fall into the total 50 ppm/ C span of such a product. The environmental resistance changes shown by these evaporated film resistors can cumulatively add up to greatly exceed their initial tolerances. Thus, resistance changes can easily reach (1% or 2%) regardless of whether the initial resistor tolerance might have been 0.1% or 1.0%. When changes amounting to 2% occur, then a total design tolerance of 6% would have to be considered in a circuit employing 1.0% units...
