New PTC and NTC thick film materials for gigahertz range temperature variable attenuators

Abstract

Negative temperature coefficient (NTC) and positive temperature coefficient (PTC) thick film thermistor pastes were developed. Using these materials, cost effective temperature variable attenuators (TVA) for use up to 18 GHz are described. These devices maintain the signal level in amplifiers, switches, delay lines, and other microwave and RF signal processing devices. By combining NTC and PTC thermistors in "T" and "/spl Pi/" configurations, the attenuation changes to compensate for temperature induced signal level changes. Judicious choice of resistance and TCR yields constant impedance over the operating range, thus avoiding unwanted signal reflections. While the temperature sensitivity of NTC and PTC chip thermistors is large, their use in TVAs requires a large and expensive inventory to provide the required resistance and TCR values. Thick film thermistor systems were developed to overcome these limitations. These include an NTC materials series with TCR values as high as -500,000 ppm//spl deg/C and resistance range coverage from 100 /spl Omega//sq. to 1M /spl Omega//sq. The complimentary PTC system features linear TCR values of 3000 ppm//spl deg/C and greater with resistance values of 5-5000 /spl Omega//sq. Process variables for these new thick film thermistor materials are discussed. Attenuation and frequency curves are presented. Impedance match, as characterized by VSWR versus frequency curves are presented. Current applications include Thermopad/sup R/ type TVAs with improved high frequency performance appropriate for mobile communications and similar uses.