Improved passive temperature-compensating attenuators have been developed for use in processing radio signals at frequencies up to about 18 GHz. In general, temperature-compensating circuits and devices are needed to minimize (preferably, to eliminate) undesired temperature dependences of the overall gains of amplifiers, filters, and other microwave-signal-processing circuits. The role of a temperature-compensating attenuator is to provide a temperature-dependent amount of power dissipation complementary to the temperature-dependent gain or dissipation of other devices in the circuit, such that the overall gain or attenuation of the circuit varies minimally (preferably, not at all) with temperature.
Active temperature-compensating attenuators that include temperature sensors, transistors, and other circuit elements are known; unfortunately, these active attenuators often exhibit nonlinear responses, giving rise to distortion of signals (intermodulation distortion and harmonics). In some applications (e.g., some microwave-signal-processing applications), it is possible to use passive temperature-compensating attenuators, which are preferable to active attenuators in that the passive devices are more reliable and smaller, and their responses to signals are more nearly linear (they do not introduce intermodulation distortion or harmonics).
The improved passive temperature-compensating attenuators were developed because users expressed a need for degrees of temperature compensation greater than those afforded by previously available passive temperature-compensating attenuators. Ideally, in a typical application, a temperature-compensating attenuator intended for use in a specified temperature range would function with no loss (no attenuation) at the highest temperature in the range and with maximum loss at the lowest temperature in the range. While none of the improved attenuators performs in this ideal manner, it is notable that they can be made to present constant input and output impedances, and their temperature coefficients of attenuation (TCAs) are tailorable. Both units with positive and units with negative TCAs have been fabricated.
The improved passive temperature-compensating attenuators are all variants of the same basic device: Each such attenuator comprises a network of thick-film thermistors (similar to thick-film chip resistors) deposited on an alumina substrate. The thermistors are made of materials that have been formulated specifically for this use and that exhibit temperature coefficients of resistance (TCRs) considerably greater than those of previously commercially available materials. The parameters of the thermistors are selected to obtain the desired temperature-dependent amounts of attenuation while maintaining constant input and output impedances.
This work was done by Joseph B. Mazzochette of EMC Technology, LLC for Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4-8
21000 Brookpark Road
Refer to LEW-16927.