A Simple RF Attenuation Measurement Technique With a Small Mismatch Uncertainty

An accurate radio frequency (RF) and microwave attenuation measurement method is described, where source and load do not have to be matched to the line impedance and where the contribution of a mismatch effect to the uncertainty is reduced to about 0.002 dB. From loss measurements carried out for variable attenuators, as well as for attenuators of fixed attenuation values as devices under test, which are fitted to quarter-wavelength lines in three different configurations, the attenuation is simply obtained and is unrelated to the source and load reflection coefficients of Gamma_G and Gamma_L, respectively. Experimental results on a 10-dB step attenuator measurement at frequencies from 1 to 17 GHz show good agreement with measurements where the source and load are impedance matched. The technique is effective even with Gamma_G and Gamma_L higher than 0.1.

I. Introduction

Attenuation is one of the fundamental measurement quantities in almost all aspects of radio frequency (RF) and microwave measurement technologies. Attenuation is defined as the ratio of power absorbed by the load without the device under test (DUT) in the line to the power absorbed by the load with the DUT inserted in the line when the source and load are perfectly matched to the line impedance. If an attenuation measurement is carried out without the source and load being perfectly matched, there will be an error, i.e., a mismatch error, in the result. In high-precision attenuation measurements, this mismatch is often the largest term contributing to the systematic uncertainty. By using RF tuners and isolation elements, a precise match of source and load to the line impedance is possible. It provides the smallest mismatch uncertainty if carefully executed but has to be carried out anew when changing the measurement frequency [1], [2].