Piston attenuator as RF signal vernier in the 10-300 MHz range: determination and correction of spurious mode errors

Abstract

Systematic errors in H11-mode piston attenuators from unwanted mode excitation have been measured for CW, and pulse modulated RF signals as well, up to 300 MHz with a new calibration circuit based on a linear RF-IF mixer. The attenuation deviation by spurious mode influence can be calculated from the determined amplitude ratio of superposed modes and can be corrected. In this way piston attenuators, originally designed for IF application, can also be used as RF verniers with 0.01 dB resolution up to 300 MHz and possibly higher with an accuracy depending on mixer linearity in the calibration circuit.