Abstract
We have developed a total-power radiometer to establish a noise standard for the national metrology standards in the frequency range of 18-26.5 GHz. The radiometer is based on the heterodyne scheme working on the intermediate frequency (IF) of 30 MHz. The IF signal is detected by null-balance method under the control of the IF variable attenuator. The RF noise temperature can be determined from the attenuation after the balance, since the RF noise power is proportional to the attenuation. We have investigated the uncertainty sources of the radiometer, such as stability, non-linearity, internal noise parameter and Y factor defined as the ratio of the measured noise power levels, and quantitatively evaluated each contribution by the sources. To improve the uncertainty of the Y factor that is one of the major uncertainty, we have adopted an accurate calibration method of the IF attenuator by directly comparing it to a precision step-attenuator calibrated by the national standard of attenuation. As a result, the type B combined standard uncertainty of the radiometer was estimated to be less than 1 % in the noise temperature range from 300 K to 30000 K.
