(5.15)Noise power from noise diodes fluctuate from diode to diode. Also the decouple factor of the coupler fluctuate. This deviation is determined during a cold-load calibration and the absolute base is set.
A cold-load is a waveguide termination insight an isolated box. Liquid nitrogen cools the termination to approximately 80K (-193°C). This is a stable noise source and the variations in noise power are very small. Equation 5.15 shows the calculation of the correction factor 'b'.
(5.15)
The noise power of a cold-load 'T_CL' is deposited in a calibration certificate. 'T_mess' is the measured noise temperature of the cold-load. This value has to be corrected with the following equations. Equation 5.16 shows the basic calculation.
(5.16)
The reflected power of the mismatched receiver is subtracted with 5.17.
(5.17)
The waveguides are also afflicted with ohmic losses. Equations 5.18, 5.19, 5.10 correct this influence.
(5.18)
(5.19)
Finally the displayed value is calculated with equation 5.20.
(5.20)
The RMC set the factor b=1 for a calibration, so that the natural value is measured. The RMC calculate the factor 'b' automatically.