Technique for Absolute Temperature Measurements at Microwave Frequencies

Abstract

Measurement of the absolute temperature of microwave noise sources under field conditions is usually achieved at the expense of tedious rf attenuation measurements and the incorporation of cryogenic equipment in the measurement system. Measurement accuracy is determined by the precision to which the effects of passive rf attenuation are known. The technique described does not require this knowledge of the effects of rf attenuation on instrument calibration. The feasibility of this technique has been established by tests performed with a modified Dicke radiometer. The technique establishes the radiometer indicator output zero as a direct reading of absolute zero and provides calibration of the indicator scale directly in degrees Kelvin. Laboratory calibration of an appropriately modified radiometer and measurement of noise temperature near absolute zero are discussed.

Analysis of the Switched Self-Balancing Comparsion Radiometer

“…(5) Narrow-band amplifier outputs due to X(t), Y(t), Za(t), and Zb(t) can be expressed as Xl(t) = Xc(t) cos wct -X8(t) sin wct Yi(t) = Yc(t) cos wct -Y,(t) sin wt (6) Zai(t) = Zac(t) COs Wct -Zas(t) sin wct Zbl(t) = Zbc(t) cos wct -Zb8(t) sin wxt where Xc(t) and XQ(t) are low-pass orthogonal processes with zero-mean and equal variance, and so are Yc(t) and Y,(t), Zac(t) and Zaa(t), Zbc(t) and Zb,(t). Now if we let Sx, (COC) = 4X HI(cc)v (7) 4ir which is the BPA output spectrum due to X(t) alone at W = Wx, then the variance of Xl(t) is given by…”

“…After that, the same principle was applied to radio astronomy [4] at X-band and an analysis of the system was made with a noise comparison source instead of CW [5]. Other methods for measuring temperature in a wide range of the electromagnetic spectrum are also presented in the literature [6]. Fig.…”