We have shown that the dominant source of noise in the radiometer output is amplifier noise temperature fluctuations. The contributions to the noise from other sources, such as amplifier gain fluctuations, reference load fluctuations and fluctuations in the ratio of DC gains, are either zero at first order or much less than the amplifier noise temperature fluctuations term under quite reasonable assumptions.
For a total power radiometer, laboratory measurements have found knee frequencies between 10 and 100 Hz; the modified correlation radiometer scheme reduces the knee frequency by more than two order magnitudes.
The value of the knee frequency depends upon several factors including the radiometer bandwidth, reference load temperature, and the intrinsic level of fluctuation in the HEMT devices; values of Hz should be reached with only passive cooling of the radiometer to 50 K. A of Hz will not significantly contaminate the PLANCK observations. For example, considering the standard PLANCK observational strategy for the channel at 30 GHz (sample time of about s per pixel and about 680 pixels, , for a scan circle with an angle of between spin axis and telescope direction), and referring to Janssen et al. (1996), we find that the maximum excess noise factor for a typical scan circle, with respect to the case of pure white noise, is For the channel at 100 GHz ( s and ) we find .
Active cooling to 20 K of the amplifiers and reference loads would, of course, allow the reduction of the knee frequency and the total noise to very low values.
A refinement of the present analysis for the determination of will be pursued in the future by including in the analytical formalism the phase shifts between the signals entering the two legs of the radiometer, by software simulations of the radiometer functions to accurately study the combined effect of all components and finally by testing it with laboratory bread boarding and precursor sub-orbital CMB experiments.