postscript
3 Susceptibility to various ...
Abstract...
Acknowledgements...
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.