Some pitfalls in millimeter-wave noise measurements utilizing a cross-correlation receiver

It is shown that the use of a hybrid junction as the power splitter in a cross-correlation receiver for millimeter-wave (94-GHz) noise measurements at low temperature (2 K) introduces an unwanted source of noise that defeats the purpose of the measurements. This is also true when isolators are introduced to prevent cross-coupling of the noise from the two receiver channels. It is noted that dual-channel receivers implementing the cross-correlation technique have been successfully used in both radiometers and radio telescopes. In those cases, however, two separate antennas aimed at the 'target' provide the necessary isolation between the two receiver channels, and no hybrid power splitter is involved. This is in contrast to the situation described here. It is shown unequivocally that the cross-correlation approach cannot produce the results desired.