Reduced Chopper Calibration

With modern sensitive receivers, the system noise under good weather conditions may be significantly lower than 300 K. Accordingly, measuring the total noise when looking at a warm load at 300 K requires a linear response of the whole detection system over a dynamic range greater than 2-3. This can be problematic.

There are several ways to bypass this limitation, for example

1. Using lower temperature (controlled or monitored) loads. This is probably the best, but it requires cooling devices somewhere.
2. If the receiver is linear, but not the backend, it is possible to place a switchable attenuator in the IF chain, preferably immediately after the conversion to IF frequency.
3. Covering partially the beam by the absorber, the other part still looking at the sky.