Noise can hide details, but it can also reveal them. If properly used, noise will tell a great deal about a device under test (DUT), including when a DUT is raw material. For example, the Microwave Noise Field (MNF) test method has the capability to test different objects that can move, as well as objects with granular or irregular composition. When properly applied, the MNF method can provide precise analysis of different types of materials to help optimize the use of those materials in industrial applications.
The MNF method, also known as active radiometry, is a means of using noise rather than continuous-wave (CW) test signals when measuring the insertion loss of different materials.1-3 The DUT or material to be tested is placed between a noise source and a radiometer antenna. The noise field is random with low coherence so that interference effects are negligible close to the radiators.
In addition, a DUT can move within the noise field without invalidating the measurements. A DUT being characterized in the noise field of the MNF method might even be considered as an object surrounded by white light, with little or no effects on the object from the white light.4
For measurements using the MNF method, the power level of the radiated noise in the noise field is very low, with noise measurements typically at nanowatt (nW) levels. A microwave radiometer is required as a detector to determine such low noise levels. The use of wideband noise for device or material testing has certain advantages over the use of measurements performed with CW test signals. In contrast to the MNF method, measurements performed with CW test signals require that a DUT be located in the far-field zone of the test signals, and that the DUT not move at all during testing.
Low-cost radiometers have been designed with the same low-noise block downconverters (LNBs) often found in satellite television receivers. Such LNBs are stable enough for measurement purposes, with good low-noise characteristics. In addition, the C- and Ku-band frequencies used for satellite television reception are internationally protected and managed communications frequency bands, preventing the generation of man-made signals (other than satellite-television signals) within those frequency bands.