For most radiometers, periodic calibration is essential to ensure sensor measurement accuracy. This paper describes the application of a unique active microwave circuit that simulates known radiated thermal temperatures over a stepped range of less than 105 K to over 300 K. The device, when connected or switched to the input of a receiver, can be incrementally stepped via a bias voltage from the coldest to the warmest temperature. It offers the potential to replace more complex and expensive external and internal calibration techniques. In addition to calibration, the device also serves to test the receiver linearity and measure receiver noise figure. The paper will identify significant advances in the area of modeling, design, development, and test of the stepped 'cold/warm' noise source. Several prototype FET circuits, using 150 and 300 micrometer InP devices, have been built and demonstrated at K-band frequencies of 18 - 22 GHz. Work is in progress to build and test the next device at 37 GHz. Tests have been conducted at Raytheon Systems, the University of South Florida (USF), and the National Institute of Science and Technology (NIST). The results illustrate good agreement between simulation and measurements. Block diagrams are included to show the method of calibrating radiometers, testing receiver linearity, and measuring receiver noise figure.