Mixer Design Radiometry System

The purpose of the radiometer project was to research and demonstrate the working principles of a radiometer system. The radiometer design team was composed of five members, each contributing a specific component to the final system. A radiometer system is a device that detects microwave emissions from noise producing objects [1]. These noise sources are typically objects that emit heat and thereby produce noise in the microwave region as expressed by Plank’s Radiation Law [1]. These objects are called blackbody radiators. The practical implementation of such a system was the construction of a microwave receiver system that would accept the incoming noise power from the source and convert the power to a proportional voltage. By this method, a radiometer directed toward point A (at room temperature) should output an increased voltage when directed toward point B (at a higher temperature--such as human body temperature). The author was tasked with the design of the down-conversion stage of the receiver, which is typically termed mixer. A mixer accepts two inputs--the desired signal (in this case noise), and a standard sinusoidal signal generator or voltage controlled oscillator. The mixer generates the sum and difference frequencies of the two input signals. In this application, the output signal of interest was the difference frequency. This signal transmitted to the output stage. The primary parameter of concern in the design of a mixer is conversion loss. Conversion loss is a measure (in dB) of how much power is lost in the conversion from the input frequency to the output frequency. The design goal for conversion loss was 10 dB. Simulations indicated a projected conversion loss of 7.9 dB. The measured results of the author’s mixer design indicated a conversion loss of 8.2 dB with an input signal of -30 dBm and an LO signal level of +8dBm. The measured results of the student-designed radiometer system indicated that the radiometer was able to sense a microwave energy change (caused by a temperature change) in the power region between -85 dBm and -90 dBm. The project was considered successful as the system design effectively demonstrated the basic principles of radiometry and Plank’s radiation law. The mixer design worked as designed/simulated and worked successfully with the intended application.