High-impedance high-speed microwave detector
The majority of today's microwave detectors are based on microwave diodes. These detectors generally present a low impedance to the microwaves they are used to detect. Input matching circuits and filtering, as well as parasitics can slow the detectors response and require larger microwave power consumption. The research presented in this dissertation develops the theory and optimized design procedure of a fast, sensitive, high impedance detector based on a negatively biased microwave Field Effect Transistor (FET) at 9GHz for the first time. A microwave FET was chosen because, as an amplifier, it's bandwidth is large enough to handle the modulated microwave carrier frequency. The gate will be able to follow the modulated microwaves and the drain will be able to follow the lower frequency video output. The circuit is negative output detector, which means that the video output voltage increases negatively as the microwave input increases. The impedance of the FET gate circuit is low at the desired carrier frequency, but the impedance of the detector can be made large by the use of a transmission line of the proper length. The detector developed has an input impedance of up to 2.9k$\Omega.$ It was found that the FET's internal filter-electrode capacitances and electron transit time through the channel limit the bandwidth and the output rise and fall times of the detector as a result are limited. This problem is addressed by choosing a FET with sufficient bandwidth. R-C time constants of the external circuits of the detector will also limit the rise and fall time. Typically rise and fall times are thought to be equivalent. In this detector circuit the FET drain impedance changes with the application of microwaves to the gate and it was found that the rise and fall times can be slightly different. The fall time of the circuit was measured and determined to be 2.053ns and the rise time 1.87ns on the average. It was also found that these times varied with the microwave carrier frequency used. Low noise design of this detector was investigated. As a result an equivalent TSS 5.9dB below that of a Schottky diode was measured.
Luglio, Juan Raimundo, "High-impedance high-speed microwave detector" (1997). Dissertations (1962 - 2010) Access via Proquest Digital Dissertations. AAI9825394.