A voltage-controlled oscillator (VCO) that operates in the frequency range from 77.5 to 83.5 GHz has been constructed in the form of a monolithic microwave integrated circuit (MMIC) that includes high-electron-mobility transistors (HEMTs). This circuit is a prototype of electronically tunable signal sources in the 75-to-110-GHz range, needed for communication, imaging, and automotive radar applications, among others.
This oscillator (see Figure 1) includes two AlInAs/GaInAs/InP HEMTs. One HEMT serves mainly as an oscillator gain element. The other HEMT serves mainly as a varactor for controlling the frequency: the frequency-control element is its gate-to-source capacitance, which is varied by changing its gate supply voltage.
The gain HEMT is biased for class-A operation (meaning that current is conducted throughout the oscillation cycle). Grounded coplanar waveguides are used as impedance-matching transmission lines, the input and output matching being chosen to sustain oscillation and maximize output power. Air bridges are placed at discontinuities to suppress undesired slot electromagnetic modes. A high density of vias is necessary for suppressing a parallel-plate electromagnetic mode that is undesired because it can propagate energy into the MMIC substrate.
Previous attempts at constructing HEMT-based oscillators yielded circuits with relatively low levels of output power and narrow tuning ranges. For example, one HEMT VCO reported in the literature had an output power of 7 dBm (≈5 mW) and a tuning range 2-GHz wide centered approximately at a nominal frequency of 77 GHz. In contrast, as shown in Figure 2, the present MMIC HEMT VCO puts out a power of 12.5 dBm (≈18 mW) or more over the 6-GHz-wide frequency range from 77.5 to 83.5 GHz.
This work was done by Lorene Samoska of NASA's Jet Propulsion Laboratoryand Vesna Radisic, Miro Micovic, Ming Hu, Paul Janke, Catherine Ngo, and Loi Nguyen of HRL Laboratories, LLC.
NPO-21214
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80-GHz MMIC HEMT Voltage-Controlled Oscillator
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Overview
The document discusses the development of an 80-GHz monolithic microwave integrated circuit (MMIC) voltage-controlled oscillator (VCO) utilizing high-electron-mobility transistors (HEMTs). This innovative oscillator was created by a team from NASA’s Jet Propulsion Laboratory and HRL Laboratories, addressing the need for electronically tunable frequency sources in the 75-110 GHz range, which are essential for applications in microwave systems, automotive technologies, next-generation fiber optic communications, and advanced imaging systems.
Previous HEMT-based oscillators had limitations, including low output power and narrow tuning ranges. For instance, earlier designs reported an output power of only 7 dBm (approximately 5 mW) with a tuning range of 2 GHz. In contrast, the new MMIC HEMT VCO achieves an output power of 12.5 dBm (around 18 mW) over a 6-GHz tuning range from 77.5 to 83.5 GHz. This significant improvement marks a breakthrough in the design of frequency-tunable oscillators.
The oscillator design incorporates two AlInAs/GaInAs/InP HEMTs: one serves as the gain element while the other functions as a varactor, controlling the frequency through its gate-to-source capacitance, which is adjusted by varying the gate supply voltage. The gain HEMT operates in class-A mode, ensuring continuous current flow throughout the oscillation cycle. Grounded coplanar waveguides are employed for impedance matching, optimizing output power and sustaining oscillation. Additionally, air bridges are utilized to mitigate unwanted electromagnetic modes, and a high density of vias is implemented to suppress undesired parallel-plate electromagnetic modes.
The document emphasizes the oscillator's efficiency, reporting a radio-frequency to direct current efficiency of 23% across its entire tuning range. This advancement not only enhances the performance of HEMT-based oscillators but also opens new avenues for research and application in high-frequency electronics.
For further details, the document references a publication in the IEEE Microwave and Wireless Components Letters, highlighting the collaborative efforts of the inventors and the significance of this technology in advancing communication and imaging systems. The work was conducted under NASA's sponsorship, underscoring the agency's commitment to innovation in aerospace technology.
