80-GHz MMIC HEMT Voltage-Controlled Oscillator

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