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It is demonstrated that SiGe bipolar technologies are well suited for voltage-controlled oscillators (VCOs) in 77-GHz automotive radar systems. For this, the design of a VCO with powerful output buffer (with good decoupling capability and high output power), comparatively wide tuning range, and reasonably low phase noise is described. To achieve the required high output power, the potential operating range of the output transistors, limited by high-current effects and avalanche breakdown, respectively, had to be exploited using adequate transistor models. The VCOs need a single supply voltage only and have been fully integrated (including resonant circuit and output buffer) on a single small (1 mm2) chip, demonstrating their low-cost potential. Experimental results showed, at a center frequency of around 77 GHz, a usable tuning range of 6.7 GHz and a phase noise of -97 dBc/Hz at 1-MHz offset frequency averaged over this range. In addition, the center oscillation frequency can be coarsely adjusted within a wide range by cutting links in the upper metallization layer. The total signal power delivered by both buffer outputs together is as high as 18.5 dBm at a power consumption of 1.2 W. Simulations let us expect a potential doubling of the output power (for two or four outputs) by extension of the output buffer. To get an impression of the maximum frequency achievable with the circuit concept and technology used, a second VCO (again with buffered output) has been developed. To the best of the authors' knowledge, the measured maximum oscillation frequency of about 100 GHz, at 12.4-dBm total output power (14.3 dBm at 99 GHz), is a record value for SiGe VCOs with buffered output operating at their fundamental frequency. The usable tuning range is still 6.2 GHz.
Date of Publication: Oct. 2004