Silicon-germanium (SiGe) semiconductor technology has long held the promise of high-frequency operation with high levels of integration. The process technology made headlines in the early 1990s with claims of device transition frequencies that could surpass expensive gallium-arsenide (GaAs) technology while using standard silicon wafers. While the technology is now making its way into a variety of integrated circuits (ICs), mainly for cellular handsets and wireless-local-area-network (WLAN) cards, its integration potential has been largely untapped. A single-chip fractional-N SiGe synthesizer from Centellax (Santa Rosa, CA), however, offers a glimpse of what this technology can achieve, using multiple varactor-tuned voltage oscillators to cover frequencies between 20 and 30 GHz and a total of more than 10,000 transistors on chip.

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