Speed in SiGe
The key to the advances is the replacement of gallium arsenide (GaAs), the high- frequency substance normally used in chips that contain VCO circuits, with silicon germanium (SiGe). This compound not only offers high speed, it enables high levels of integration. In this particular case, the VCOs are fully monolithic and contain no external components such as inductors and varactor diodes!
Equally important is the fully-differential architecture involved in the VCOs' design. This architecture minimizes noise coupling from digital parts of a highly-integrated chip into a sensitive analog VCO. The added circuitry of a fully differential architecture typically comes at the cost of increased power levels, but SiGe achieves this result with minimum increase in power consumption.
A significant trial of the new technology came recently when an IBM VCO was tested at 17.1 GigaHertz, an ultra-high transmission frequency recently allocated for wireless uses in Europe (HiperLAN). The record-setting VCO, operating on a single 3.3V supply, could be tuned over a 600 MHz range and exhibited phase noise of -104 dBc/Hz at a 1 MHz offset from center with an output power of -5 dBm and dissipating only 65 mW.
Another VCO, tuned for a new American standard of 5.x GigaHertz (U-NII), has also performed exceptionally well, with a tuning range of 840 MHz and a phase noise of -115 dBc/Hz at 1 MHz offset at the center frequency of 5.6 GHz.