21.6 A 2.4-to-5.3GHz dual-core CMOS VCO with concentric 8-shaped coils

Despite recent attempts to relax the phase-noise demands on voltage-controlled oscillators (VCOs) for cellular communications [1], mainstream radios require harmonic VCOs capable of a very low phase noise with moderate power consumption, associated to a large tuning range (TR) and a high insensitivity to interfering signals. Ideally, the TR should be in excess of one octave, since this allows the easy synthesis of all frequencies below those directly generated by the VCOs via repeated frequency divisions by 2. At the same time, the oscillation spectrum should be affected as little as possible by spurious (common-mode) magnetic fields impinging on the inductor coil in the VCO tank. This is a crucial requirement in modern radios, where there are more PLLs active at the same time, and particularly when (non-contiguous) carrier aggregation is implemented, since in this case the signal bands may be very close to each other. If an individual PLL is used for each band, the VCOs may oscillate very close to each other, or at frequencies that are harmonically related to each other, posing a very serious issue of mutual pulling through the respective magnetic field. And even if a single VCO is used [2], or two (or more) VCOs that are not harmonically related [3], it is nevertheless a good practice to design the tank inductor as insensitive as possible to external magnetic fields, which abound in and close to the radio IC.