Enhanced pnoise Algorithm to Compute Phase-Noise for VCOs with Bandgap Voltage Reference

Accurate phase-noise characterization is critical in the design of RF and microwave communication systems. SpectreRF’s shooting PSS/Pnoise analysis has been the golden simulator for the phase-noise simulation, and close correlation between the simulation results and silicon measurement was well reported by many research and industrial institutes.

As the IC voltage references, such as bandgap reference, are widely used as DC biasing circuitry in deep-submicron low noise VCO design because of its insensitivity to temperature and load variations, the phase noise at low offset frequency can be dominated by the thermal and flicker noises generated by the voltage reference.

The shooting Pnoise fails to simulate the effect of the supply noise on phase noise because it solves the VCO’s small signal response in two separate steps. First it solves the phase-modulation (PM) response using the phase sensitivity function calculated from the extraction of the dominant eigen-mode. Second it solves the amplitude modulation (AM) response by subtracting out the PM contribution from the perturbation equation.

The method is problematic for perturbation, which decays slower than the oscillation period, such as the noise from bandgap voltage reference with large RC-time-constant. For the slow-decaying perturbation, there is no frequency-independent separation between AM and PM responses, thus they need to be solved simultaneously.

To address this issue and accurately simulate those type of circuits , Cadence has enhanced harmonic balance Pnoise analysis. Using the "augmented" option available in MMSIM71 ISR5 and above, the end-user can now predict accurately the phase-noise of such circuits. The same solution will be available in MMSIM71 ISR6 for the shooting method.

Helene Thibieroz