A sigma-delta based open-loop frequency modulator

A constant-envelope, continuous phase modulation architecture is presented in which a sigma-delta modulator is combined with an open-loop modulator. Compared to existing architectures, Σ-Δ based open-loop modulation can be implemented with minimal analog circuitry. The architecture is demonstrated with a discrete implementation performing frequency modulation at a carrier frequency of 120 MHz. By increasing the Σ-Δ modulation frequency, the quantization noise is filtered out by the VCO and disappears beneath the measurement noise floor.