Coupling Filter Calibration

• One approach to deriving the “coupling filter” is to determine a “single mode” loop filter, $L(z)$, whose magnitude response at the various modal frequencies is equal to the respective “per sample” attenuation rates.

• The coupling filter can then be computed as
    $$\frac{2 \left(1 - L(z)\right)}{1 + N + (1-N) L(z)},$$ (5)
  where $N$ is the number of modes connected to the coupling filter.