We consider the problem of distributed primary and secondary frequency control in inverter-based microgrids with meshed topology. A dynamic phasor modeling approach is presented that accurately represents the dynamics of the grid while being suitably simple to be used in control design. Then, we propose a novel frequency-domain, fixed structure, multivariable control design method based on a convex approximation of quadratic constraints. The method is then used to design a distributed controller for primary and secondary frequency control in an inverter-based distribution grid that guarantees stability and satisfies various performance goals.