The phase-locking of multiple spin-torque nano oscillators(STNOs) is considered the primary vehicle to achieve sufficient signal quality for applications. This book highlights the resonator's design and its need for feedback for phase locking of STNOs. STNOs can act as sources of tunable microwaves after being phase-locked together. External feedback from a coplanar waveguide placed above an STNO helps ensures coherent single domain oscillations. STNOs placed within magnonic crystal cavities also demonstrate coherent oscillations. Arrays of such cavities provide a route to scale power levels from such nano-oscillators. The book presents numerical and micromagnetics to validate the design. Dr. Nikhil Kumar C S received his B.Tech in Electronics and Communication Engineering from the IES College of Engineering, University of Calicut, Kerala in 2007 and was awarded his Master of Science by Research in Electrical Engineering, IIT Madras in 2014 with a thesis titled Analytical models for magnonic devices and with his Doctor of Philosophy in Electrical Engineering, IIT Madras in 2020 with a thesis titled Magnonic Devices with Feedback Stabilization. His research interests are in the areas of Magnonic Devices, Spin Torque Devices, Magnon Spintronic Devices.