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A voltage regulator module (VRM) is a form of switching power supply, a self-contained digitally-programmable buck converter for reducing DC voltages down to levels required for today's sporty microprocessors, as well as CMOS circuits of interest to RF engineering (perhaps an E-band radio on 65nm CMOS running at 1.2 volts, with envelope tracking?) In power distribution network (PDN) design, flat impedance across frequency is the goal. Multiple resonances can set up "rogue waves" during load transients, which can damage the device you are trying to power up.
Below is a Keysight how-to video featuring Steve Sandler, managing director and CTO of Picotest. The video focuses on designing for constant output impedance over frequency, and power supply rejection ratio (PSRR). Current mode versus voltage mode control (spoiler, current mode is better...), and shunt and series feedback (series is preferred). Measurement-based component models are described using shunt two-port data measured on a E50612B impedance analyzer in low-cost test fixtures. Polymer capacitors are compared to ceramic. Learn about rogue waves caused by multiple resonances in output impedance across frequency in poorly-designed VRMs. Some tolerance analysis is provided to show what may happen when component values drift from nominal. Finally, there is a download to an Advance Design System state-space VRM module workspace so you can dive into VRM analysis yourself.
How to Design for Power Integrity: Selecting a VRM by Steve Sandler
As a microwave designer, you will also want to know about power efficiency, ripple, operating frequency and physical size, all topics for a later date.