Phase-locked loop (PLL) circuits are found in a variety of high-frequency applications, from simple clock purification circuits to local oscillators (LOs) for high-performance radio communication links and ultrafast switching frequency synthesizers in vector network analyzers (VNAs). This article will refer to these various applications to introduce some of the building blocks of PLL circuits to guide the selection of components and the trade-offs within each different application, which is helpful for both novices and PLL experts. This article refers to the ADF4xxx and HMCxxx series of PLLs and voltage-controlled oscillators (VCOs) from Analog Devices and uses ADIsimPLL (ADI's internal PLL circuit simulator) to demonstrate different circuit performance parameters.