Frequently Asked Question
Should I tie my loop filter to ground or PLL supply?
The orientation of the loop filter depends on the design of the VCO. A well-designed supply provides a low impedance path to ground. Therefore any supply will serve as an AC ground. However, supply variations with respect to the VCO ground can affect the spurious performance of the system. Often, the VCO is ground referenced, so the loop filter should also be referenced to ground. In some cases the VCO will be referenced to a supply (such as from a regulator). In that case, tying the loop filter to the supply MAY provide improved spurious performance.