Minimizing coupling of power supply noise between digital and RF circuit blocks in mixed signal systems

Isolation of supply noise between disparate circuit blocks is crucial. When powered by the same voltage supply, the switching noise created at the supply node of the digital devices can couple into the power path of the RF circuitry and cause significant performance degradation. Electromagnetic bandgap (EBG) structures, ferrite beads, and split planes are all commonly used to mitigate this problem, but each have drawbacks which can be detrimental to signal and power integrity. Furthermore, previous works in [1] and [2] have shown that by utilizing a power transmission line (PTL) in place of a power plane, one can significantly reduce the effect of switching noise in high speed digital I/Os by preventing the occurrence of return path discontinuities. The method proposed here extends the concept of the PTL to mitigate the effect of supply noise coupling between a set of digital I/O buffers and an RF low noise amplifier (LNA). In this work, the approach is to place a notch filter with a bandstop frequency corresponding to center frequency of the LNA in the power supply path of the LNA. Therefore, any frequency content of the switching noise close to operating frequency of the LNA is prevented from entering into its supply node. A board-level test vehicle was built to demonstrate this concept with off-the-shelf components. Through theory, simulation, and lab measurements, is has been shown that utilizing this method can reduce the amount of the switching noise that couples into the output of the LNA by 84%.