RF induced communication errors in RFFE MIPI controlled Power Amplifiers

As the number of RF components increases inside mobile devices, the industry has rapidly adopted and utilized MIPI's RFFE serial bus specification [1] to communicate with these devices and reduce the amount of control line routing between components. However, the use of a serial bus within the RF front end increases the risk that RF energy, particularly from the Power Amplifier (PA), can corrupt the RFFE bus communication. RF coupling to areas such as the RFFE serial clock (SCLK) signal seen within the PA's RFFE state machine can result in communication errors with the PA. This coupling can be to the SCLK signal itself, or to the PA's CMOS controller's ground reference. PA designers must pay special attention to the positioning and design of the CMOS controller and the internal routing of signals to the controller to avoid these problems. This paper provides a detailed analysis of how RF energy coupling to the internal CMOS controller can create “glitches” on the SCLK signal that cause communication errors. A simple example is provided to illustrate how choices in PA module layout can significantly impact these issues. A natural extension of these concepts applies to phone or radio board layouts, too.