Prediction of delays induced by in-band RFI in CMOS inverters

Delays induced by radio frequency interference (RFI) in CMOS inverters are measured under radiated and capacitively coupled interference. Experimental and theoretical investigations of the effects of the RFI coupling mode (capacitive versus inductive) on the amount of induced delay are presented and a worst-case coupling mode is identified. A formula to predict delays caused by in-band low-level RFI in CMOS inverters is introduced. This formula uses experimentally determined parameters which are dependent on the coupling mode. The change in delay is computed as a function of the induced voltage disturbance, which in turn can be computed from the incident field using linear frequency-domain analysis. The formula accounts for the dependence of the induced delay on the phase and amplitude of the RFI signal as well as on the slew rate of the logic transitions. A delay growth phenomenon in a string of inverters is identified and characterized. A correction to the delay prediction formula is proposed in order to take this growth into account in worst-case predictions