Modeling, Measurement, and Analysis of Audio Frequency Ground Integrity for a TDMA Smartphone System

In this paper, we first propose a modeling method of arbitrary-shaped multilayer printed circuit board (PCB) ground planes and chassis of smartphones in the audio frequency range (20 Hz-20 kHz). The model is proposed to quickly and accurately analyze audio frequency ground voltage noise at an audio circuit that is induced from a time-division multiple access (TDMA) RF power amplifier (PA) while it is switching at an audio frequency during a call. Conventional 3-D electromagnetic (EM) simulation consumes enormous computation time and resources. However, the proposed method models the ground network as a resistive ground distribution network based on the finite-difference method and saves the computation time and resources. We verified the proposed modeling method by comparing ground noise coupling levels obtained from the proposed model, 3-D EM simulation, and measurement in the time and audio frequency domains. The proposed model showed 3.4% and 22.8% differences in the time domain and 3.8% and 14.8% differences at the 217-Hz fundamental frequency in the frequency domain, respectively, compared to 3-D EM simulation and measurement. Furthermore, we used the proposed model to analyze the ground noise coupling between audio circuits and TDMA RF PAs integrated on various designs of PCBs. To verify the correlation between the analysis and the actual performance degradation of audio circuits, we fabricated the PCBs, mounted commercial audio Delta-Sigma modulators and RF PAs, and measured the output of the modulators. The correlation is verified from the modulator on the PCB with the least ground noise coupling showing a spurious-free dynamic range 40.5 dB higher than the modulator on the PCB with the largest ground noise coupling.