Modeling, measurement, and analysis of audio frequency ground integrity for a TDMA smartphone system

Abstract

In this paper, we first propose an audio frequency ground distribution network (GDN) model of an arbitrary shaped smartphone multilayer printed circuit board (PCB) and chassis. The proposed model enables fast and accurate analysis of audio frequency ground noise induced in the audio circuit from the switching time division multiple access (TDMA) RF power amplifier (PA) of a smartphone. The GDN is modeled as a distributed resistance model. The proposed model is verified by comparing the ground noise coupling in a test vehicle obtained from the model, three dimensional (3-D) electromagnetic (EM) simulation and measurement in the time domain and frequency domain (20 Hz ~ 20 kHz). The proposed model has been successfully verified by showing differences of 3.4 % and 22.8 % of the ground noise level in the time domain and 3.8 % and 14.8 % of the ground noise level at a 217Hz fundamental frequency in the frequency domain, respectively, compared with 3-D EM simulation and measurement. Using the proposed model, we analyze audio frequency ground integrity (GI) of various PCBs of audio circuits and TDMA RF PA integrated systems and discuss grounding design for low audio frequency ground noise coupling to audio circuits. Furthermore, we fabricate the PCBs, mount commercial delta-sigma ($\Delta\Sigma$)modulators and RF Pas switching at 217 Hz and measure the performance of the modulators. The modulator mounted on the PCB with the least ground noise coupling has a spurious free dynamic range 26.9 dB less than the modulator mounted on the PCB with the largest ground noise coupling. Through the experiment, we verify the audio frequency GI analysis made using the proposed modeling correlates with the actual performance degradation of audio circuits.