An experimental investigation on typical low frequency amplifiers nonlinear response to microwave/quasi-millimeter wave interference

Wireless communication devices such as mobile phones, radio-frequency identification (RFID), electric article surveillance (EAS), and contact less IC cards would be essential components to make up the ubiquitous society. In addition, new wireless devices such as mobile WiMAX and wireless USB have been developed. However, radio waves from these systems have the possibility of affecting other electronics devices and causing unwanted malfunctions in certain circumstances. Accordingly, investigations of electromagnetic interference (EMI) have more importance these days. In particular, EMI effects on medical equipment, including implantable medical devices and other electric instruments used in hospitals, should be precisely investigated because these are used widely in general circumstances. We have been conducting the investigation of the EMI on implantable medical devices [1], [2]. The EMI due to microwave/quasi-millimeter waveband radio waves on low-frequency electronic equipment is mainly caused by nonlinear characteristics of their internal circuits [3]. This is because the envelope of high-frequency signal is detected by the nonlinear response of circuit elements. This nonlinear response is possibly occurred when the frequency of exposure signals is higher than the cut-off frequency of circuit elements. In this paper, a typical low frequency amplifier nonlinear response to microwave/quasimillimeter waveband radio waves is presented. To investigate typical characteristics of interference signals generated by the nonlinear response, interference signal levels depending on frequencies and field strengths of exposure signals are experimentally obtained. Firstly, six types of low frequency basic amplifiers consisting of a single transistor or an operational amplifier IC are fabricated. Secondly, to examine the detection level depending on field strength of a 1 GHz exposure signal, the interference signal is measured. The exposure signal is a sine wave with 80 % amplitude modulation at 1 kHz. Then, to obtain the detection level depending on frequency of exposure signal, EMI experiments using a 1 GHz-25 GHz amplitude modulated signal are conducted. Finally, as practical medical equipment operating at low frequency band, measurement of interference signal using the commercially available hearing aids are carried out to obtain the EMI characteristics of actual devices.