Understanding the Parasitic Effects on the Microwave Interferences at Hearing Instrument Level up to 6GHz

Abstract: The use of electronic devices which radiate high power electromagnetic waves has seen a significant increase in the last 20 years. These radiated signals heavily effects electronic devices close to strong radio frequency (RF) or microwave (MW) transmitters . Mobile phones are a good example among electronic devices which cause signal interference in hearing instruments (HI). Changing the design of mobile phones in order to have them transmitting at lower frequencies or lower RF power is not possible. Looking back to 1993, HI electronics had to be immune against E-field levels of 3V/m and 2V/m in the frequency bands 800 - 960 MHz and 1400 – 2000 MHz. In 2015, however, they had to be immune against E-field levels of 90 V/m, 50 V/m and 35 V/m in the frequency bands 700 - 1000 MHz, 1400 – 2000 MHz, 2000 – 2700 MHz. For the remaining f-bands in the range 80 MHz – 6000 MHz, they have to withstand 10 V/m (acc. to the medical device EMC standard IEC 60601-1-2, ed.4). Currently, about 3 percent of the population in USA and EU wear a HI. A large part of these people have got their own mobile (or smart) phone and do not like to be disturbed by a loud buzzing sound coming from their HI when they do a call with their mobile. For this reason, HI immunity against strong RF/MW signals is a MUST and the EMC standards of the HI industry are much more demanding compared to the medical device EMC standard IEC 60601-1-2, ed.4. To deal with the problem and accept the challenge to design and manufacture reasonably immune RF devices requires a significant amount of analysis and modeling work in the RF/MW domain. The essential requirement is to (re-)design these complex small devices such that they are more immune to external RF/MW signals. A way to achieve this goal is to filter the RF/MW signals (which are unavoidably picked up also by small devices like a HI) close at source (directly inside the HI). Another approach is to further minimize the unwanted interaction between the RF/MW transmitter and the small electronic device. This can be done by lowering the parasitic RF pickup of the small electronic device. For example, a HI can be considered in an environment consisting of many microwave antennas (PMWA) which all contribute to the unwanted RF pickup. To further improve the HI immunity against RF/MW signals, the HI PCB (flexprint, interconnecting to the PMWA) has to be redesigned to get a poor PMWA impedance match. The two methods, RF filtering and impedance mismatch, have to be combined to get a good result. A 3rd approach would be to improve the RF/MW immunity of the audio-amplifier or the microphone on the integrated circuit itself, but such an approach is in most cases far beyond the scope and opportunities of the HI designer. All the above analysis work can only be done with the help of state-of-the-art simulation tools (e.g. CST microwave studio for 3D-EM simulations, AWR Design Environment for circuit impedance matching) and expensive equipment for RF/MW measurements (e.g. a network analyzer running up to 6 GHz, a GTEM cell for antenna measurements). In the master thesis report “Understanding the Parasitic Effects on the Microwave Interferences at Hearing Instrument Level up to 6GHz” (by Thomas Sofya, University of LUND) describes how the immunity issue of HI was tackled. While the findings of this work are of considerable value to the HI industry, their application is evidently beneficial in the design and manufacture of Audio Electronics as a whole.