Electromagnetic radiofrequency interference with Doppler equipment.

Abstract

Doppler ultrasound equipments incorporate radiofrequency (RF) receivers operating at the microvolt level and are liable to interference from radiated fields and mains disturbances. The most significant interference routes are: radiation from radio broadcast, paging, communication and diathermy picked up on the transducer and patient acting as an aerial; and mains disturbances from diathermy, x-ray sets, motors and thyristor controls, etc, reaching the equipment interior. Direct mains interference can be reduced by proper design incorporating a mains filter, a low RF leakage enclosure, a ground plane, careful layout and further screening of the receiver circuits. Pick-up via the transducer occurs even if the signal leads are completely screened because an RF potential can exist between transducer and equipment enclosure due to the considerable impedance at RF of any wire or cable, typically 70 omega at 5 MHz for 2 m. This potential, reduced by about 50-60 dB, appears at the receiver input because of imperfect common mode rejection of the coupling cable. As a result, induced voltages above 100-300 microV may cause problems. It is shown that such levels can easily result from the interfering field strengths of 1 mV m-1 or more that may be experienced in a hospital environment. On the other hand, field strengths of 300 microV m-1, as generally allowed by regulatory standards at 3 m from interfering sources, should not cause much effect. Other interference can arise from modulation of a strong RF signal by mains frequency power components in the Doppler equipment and from associated computer circuits. These require careful layout to reduce electric and magnetic coupling, decoupling and filtering of power supplies and components, screening of RF circuits and particular attention to the reduction of power and computer signal currents flowing in RF signal earth paths. Finally, some initial simple acceptance tests for interference susceptibility are proposed, based on the application of 300 microV of RF signal between transducer body and equipment enclosure and 10 mV to the mains supply.