A low frequency electric field probe for near-field measurement in EMC applications

A low frequency electric field probe with high sensitivity was designed for locating radiated emissions source in electronic systems from 10 kHz to 100 MHz. This probe consists of a dipole at probe tip loaded with an op-amp built instrumentation amplifier, whose output then is amplified by a cascaded RF amplifier. The probe dipole is capacitively loaded by the instrumentation amplifier, resulting in a flat frequency response with high sensitivity at low frequency. The instrumentation amplifier also converts differential dipole output to single-ended signal without the need of an external hybrid. Additionally, the instrument amplifier suppresses common-mode noise picked up by the probe dipole. Measured data from probe above a microstrip trace shows flat frequency response of wanted field coupling and 30 dB – 40 dB suppression of unwanted field coupling from 10 kHz to 100 MHz.