A Low Frequency Current Probe System for Making Conducted Noise Power Measurements

A technique for accurately measuring conducted radio frequency interference has been developed at the U.S. Naval Civil Engineering Laboratory. The technique involves the use of two current probes simultaneously to determine the impedance levels of the lines or equipment under measurement. The noise currents of the lines or electrical equipment are measured directly using one of the two probes. Knowing the noise currents and the impedances through which it flows provides the necessary information for computing noise voltages and noise power. Further, it allows prediction of the noise power delivered by the measured device into any known impedance connection. The bandwidth covered by this technique is 20KC-30mc. This range is covered by two instruments, the first covers 20KC-2mc and the second 2mc-30mc. The high frequency instrument has been described before and will not be discussed here. The low frequency instrument is new and has several unique features. The main feature being a direct read out of both the impedance magnitude and the phase angle of the system being measured. This is accomplished by transistorized circuits which drive two front panel meters, one for impedance magnitude and one for phase angle and its polarity. By providing the phase information it is possible to specify the noise observed in terms of noise power. This technique removes the many ambiguities which are normally associated with conducted noise measurements using line impedance stabilization networks.