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

The two current probe technique of measuring conducted radio-frequency interference is a radical departure from conventional measurement techniques. The technique evolved from a dissatisfaction with present techniques, due to the lack of sufficient information obtained from measurements concerning the source of the interference. Normally, the wires concerned when testing for conducted interference are 60-Hz power lines of the two conductor type. If a pair of conductors comprising a power cord is considered as a transmission line, the factors which determine its performance at radio frequencies are primarily the wire length and loading. In practice, power lines do not have readily described configurations, which makes a transmission-line analysis extremely difficult. As a result, no attempt to analyze the situation is made at all. Usually, a line impedance stabilization network (LISN) is inserted into the line and the noise voltage developed is measured across a 50-ohm internal resistor which is effectively placed across the line. Thus, the only information obtained is a noise voltage which existed across a resistor at the time of measurement, but is nonexistent after the test. The noise voltage existing across the line after the LISN has been removed remains unknown. As a matter of fact, it cannot even be calculated, since the line characteristics are unknown. Thus insufficient information is the reason for pursuing the current probe techniques. The two current probe technique of measuring conducted interference overcomes the limitations of the LISN technique by supplying more information about the line.