SCIN: Shield Current Induced Noise

Introduction My previous note focused on pin 1 problems as an open door for RF into audio equipment. But RF can also enter equipment on the signal conductor(s) if the equipment fails to block it with an effective low pass filter. RF is induced on a shielded balanced pair by at least three mechanisms: • Imbalance in the magnetic coupling between the shield and the two signal conductors (Shield-current-induced noise). • Voltage gradients resulting from imbalance in the capacitances between the two signal conductors and the shield. • Coupling of the electric field through tiny openings in the shield. In this note, we’ll focus on the first of those mechanisms. For the same 1994 paper that introduced us to the “pin 1 problem,” subsequently published in the June 1995 issue of the Journal of the AES, Neil Muncy set up an experiment to study what happens when current flows on the shield of audio cable. With a power amplifier and output transformer driving the shield of audio cable with 100 mA sine and square waves at 60 Hz, 600 Hz, and 6 kHz, he measured the voltage induced on the signal pair. He called this voltage “shield-currentinduced noise” (SCIN), noting that its amplitude was proportional to frequency and that it was due to the imbalance in magnetic coupling between the shield and the two signal conductors. He measured cables with a variety of shield constructions, and observed that cables with certain types of shield construction had much greater SCIN than others.