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I've come across a setup for measuring AC power line spectrum.

Here is the spectrum graph which shows 60Hz and harmonics in a power line voltage:

enter image description here

And below is the setup and wiring for this measurement:

enter image description here

If I would do it I would use a step down transformer like 230V to 5V, then connect the primary to 230V mains and then couple the secondary to an oscilloscope and look at the FFT on the scope screen. I guess using a spectrum analyzer gives a better result than a scope's FFT function which I can understand.

But there are some points which I don't understand about the diagram above:

1.) Why are the primary windings are not connected to Line and Neutral but connected to Line and SG? If SG is the safety ground, what is it directly connected to? To the mains earth? How does it work? (Is the spectrum analyser earth grounded?)

2.) What is meant by 10Hz - 10MHz transformer? (I'm not used to see transformers by frequency but by primary and secondary nominal voltages.)

3.) What could be the reason for 1nF capacitor?

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