Mains frequency noise components

Question

I'm currently doing a lot of work in measuring transfer functions of circuits. One thing that crops up a lot is mains frequency noise:

As you can see there is a huge 11 dB leap at 50 Hz, followed by harmonics all the way up past the end of the amplitude response.

There also seem to be some side-bands around the 50 Hz. By this I mean observing the TF close to 10 Hz it is quite clean, but upon approaching 50 Hz there is an increase in noise. I don't have enough resolution to really see what this looks like properly but to me it seems like side-bands.

My question: what causes the distortion in the mains noise?

Is the power already heavily distorted when it reaches my sockets? Or is caused by the circuit? (Which for reference is a DAQ measuring an RC circuit). Is that intermodulation distortion around 50 Hz or just time-varying behaviour of the mains power?

EDIT:

To obtain the above figure, an RC circuit of 2 MΩ and 56 pF is being driven with a wide-band multi-sine signal of frequencies between 1 Hz - 192 kHz for 1 second. 100 seconds are concatenated which are then averaged for noise suppression. The FFT of the averaged output signal is then found, and divided by the FFT of the input signal (deconvolved). This results in the transfer function of the RC circuit.

However there is significant mains interference on the measured signal which has not been removed from averaging. This appears in the transfer function as a peak at 50 Hz and several harmonics. There is also disturbance around the 50 Hz peak.

Answer 1

If the question is why do you see harmonics of the line frequency popping up as EMI in your measurements of a transfer function the the answer follows:

Harmonics on the AC line are generally caused by nonlinear loads. For example, a bridge rectifier followed by a capacitor filter will cause the following kind of current waveform, as current flows only when the line voltage exceeds the capacitor voltage:

In many parts of the world regulations require power factor correction, or more precisely harmonic elimination circuitry to mitigate (but not necessarily eliminate) the problem, but lower power (<75W for example) equipment is exempt. So in aggregate this equipment can still cause harmonic distortion on the lines.

The IEC standard that applies is IEC 61000-3-2, which specifies the maximum allowable value for harmonic currents from the second harmonic up to the 40th harmonic current.