Q:

Plug-in supply with ac output

I have a plug-in-supply unit with "220V ac 50Hz" input and "12V rms" output. 
My question: Does anybody know how the waveform and the frequency looks like for such a off-the-shelf device?  
EDIT: It is an italian device called "Transformatore elettronico" and the weight is only 70 grams.  So it is kind of switched power supply - however with an ac output. 
That was the background of my question.  

A:

It would likely be a SMPS intended for driving Halogen lamps with 12Vac or compatible lamps. 
The primary is FW rectified then creates a T1 relaxation oscillator to drive a halfbridge and step-down transformer at higher f such that a smaller core can be used.  The Vdc is split by two plastic caps to create a bipolar hi Vdc supply with centre tap of the caps to transformer input on one side and L C filtered to driver output with complementary switches. 
T1 provides the ultrasonic switching clock rate (eg>20kHz) with positive feedback, while the diac triggers AM to modulate this carrier at twice the line frequency to drive T1 with more Ac current. 
e.g.

The waveform would look pretty ugly depending on load and rich in harmonics of 1f, 2f with carrier fosc.   EMI??  but it just serves to drive lamps.