>Can anyone help me and show me how to display low frequency
>signals in oscilloscope (what are the settings? triggering,
>level....) in order to have a stable signal on the screen
>not a moving spot.

You need a storage oscilloscope, typically known these days as a "digital 
storage oscilloscope" or DSO. The waveform is then stored digitally an will 
form a stable display on the screen.

Suitable cheap units for this work would be the Tektronix TDS220 or TDS1000 
series. Many other manufacturers also make suitable units. Taken to the 
extreme, if you do not have something handy there are units that interface 
to PCs to achieve the same functionality. 

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