measure mains with oscilloscope
1. In DC mode the display will show the exact waveform. In AC mode, the dc components will be removed and only ac components will be displayed. This mode is useful when we are monitoring the ripples/spikes of a power supply or when u are checking transient response (in case of a DSO). GND mode will not display anything other than the horizontal ground trace. At this time you can position the trace anywhere in the display using vertical position knob as per your requirement
2. Basically in CRO, the frequency is measured based on zero crossing principle. In DSO, by finding the highest component in their frequency spectrum. Both methods will not display the frequency value correctly when the signal is not sinusoidal. It is because, the non-sinusoidal signal will have many other frequency components other than its normal frequency. Noise in ground also will affect the readings.
The best way to measure the frequency is by finding the time period and taking its inverse
3. DMMs can measure only the rms value of a signal. Its not the peak value. In scopes you can see the peak voltage. It depends on your requirement that which equipments should be used. If you want peak values use scope and if you want rms, use DMM...
- SUjO