RTE/RTO/RTP - Wrong amplitude when digital filter is used

Question

A sinus signal with i.e. 10 V peak-peak, frequency 1 MHz is applied. If I choose a vertical scale which is too small the peaks of the waveform are not displayed as they are out of range.

Now I activate the digital filter in the “Vertical” menu à “Channel”. The frequency of the filter is also set to 1 MHz. Now even at very small vertical scales (i.e. 50 mV/div) the waveform is displayed completely, see screenshot. But the amplitude can not be correct. What goes wrong here?

Answer

In the block diagram you can see that the digital filter is located after the A/D converter (marked in red):

A signal of 10 V peak-peak in combination with a vertical scale of 50 mV/div overdrives the A/D converter. The data is not plausible anymore, so the amplitude is wrong.

In general the A/D converter is overdriven when the waveform touches the top or the bottom of the display. Then the waveform and any kind of measurement is not correct any more.

We recommend to do the following settings:

• Apply the signal
• Make sure that the digital filter is not active
• Choose a vertical scale that the waveform is displayed completely
• In the Channel menu you may set a vertical scale to i.e. 1.234 V/div so that the waveform is displayed as big as possible.
• Now activate the digital filter
• If you want that the waveform in enlarged, do not choose a smaller vertical scale. Please use the zoom function for that.