Ken, Erik, thanks for your reply. I have checked it again with a square wave from a VCO type oscillator (LTC1799). Exactly the same picture. Yes, it is aliasing. The aliased signal part after A/D conversion maps back to the frequencies below the sample frequency. If the mapped back harmonics of the ...

Dear folkes, I am using a 3204 with 50 MHz bandwith and 50 MHz sampling rate for amateur radio purposes. Taking a look at a MHz square wave from a DDS generator gives very strange results. If the signal is not a multiple of the sampling frequency, I got large peaks below and above the base frequency...