Help to analyze FFT spectrum

[kickbeer]

Hi,

There is a phenomonen that i don't understand about the FFT spectrum. I just executed a FFT spectrum of the output of an ideal DAC which is connected to ADc under test.As you can see below, my fundamental signal (inpu signal) is 2 MHz and is the longest in the spectrum. The sampling rate of my ADC is 25 MHz. At 23 MHz (25MHz-2MHz) and at 27 MHz (25MHz + 2 MHz), it seems that the fundamental signals re-appear but they are shorter than fundamental signal. It happen again at (50MHz-2MHz) and at (50MHz +2MHz) and so at every factor of 25MHz(plus and minus 2MHz) Anyone understand about this?? Really need help. Thx in advanced

 

[FvM]

1. A sampled signal's spectrum is periodical in frequency with +/-n*fs and mirrored at fs/2 (aliasing effect)
2. The stepped DAC output (first order hold element) is adding a sin(x)/x frequency characteristic