# Amplitude Response at greater than half the sampling frequency

I am hoping to clear up some confusion I have. In a lab I am taking, we analyzed the amplitude response of a simple system. We found that as we increased the input signal frequency to greater than half the sampling frequency, the output signal began to flat line. The picture below show the corresponding Amplitude Response. I am aware of the Nyquist theorem, and I thought that the reason the output signal flatlined at frequencies greater than 8 kHz was because the sampling frequency was 16 kHz.

But during a second experiment of generating a sine wave, I discovered the concept of folding about the Nyquist frequency. Now in this case, increasing the generated signal frequency to greater than the Nyquist frequency caused the output signal to not decrease in amplitude, but to become symmetric about the Nyquist frequency.

E.g.

Generating a sine wave of 14 kHz produced an output of a sine wave of 2 kHz.

How are these two concepts related? It seems in the first example, going above the Nyquist frequency caused the output signal to flatline. In the other case, generating a signal above the Nyquist frequency didnt cause the output to flatline, but caused the output signal to only have a different output frequency.

• You are talking only about the output sample frequency and not the input sample frequency? – fibonatic Feb 23 '19 at 13:31