adequate sampling frequency in measuring acoustic emission I am measuring acoustic emission signals. The a.e. frequency range is 20kHz to 1 MHz. The sampling frequency that I,set was 100kHz
Samples per second. I must be able to detect a maximum of 50 kHz signal using it. But when I do fft I get a maximum of only 300Hz. 

Can anyone tell me where I would have gone wrong?
How are you determining your maximum frequency?

If you sample at $f_s$ the frequency resolution (the frequency difference between frequency components) of your FFT is $f_s/N$, where $N$ is the number of samples collected. And the maximum resolvable frequency is the Nyquist frequency corresponding to the sample rate. For more information see here