Relation of FFT with sampling frequency

Hi,

I have a time series record ,which is sampled at 10kHz. For the signal processing i need to change it to 20kHz as other signals are at that frequency.

I tried the resampling with resample command. resample(x,20000,10000); When i do a power spectrum analysis or fft i can see the same signal sampled at two different frequencies are showing different spectrum. I was expecting both the FFT will yield same result as the energy content of the signals are same. Why it is happening like that?? When i checked with a simple sin wave sampled in two frequencies, the spectrum looked different.

Please Help

[Honglei Chen]

I didn't quite get what you try to describe. Do you have a picture you can upload or a simple code to reproduce the plot?