FFT frequency shift during downsampling

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A VP
A VP 2020-4-17
评论： A VP 2020-4-19
I have a signal at 1000Hz and I am calculating FFT of the signal. I see it's first order contents at 11Hz. However when I downsample the signal to 100Hz and re-run FFT, I do not see a peak at 11Hz. Instead I see a peak at 9.6Hz.
Can anyone explain why I see this behavior when I downsample a signal?
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David Goodmanson
David Goodmanson 2020-4-18
Hey Kalyan,
In the link they appear to be transforming a function cos(30*x) to the spacial frequency domain. In the time domain this would be cos(30*t) = cos(w*t) so w = 30. That means f = w/(2*pi) which is around 5, which is where their peak is. I don't see what the problem is.
A VP
A VP 2020-4-19
Hi David Goodmanson,
Yes, I am doing an FFT of a 600s signal sampled at 100Hz. [total : 600001 samples]. I am looking at FFT of every 5seconds [500 samples] and have used a Hanning window with a 50% overlap.

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