Improvement of SNR by sampling different number of cycles using FFT of a fixed length singal
Ok, so, what you need to understand is that FFT has particularities that if not taken into account produces erroneous conclusions out of its results. Firstly, both of your signals have the same SNR. Why? The way you described the problem led me understand that you tried to put two identical signals into different process paths to understand why they came out differently. Am I right? So, what are you really trying to ask is “Why both paths led to different results?”. If that’s the truth, allow me:
- FFT can generate lobes around the real frequency in many cases: 1. when you are trying to see a frequency that is not part of the indexes when you selected the FFT lenght; 2. When you don’t use a proper window function prior to FFT analysis; 3. When you have a small window relative to the frequency you’re trying to see and you produce a discontinuity. This is because the FFT algorithm considers that everything you have in your analysis window is a periodic pattern. Probably the 3rd reason is letting you believe that your second measurement has a higher SNR when the truth is your window has only 10 periods of your frequency, and it’s much likely that you have a discontinuity between the beginning of it and the end. If you fix this discontinuity you should see a clear FFT in your second case.