It sounds to me as if you're trying to move the signal to the center of a FFT bin so that you don't have to deal with scalloping loss. Is that correct? If so, I strongly recommend that you skip all of this resampling and just use a "flattop" window in the FFT. That will allow you to measure the maximum amplitude with very high accuracy (roughly within 0.01 dB) regardless of where the signal resides within the spectrum.
If that's not satisfactory, then I recommend that you skip the "having a sample rate that is a power-of-2 Hz" and just move the signal to the center of a frequency bin with whatever sample rate you have. The bin frequencies are simply (sample rate)/N, where N is the number of samples in the FFT. So with a sample rate of 16 kHz and a 512 pt FFT, you'd select a frequency that was an integer value of 16000/512 = 31.25 Hz. So, if you want to move it to, say, the 10th bin, move it to 31.25*10 = 312.5 Hz. Done. Good luck! Gary ************************************ I am reading Digital RF data at 16k samples/second, and my goal is to get the power of the maximum frequency once per second. I start by resampling to a power-of-two rate, then translating to move the desired frequency to the center with further decimation, then doing an FFT, converting to log power and finally extracting the power of the loudest bin. The decimation and FFT size are calculated to yield one FFT per second. I'm not sure if I'm doing this correctly and I've attached the flowgraph. (a) Should the FFT use only a power-of-two bin size? I am resampling to go from 16k to 8192 samples/second so that I ultimately decimate to 512 samples/second rather than 500 samples/second. Is that the right thing to do? Should I resample up to 16384 samples/second rather than down to 8192? Or should I just use a 500 bin FFT? (b) Given that the final sample rate and FFT depth are equal (512), I expected to get one vector per second, and thus one maximum value per second. Instead, I see a much slower update rate, about once per five seconds. I added a QT vector sink and while that takes a long time to get started, once going it does update about once per second. Am I doing something wrong that the maximum value doesn't update with each new vector? Thanks, John
