After extensive research I found some information hinting at my freq array being wrong. I thought that the data spanned -F/2 to F/2. It seems that I should only plot the first half of the fft vector? Can anyone confirm or shed some light on this?
One other thing I thought the magnitude should be 10 for 10* sin(2*pi*10*t), my magnitudes are much higher. delic wrote: > > I wrote a script that I anticipating seeing a spike at 10Hz with the > function 10* sin(2*pi*10*t). > I can't figure out why my plots do not show spikes at the frequencies I > expect. Am I doing something wrong or is my expectations wrong? > > > require(stats) > layout(matrix(c(1,2,3), 3, 1, byrow = TRUE)) > #SETUP > n <- 256 #nextn(1001) gives next power 2 > F <- 100 #Hz -50 to 50 Hz > dt <- 1/F > T <- n*dt > df <- 1/T > t <- seq(0,T,by=dt) #also try ts function > t<-t[1:length(t)-1] > freq <- 5 #Hz > > #SIGNAL FUNCTION > y <- 10* sin(2*pi*10*t) #10*sin(2*pi*freq*t) > #FREQ ARRAY > #f <- seq(-F/2,F/2,by=df) > f <- F/2*seq(-1,1,length.out=n+1) > f<-f[1:length(f)-1] > #FOURIER WORK > Y <- fft(y) > mag <- sqrt(Re(Y)^2+Im(Y)^2) > phase <- atan(Im(Y)/Re(Y)) > Yr <- Re(Y) > Yi <- Im(Y) > > #PLOT SIGNALS > plot(t,y,type="l",xlim=c(0,T)) > plot(f,Re(Y),type="l") > plot(f,Im(Y),type="l") > -- View this message in context: http://www.nabble.com/Fourier-Transform-fft-help-tp25475063p25477281.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
