fft amplitude in mV
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From: Martijn Roelofsz on 12 Jul 2010 08:45 I am trying to plot an fft of a known sinusoid signal. The input signal is a 20mVtt, 200Hz sinus. Duration is 0.2sec. 500kHz samplerate. When i calculate the fft I expect a spike at 200Hz with a magnitude of 10(in this case mV). y = fft(X); m = abs(y); plot(m); The fft shows a amplitude of 4.6922 at 200Hz. Why is it not 10(or very close to 10) and how can I scale it back to mV again?
From: Lothar Schmidt on 12 Jul 2010 09:13 Am 12.07.2010 14:45, schrieb Martijn Roelofsz: > I am trying to plot an fft of a known sinusoid signal. > The input signal is a 20mVtt, 200Hz sinus. Duration is 0.2sec. 500kHz > samplerate. > When i calculate the fft I expect a spike at 200Hz with a magnitude of > 10(in this case mV). > y = fft(X); > m = abs(y); > plot(m); > > The fft shows a amplitude of 4.6922 at 200Hz. Why is it not 10(or very > close to 10) and how can I scale it back to mV again? one keyword interesting to you is leakage
From: Greg Heath on 12 Jul 2010 13:04 On Jul 12, 8:45 am, "Martijn Roelofsz" <martijn.roelo...(a)nlr.nl> wrote: > I am trying to plot an fft of a known sinusoid signal. > The input signal is a 20mVtt, What does "tt" mean? >200Hz sinus. Duration is 0.2sec. 500kHz samplerate. Fs = 5e5 tmax = 0.2 dt = 1/Fs % 2e-6 % t = 0:dt:T-dt; % t = dt*(0:N-1); T = tmax+dt % 0.200002 N = T/dt % 100001 > When i calculate the fft I expect a spike at 200Hz with a magnitude of 10(in this case mV). > > y = fft(X); y = fft(X)/N; % will give the correct spectrum scale > m = abs(y); > plot(m); > > The fft shows a amplitude of 4.6922 at 200Hz. Why is it not 10(or very close to 10) and how can I scale it back to mV again? It's not clear how you obtained that number. Hope this helps. Greg Greg
From: Martijn Roelofsz on 13 Jul 2010 02:26 Greg Heath <heath(a)alumni.brown.edu> wrote in message <0f2e8b34-9dc2-44b9-9e16-dcc474e036af(a)i1g2000vbk.googlegroups.com>... > On Jul 12, 8:45 am, "Martijn Roelofsz" <martijn.roelo...(a)nlr.nl> > wrote: > > I am trying to plot an fft of a known sinusoid signal. > > The input signal is a 20mVtt, > > What does "tt" mean? > > >200Hz sinus. Duration is 0.2sec. 500kHz samplerate. > > Fs = 5e5 > tmax = 0.2 > dt = 1/Fs % 2e-6 > > % t = 0:dt:T-dt; > % t = dt*(0:N-1); > > T = tmax+dt % 0.200002 > N = T/dt % 100001 > > > When i calculate the fft I expect a spike at 200Hz with a magnitude of 10(in this case mV). > > > > y = fft(X); > > y = fft(X)/N; % will give the correct spectrum scale > > > m = abs(y); > > plot(m); > > > > The fft shows a amplitude of 4.6922 at 200Hz. Why is it not 10(or very close to 10) and how can I scale it back to mV again? > > > It's not clear how you obtained that number. > > Hope this helps. > > Greg > Greg tt is the top to top voltage. In this case 20mV.
From: Steve Amphlett on 13 Jul 2010 02:43
"Martijn Roelofsz" <martijn.roelofsz(a)nlr.nl> wrote in message <i1f2ok$1cv$1(a)fred.mathworks.com>... > I am trying to plot an fft of a known sinusoid signal. > The input signal is a 20mVtt, 200Hz sinus. Duration is 0.2sec. 500kHz samplerate. > When i calculate the fft I expect a spike at 200Hz with a magnitude of 10(in this case mV). > > y = fft(X); > m = abs(y); > plot(m); > > The fft shows a amplitude of 4.6922 at 200Hz. Why is it not 10(or very close to 10) and how can I scale it back to mV again? Putting leakage to one side, you are mainly not using the number of points to scale your transform. N=length(X); m=abs(y)*2/N; (Note that the DC and Nyquist frequencies should only be scaled by 1/N, but that detail isn't hugely important here.) |