# The scaling of FFT magnitude and phase

In Matlab, I simulated and graphed out the FFT of a signal composed of three pure sinusoids of distinct magnitudes and phases. Figuring out the scaling of the frequency axis is fairly straightforward - the number of bins in the frequency domain is equivalent to the sample size in the time domain. So bin #31 corresponds to $$31 \times F_s/N = 31 \times 1000/1500 \approx 20 \operatorname{Hz}$$.

What I don't understand is the scaling of the y-axis (magnitude or the phase). Given the definition of DFT and the math derivation from FS to DFT, there has to be a scaling factor of $$\frac{1}{NT_s}$$ between the digital amplitude and the analog amplitude:

$$C_k \,=\, \frac{1}{T_0} \int_{-\frac{T_0}{2}}^\frac{T_0}{2} x(t) \, e^{-j \omega_0 kt} \operatorname{dt} \,=\, \frac{1}{T_0} \int_{T_0} x(t) \, e^{-j \frac{2\pi}{T_0} kt} \operatorname{dt} \,=\, \frac{1}{NT_s} \sum_{n=0}^{N-1} x(nT_s) \, e^{-j \frac{2\pi}{NT_s} k nT_s} \,=\, \frac{1}{NT_s} \underbrace{\sum_{n=0}^{N-1} x(nT_s) \, e^{-j \frac{2\pi}{N} k n}}_{\text{DFT}}$$

Take bin #31 as an example, the analog amplitude that corresponds to $$20\operatorname{Hz}$$ is $$3$$ (or two spectral lines each with height $$1.5$$ at $$\pm 20 \operatorname{Hz}$$ because $$\cos(\theta) = 0.5e^{j\theta}+0.5^{-j\theta}$$). If I scale the unscaled magnitude of $$x[31]$$ I get $$2250 \times \frac{1}{NT_s} = 2250 \times \frac{1}{1500\times 1000} = 0.0015$$, which is obviously wrong. If I want to get to the value of $$1.5$$, I have to divide the digital magnitude by $$1500$$, which is exactly my sampling rate .... why?

1. Why do I rescale the magnitude by $$1/N$$ whereas the math derivation suggests a scaling factor of $$1/(NT_s)$$?

2. If I have to scale the magnitude by $$1/N$$ I would have thought the phase will need the same scaling - after all, both the magnitude and phase originate from the same complex spectrum equation. But to my surprise, the FFT gives me the exact phase value without any rescaling - at bin #31, the phase is $$0.2$$! How is that possible?

Matlab code:

fs=1000;                    % sampling @1000Hz
t=0:1/fs:1.5-1/fs;          % from 0-1.499s in 0.001s interval so we have 1500 samples

f1=20;
f2=30;
f3=40;

x=x1+x2+x3;                 % x has 1500 samples; x is a real number
X=fft(x);                   % X has 1500 "bins" of frequencies
X_mag = abs(X);             % get magnitude spectrum from the complex spectrum; X_mag is a real number

figure(1)                   % unscaled magnitude spectrum
plot(X_mag);

figure(2)                   % scaled magnitude spectrum
plot(X_mag/1500);

figure(3)                   % unscaled phase spectrum
plot(angle(X));


FFT spectrum graphs: