DAC SNR Matlab 仿真 help？

那位帮忙解释一下以下Code的意义。
clear all;
N=2^12;% The input sequence is made by 2^12 points
Nbit=8;% 8-bit DAC
Ntransient=11;
Ntot=N+Ntransient;
nper=311;% prime integer number of sine waveforms
Fs=1;
bw=0.5;
Fin=nper*Fs/N;% Input signal frequency (Fin=nper*Fs/N)
Vref=0.5;% Full scale for an input sinewave
Amp_dB=-0;% Amplitude in dB
Ampl=10^(Amp_dB/20)*Vref;% Input signal amplitude
finrad=Fin*2*pi;
alphadx=0.0001;% gradient in the resistivity
den=2^Nbit-1+alphadx*2^Nbit*(2^Nbit-1)/2;
for i=1:Ntot,
x=Vref*sin(finrad*(i-1))+Vref;% Unipolar range
k=round(x*2^Nbit);
vout=(k+alphadx*k*(k+1)/2)/den;
y(i)=vout;
end
y=y-mean(y)% Output with zero average
%--------------------------------Graphics---------------------------------%
figure(1);
clf;
plot(y,'r');
grid;
xlim([1600 1900]);
title('Sinewave with distortion');
w=ones(1,N);
f=Fin/Fs;% Normalized signal frequency
fB=N*(bw/Fs);% Base-band frequency bins
[snr,ptot]=calcSNR(y(1:N),f,fB,w,N);
ptot=ptot-max(ptot);% Normalize total spectrum
figure(2);
clf;
plot(linspace(0,Fs/2,N/2), ptot(1:N/2), 'r');
grid on;
title('PSD of the Output')
xlabel('Frequency [Hz]')
ylabel('PSD [dB]')
axis([0 Fs/4 -120 0]);
text(Fs/10,-20,sprintf('\\bfSNR = %3.1fdB@\\alpha\\DeltaX = %1.0e\n'...
,snr,alphadx),'Fontsize',12);
text(f*2,ptot((f*N+1)*2-1)+3,sprintf('\\bf%2.1fdB',ptot((f*N+1)*2-1)),...
'Fontsize',8,'HorizontalAlignment','center');
Ntransient表示什么意思?
Ntot=N+Ntransient表示什么意思?
Fin为什么为nper*Fs/N?

simpower system