what does this instruction means?

                                                                                                         %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%                              Example 4.11                               %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%                                                                         %
%                           Pipeline Converter                            %
%                                                                         %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
clear all;
                                                                                                %-------------------------------------------------------------------------%
%                         DAC Static parameters                           %
%-------------------------------------------------------------------------%
                                                                                       %-------------------------------- Stage 1 --------------------------------%
V_DAC_L1=-0.5;
V_DAC_H1=0.5;
GAIN1=2.0;
VthL1=-0.25+eps;
VthH1=0.25+eps;
                                                                                       %-------------------------------- Stage 2 --------------------------------%
V_DAC_L2=-0.5;
V_DAC_H2=0.5;
GAIN2=2.0;
VthL2=-0.25+eps;
VthH2=0.25+eps;
                                                                                            %------------------------------- Stages 3-9 ------------------------------%
V_DAC_L3=-0.5;
V_DAC_H3=0.5;
GAIN3=2.0;
VthL3=-0.25+eps;
VthH3=0.25+eps;

Vth10=0;

                                                                                             %-------------------------------------------------------------------------%
%                             Dynamic parameters                          %
%-------------------------------------------------------------------------%
                                                                                       %-------------------------------- S&H Block-------------------------------%
srH=250e6;
f_TH=600e6;
betaH=1;
tauH=1/(2*pi*betaH*f_TH);
gainH=1;
                                                                                        %-------------------------------- Stage 1 --------------------------------%
sr1=10000e8;
f_T1=6000e6;
beta1=1/2;
tau1=1/(2*pi*beta1*f_T1);
gain1=1;
                                                                                       %-------------------------------- Stage 2 --------------------------------%
sr2=20000e8;
f_T2=6000e6;
beta2=1/2;
tau2=1/(2*pi*beta2*f_T2);
gain2=1;
                                                                                          %------------------------------- Stages 3-9 ------------------------------%
sr=25000e8;
f_T=6000e6;
beta=1/2;
tau=1/(2*pi*beta*f_T);
gain=1;
                                                                                                 %-------------------------------------------------------------------------%
%                         Simulation parameters                           %
%-------------------------------------------------------------------------%
                                                                                       Ts=1e-8;
Tmax=Ts/2;
Vfs=2;
N=2^12;
Ntransient=20;         
Tstop=Ts*(N+Ntransient);
nper=73;
Fs=1/Ts;
Fin=nper*Fs/N;                 % Input signal frequency (Fin = nper*Fs/N)
f=Fin/Fs;                      % Normalized signal frequency
bw=Fs/2;
                                                                                       Amp_dB=-0;                     % Amplitude in dB
Ampl=10^(Amp_dB/20)*Vfs/2;     % Input signal amplitude
                                                                                     finrad=Fin*2*pi;
                         %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%                           Launch Simulation                             %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
options=simset('InitialStep', 1, 'RelTol', 1e-3, 'MaxStep', 1,...
    'Fixedstep', 1);
sim('Ex4_11',Tstop, options);           %Starts Simulink simulation
                                                                                               %-------------------------------------------------------------------------% 
%                             Graphic Outputs                             %
%-------------------------------------------------------------------------%
w=hann(N);
f=Fin/Fs;                             % Normalized signal frequency
fB=N*(bw/Fs);                         % Base-band frequency bins
yfft=y(1+Ntransient:N+Ntransient);
[snr,ptot]=calcSNR(yfft',f,fB,w',N);
ptot=ptot-max(ptot);                  % Normalize total spectrum
                                                                              figure(1);
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/2 -140 0]);
sfdr=max(ptot(nper+4:N/2)) 
snr

i got the answer sfdr=-74.2306. but supposedly, the answer should be 70.9..can you tell me which part that i should do the correction.this is the main file

C.J. Harris 2013-3-22

编辑：C.J. Harris 2013-3-22

My guess is that the values in the array ptot are not what you expect. In your example above you are taking the maximum value in the array ptot between element 77 and 2048. In your case that maximum value is -74.2306, there is nothing you can do to 'correct' this, apart from changing the values in the array ptot.

lotus 2013-3-22