【求助】怎样去除FFT的频率折叠问题

[hglf117]

clear ;
clc ;
f0 = 600;%初始采样频率为600Hz
df = 200;
f1 = 100; %信号频率100Hz
f2 = 750;%信号频率750 Hz
k = 5;
for i = 1:k
fs = f0 + (i-1)*df;
dt = 1/fs;
t = 0:dt:1024*dt;
x1 = sin(2*pi*f1*t);
x2 = 2*sin(2*pi*f2*t);
X = x1' + x2';
Y = fft(X);
y = abs(Y(1:1024));

subplot(k,1,i);
plot(y);
ticks = 0:1024*100/fs:1024;
label = 0:100:fs;
axis([1,1024,0,1000]);
set(gca,'xtick',ticks);
set(gca,'xticklabel',label);
grid on;
end

其中750Hz的信号不想采进来的，怎样能够去掉？