figure('position',[10 10 800 500],...
    'name','sampling frequency and aliasing');
axes('position',[0.1 0.1 0.65 0.8]);

a1=uicontrol('style','text',...
    'Fontsize',12,...
    'position',[150 500 100 40],...
    'string','sampling frequency(Hz)');
a2=uicontrol('style','popupmenu','string','8000|5000|4000|3000|2000|1000',...
    'position',[150 470 100 20]);

Hc_close=uicontrol(gcf,'style','push',...
    'position',[150 30 100 25],...
    'string','Close',...
    'Callback','close');
set(a2,'Callback','sam_frequency_sub');
sam_frequency_sub;