function d=q_psfd_cal(data,psf,fs)
% command object: calculate power spectral density of frequency specified by psf
% command format: d=q_psfd_cal(data,psf,fs)
% command parameters: data, one array of time data
%                     psf, focused frequecy
%                     fs, sample frequency
% other descriptions: 

[tf,f,t]=spectrogram(data,kaiser(fs,2),fs-1,fs,fs);
temp=abs(tf);
d=mean(temp(psf+1,:),1);