Zero-pad to chirp signal I have generated 16 chirps in experimental study:



and the signal was picked up by some electronic sensor, and then I have calculated the spectrum of the received signal:



Following that I have calculated the cross spectrum density between the sound emitter and receiver and did some post processing, i.e.:

x=voltage1(:,2);    
y1=vertgeo1(:,2);

Fs=1000;
window=hamming(512);
noverlap=512/2
nfft=512;Fs=1000;

[Pxy1,f]=cpsd(x,y1,window,noverlap,nfft,Fs);


From this code the number of the points that shows the frequency resolution, essentially the number of the point that f contains is $1\times 256$.

I would like to know how I can increase this to $1025$ points.
No you don't need to zero-pad each chirp. MATLAB's cspd function divide the signal into segments of length nfft. Would you want is to zero-pad each segment. Luckily for you, the cspd function can do it for you. Take a look here:


  If $\tt nfft$ is greater than the signal length, the data is zero-padded. If $\tt nfft$ is less than the signal length, the segment is wrapped so that the length is equal to $\tt nfft$.


So what you should do is increase the value of nfft from $512$ to $2048$.