variable sampling frequency-FFT

Hello all, First a little bit of background: I have constructed a continuous time signal from the output of a Sigma-Delta modulator whose output is varying frequency.. Therefore, this signal has varying sampling frequency(which can take 4 values)..I am trying to find the FFT(rather plot the spectrum) of that signal... For that I looked around in the forums and found that we use NDFT for that... but all the discussions I have found are extremely confusing.. Can anyone kindly clarify me of the procedure to follow to obtain the spectrum of such a signal..

Thanks in advance..

[Matt J]

It's not clear why you think that the sampling frequency varies just because the continuous signal's frequency varies (if that's in fact what you're saying). The two are normally independent of each other.