IDFT - influence of sample numbers I'm quite a newbie in dsp domain, and there's something I can't understand clearly.

I'm doing a IFDT of a spectrum regularly spaced with complex values : the spectrum is a complex conjugate symetric frequency domain sequence so the output of the IFDT should be real.
It is when using a sampling rate of 50MHz with 2048 samples.
But when I change the number of samples (for exemple 1024), it does not work anymore.

Can someone explain why it won't work ? I'm clearly missing something...
How are you changing the number of samples ? If you are just truncating the 2048 samples $X$ to 1024 samples $X^t$, then the samples won't be complex conjugate anymore. 

First sample of the 2048 length sequence will be considered as DC and then the remaining should be conjugate symmetric around $N/2$. That means, $X[k] = X^*[2048-k], \forall k \in \{ 1, 1023 \}$ and $X[1024]$ must be real. Then IDFT will give real $x[n]$.

When you want to downsample it to 1024 samples, then, first of all there will be aliasing in time -domain but still you can get real time domain samples if the downsampled 1024 length $X^t$ maintains the above explained conjugate symmetry.