Why is there a shift in PSD peak (using waveform chart) for a sampling rate of 20 MS/s and samples to read be 200000?

[Shruti92]

Hello!

I have been applying STFT spectrogram on the real time acquired signal and is willing to plot it's power spectrum and the spectrogram using waveform chart in the front panel in labVIEW. But I have witnessed that my frequency information is getting shifted towards 0 Hz though I expect detected frequency in the order of KHz. The signal carries a frequency of 3 KHz and as per to theory the, for the sampling rate of 20 MS/s and samples to read of 200000, 3 KHz frequency can be easily acquired. 

But why there is a psd peak near 0 Hz and not at in KHz range using waveform chart in labVIEW?  Kindly, clear the doubt. Thanks in advance.

[santo_13]

Without looking at the data we could only speculate that the signal has more near DC offset than the kHz tone.