I met a problem when testing a signal using N9020A spectrum analyzer. The SNR of signal is different under different sweep mode. When using swept mode, SNR is obviously higher than FFT mode. Especially for the signal frequency lower than 3MHz. When signal frequency is higher, this difference become smaller. The interest thing is that when I collect the signal using my ADC board and done the FFT in software, it has exactly the same SNR as FFT mode of spectrum analyzer. Means the SNR is about 15dB lower than swept mode. Would you please let me know that what happened to these two modes and which gave the real SNR?