On the SNR slide, the Noise floor looks like it is around 100dB, so how is SNR 57dBc?

On the SNR slide, the Noise floor looks like it is around 100dB, so how is SNR 57dBc?

  • +1
    •  Analog Employees 
    on Dec 7, 2020 7:39 PM

    This is a great question, I had to look it up myself as I come from a low frequency background. SNR is the signal power compared to the noise power for a certain signal bandwidth. The noise floor power is typically the integration of all the noise within a given frequency bandwidth excluding the fundamental signal and its 6 harmonics. Which means that the larger the frequency bandwidth the worst the noise power is which translates into worst SNR. This is clearly shown by looking at the noise power equation: Pnoise = 10log(Pnoise) dB/Hz + 10log(BW) Hz. 10log(1GHz) is 90dB. So looking at the AD9625 example, the NSD at 1.25GHz is  -149dB/Hz so the SNR = Psignal – Pnoise = -1 + 149 - 10log(1.25G)) ~ 57dBc. If the frequency bandwidth of interest is much lower than 1.25GHz, a LPF can be used to limit the bandwidth and in turn improve the SNR value.