I was trying to understand the noise in switiching regulators and took 8192 samples of a 5V DC supply at 5Khz. If I calculate the 8k point FFT of this sequence, divide the absolute value by 8192 and square the expression, I will get the power right? It comes out as 14dB approximate. If I connect the dc supply then to a 50 ohm spectrum analyzer, would I see 14dB at zero frequency or 10*log10(5*5/50) = -3dB?
Say x is the L= 8192 long data vector of the 5V DC supply sampled at some frequency and I calculate Sx as Sx=abs(fft(x)/L).^2
The plot of fftshift(10*log10(Sx)) shows approx 14dB at center which is the power at DC or zero frequency as per the periodogram estimate definition
I am trying to understand if this will be the value that will show up in a 50 ohm spectrum analyzer at zero frequency if I hook up the same DC supply to it.
I get that dB is a relative value. But what I am looking for is what is the power a spectrum analyzer at 0Hz would show when I connect a 5V DC supply to it in dB (relative to 1W).