# available noise power consistency check - RF Cafe Forums

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SmokingJoe

Post subject: available noise power consistency check Posted: Sat Sep 10, 2005 8:42 am

Lieutenant

Joined: Sat Sep 10, 2005 8:04 am

Posts: 1

Hi all,

I was reading up on noise power and etc, and I understand that 4KTR*delta_f is the maximum power transmitted(available noise power) for a resistor with a matching load resistance. Unless I miss read between the lines, if the load resistance is NOT = to the source resistance, then the above equation is no longer valid.

Now the problem I ran into is this: after a few more pages, I found a simple Thevenin eq circuit containing Source V and R, and a load C. Now Vo^2 is the noise power across the Capacitor and Vs^2 is the noise power input. Thus, Vo^2 = Vs^2 / (1 + jwRC)^2. Now my first question is:

I thought P(load) = Vs^2 x Resistance(load) / Impedance(total), and since the output impedance is modeled by just the capacitor, there shouldn't be a real resistance value at all, but the above equation applies (1/sC) as the output resistance and I don't know why. I thought (1/sC) is the complex part of the impedance and should not be utilized in the above Noise power equation?

And then the book replaces Vs^2 with 4KTRdf, so:

[b] I thought 4KTRdf is only valid when the R_load matches the R_S, so what makes the author perform the above subsitution?

_________________

I am a fourier illiterate...

Posted  11/12/2012