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available noise power consistency check - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.

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


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I miss read between the lines, if the load resistance is NOT = to the source resistance, then the above equation is no longer valid.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
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