# Available Noise Power Consistency Check - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

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I miss read between the lines, if the load resistance is NOT = to the source resistance, then the above equation is no longer valid.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

RF Cafe Software

Wireless System Designer

RF & EE Symbols Word
RF Stencils for Visio
Calculator Workbook
RF Workbench
Smith Chartâ„¢ for Visio
Smith Chartâ„¢ for Excel

1996 - 2022
Webmaster:
Kirt Blattenberger,
BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:
AirplanesAndRockets.com

Try Using SEARCH
to Find What You Need.
There are 1,000s of Pages Indexed on RF Cafe !