# mixed noise figure

Discussion in 'Wireless & RF Design' started by yottavolt, Sep 28, 2009.

1. ### yottavolt Thread Starter New Member

Jul 25, 2009
1
0
I'm new to telecomm and I was wondering how a SNR would be calculated if I knew the signal voltage and the noise power? Both the input and output signal is measured in voltage but the input and output noise is measured in watts. Also, this is through a 300ohm antenna amp (booster). Would the ratio be V^2/P or V^2*R/P? Or is there something I'm missing? I'm also looking for the noise figure in dB (so log10(SNRi/SNRo)). Thanks in advance if you can tell me if it's that simple above or if there is something else.

Last edited: Sep 29, 2009
2. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
You can get input and output power from voltage if you know the source impedance and the load impedance. For example transmitter outputs are often measured in dBm into 50 ohms. dBm is power with respect to a milliwatt.

For example +16 dBm into 50 ohms would be
Code ( (Unknown Language)):
1.
2. 10^(16/10) = 39.8 mWatts
3. V = SQRT(.0398*50) = 1.41 VRMS or ~4V P-P
4.
You can of course run that calculation in reverse. Given an RMS voltage and an impedance you should be able to calculat the power. If your amp expects a 300 ohm source and load you can adjust things accordingly.

As for noise figure:
Try the following article:
http://en.wikipedia.org/wiki/Noise_figure