# S/N-ratio

Discussion in 'Homework Help' started by boks, Nov 2, 2008.

1. ### boks Thread Starter Active Member

Oct 10, 2008
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0
An ideal voltage source of 1mV is in series with a 1 kOhm resistor.The temperature in the resistor is 300 H, and the bandwidth of the system is 100 kHz. What is the S/N ratio of the output signal?

I know that the S/N ratio is 10 log (Ps/Pn) dB where Ps and Pn s signal- and noise power. Ps = V/R^2, right? But what is Pn?

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Calculate the power delivered to the load with full bandwidth and the power delivered to the load with the limited bandwidth. Then subtract the second from the first to get Pn. I am not sure about that but i think its like that.

3. ### steveb Senior Member

Jul 3, 2008
2,433
469
http://en.wikipedia.org/wiki/Thermal_noise

The problem seems to want you to calculate the resistor thermal noise (Johnson Noise). This noise increases as the resistance and bandwidth each increase.

The above web-site talks about the physics of Johnson Noise and gives a formula for Vn, the RMS value of thermal noise in a resistor. Thermal noise is white noise which extends over all frequencies. So, the bandwidth is important since it limits the amount of noise detected.

Perhaps a better formula for you is 20*log(Vs/Vn), where Vs is the 1mV signal and Vn is the thermal noise in the resistor.

Note that I'm not an expert on noise either, so just take this as my best guess as to what the problem is asking.