S/N in dB question.

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CIVEDM

Joined Jan 28, 2014
38
Av = 34dB, NF = 12dB, Vi = 40mV, Ni = 1mV

I already solved this problem. The professor wanted the Vo and So in voltage. I got So = 2V and No = 200mV. I calculated the S/N = 20dB, which I know if correct. If I convert the So and No to dab I get So = 6dB and No = -14dB. The only way to get 20dB from that is to: 6dB - (-14dB), but I don't know why..... First is that right? If so why subtract the noise from the signal?
 

JoeJester

Joined Apr 26, 2005
4,390
The Av is a distractor as it will amplify the noise and the signal the same. 20 log (40) =32 dB. 32 dB- 12 dB = 20 dB.

dB use to be common, as all you needed to do was add or subtract. I can remember a receiver sensitivity was spec'd at -106 dBm.
 
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