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dB or not dB? Everything You Ever Wanted to Know About Decibels But Were Afraid to Ask

Authored on: May 15, 2014 by A. Winter

Technical Paper / Application Note

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True or false: 30 dBm + 30 dBm = 60 dBm?

Why does 1% work out to be -40 dB one time but then 0.1 dB or 0.05 dB the next time? These questions sometimes leave even experienced engineers scratching their heads. Decibels are found everywhere, including power levels, voltages, reflection coefficients, noise figures, field strengths and more. What is a decibel and how should we use it in our calculations? This application note is intended as a refresher on the subject of decibels.
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Jeff.Petro Posted Jul 13, 2011

Excellent refresher.

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zeeglen Posted Jul 13, 2011

Good info, but ignores optical measurements. For example, a 3dB optical power loss equates to a 6dB voltage reduction when the photons are converted back to an electrical signal.

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ForrestE735 Posted May 20, 2015

Zeeglen, I was convinced by a coworker never to use dBs with voltage because of that confusion. Bells are defined as a power ratio so you should never use them with voltages unless you convert back to power (squire the voltage and divide by impedance) in which case it is still 3dB. And I hope I am not spouting error myself.

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AnalogEE92056 Posted Sep 15, 2014

Can't download or View

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mortizo29772 Posted Sep 15, 2014

disable pop up blockers

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dtrescott Posted Sep 22, 2014

Or right click an open in new tab/window

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dlamm447 Posted Sep 20, 2014

Very good paper. I'll have to check out the free smartphone apps listed on page 35. My Chrome browser had a problem using the Download to Print link directly, but a simple right-click and a SAVE LINK AS got me the PDF document.

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