Decibel Values from Volts Conversion? - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 02, 2013, 03:58:07 pm
content: i cant figure this out and wiki reference is confusing. i need a straight forward answer. how to convert volt value to decibel? () for example, i feed a pure 1Vpp sine into spectrum analyzer, what reading in decibel should i expect the SA will give? and then let say the signal is not pure it got harmonic 1mVpp, then what decibel it is? thanks.

author: mamalala
author_rank: Unknown
timestamp: January 02, 2013, 05:02:28 pm
content: dB is not an absolute measurement like volts or amperes. It is always referenced to something. There simply is no straight conversion possible. Plus, the waveform itself plays an important role as well.http://www.mogami.com/e/cad/db.htmlGreetings,Chris

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 02, 2013, 05:13:33 pm
content: ok watched the dave video. its dBV = 20 * log(volt). otoh, for dBm it should be 10 * log(V^2/100) for 50ohm terminated line (50ohm source + 50ohm destination = 100ohm). this is from watching Shariar (the signal path) video his rigol SA and FG is in dBm unit. so when he's outputting -3dBm it means 223mVrms or 316mV amplitude or 632mVpp sine signal. right, thank you everybody.

author: aghp
author_rank: Regular Contributor
timestamp: January 02, 2013, 05:31:13 pm
content: This Minicircuits pdf table is handy.http://217.34.103.131/pages/pdfs/dg03-110.pdf".... -3dBm it means 223mVrms or 316mV amplitude or 632mVpp sine signal. right, thank you everybody."Wrong.-3dBm sinewave is around 160mV rms (in "50" ohm system)and it is around 453mV p-p(0dBm is around 225mVrms)(Exatly 50.00 ohm system 0dBm (1mW) is 223.607mV rmswhy RF specialist Minicircuits use 225mWHere is something, just for "fun" becouse real world is not idealhttp://www.jensign.com/RG58U/)

author: robrenz
author_rank: Super Contributor
timestamp: January 02, 2013, 06:10:24 pm
content: From my Radio shock data book1 dBm = reference level of 1milliwatt , 600 OhmsVU also = reference level of 1milliwatt , 600 Ohms1 dBv = reference level of 1V1 dBw = reference level of 1W1 dBk = reference level of 1kW1 dBvg = Voltage gainThere are 2 different formulas for dB of current or voltage and dB of power

author: aghp
author_rank: Regular Contributor
timestamp: January 02, 2013, 06:23:57 pm
content: Quote from: robrenz on January 02, 2013, 06:10:24 pmOf course if there is 600ohm also then need more voltage if want 1mW power. It needs in 600ohm system 0.77Vrms(this is why normally always need take care what impedance we are talking. 1Mohm system we need 31.6Vrms for 1mW aka 0dBm)

author: dr_p
author_rank: Frequent Contributor
timestamp: January 02, 2013, 07:52:07 pm
content: AFAIK, the reference of the 0 dB value is the voltage level which delivers 1 mW of power in a 600 ohm resistor (old standard reference impedance in telephone circuits). It translates into some 0.775V RMSThis being the "regular" dB, not dBm or dBa or any other.

author: robrenz
author_rank: Super Contributor
timestamp: January 02, 2013, 07:59:52 pm
content: dB on Wikipediahas a nice listing of all the suffixes/reference levels for various engineering disciplines.

author: aghp
author_rank: Regular Contributor
timestamp: January 02, 2013, 08:54:20 pm
content: Quote from: robrenz on January 02, 2013, 07:59:52 pmWikipedia is just for fun.Example in external links there is this...https://sites.google.com/site/dbmmadeeasy/home...keep fun!maybe there need be some source criticism before write wikifunmedia. But many peoples trust wikipedia as "truth". Yes, there is right things and wrong things.. but what if reader do not know and he just trust becouse it must be truth becouse it is in wikipedia and wikipedia is truth becouse wikipedia is truth." If you substract 3dB from the voltage, the power wil Sqrt(3), voltage will half."and so on...Perhaps I need do all my lab works agen.. and write all equipments specifications agen... or...Perhaps I still want look McCrawHill or others and just let wikipedia live its own fun life.

author: FenderBender
author_rank: Super Contributor
timestamp: January 03, 2013, 12:11:15 am
content: If you have the Art of Electronics, it's explained pretty well. It's right in the beginning.

author: Sm58
author_rank: Contributor
timestamp: January 03, 2013, 10:06:02 am
content: Quote from: dr_p on January 02, 2013, 07:52:07 pmWhat you describe is exaktly what dbm is.There is no "regular" dB. Well there is, but that means nothing more then a logarytmic relation between two things. It is usualy then named as DB or DBr (r=relativ)A absolute DB scale use to have a referenz level. The Problem is, that there are so many different references...-DBspl (sound preassure level)Ref: 0.000002 pa (pascal) = 0 DBsplAcoustic loudness meassurement-DBmRef: 1mW (into a 600 Ohm terrmination--> 0.775 Volt)ITU standart in telefone circuits. It was the bestCompromise to delifer the most power ouver Long distances.-DBV (Capital V, --> Volt)Ref: 1 Volt = 0 DBVConsumer audio level.DBuRef: 0.775 Volts = 0 DBuProfessional Studio and lifesound level.If such a circuit is terrminated with 600 Ohm, it corresponds with DBmDB A/B/C curveRef: None !!It is just a a filter added in the frequency Domain.There are many many more...So it is really important to know what reference is used. Otherwise you compare Apples and oranges.

author: dr_p
author_rank: Frequent Contributor
timestamp: January 03, 2013, 11:01:58 am
content: Quote from: Sm58 on January 03, 2013, 10:06:02 amYou're right, I wasn't paying attention.

author: EEVblog
author_rank: Unknown
timestamp: January 03, 2013, 11:45:16 am
content: Quote from: Sm58 on January 03, 2013, 10:06:02 amBe careful, dBm doesn't have to referenced to 600ohms. It can be referenced to whatever circuit impedance the value is referring too. For spectrum analyser and RF stuff, dBm is usually 1mW referenced to 50ohms.Dave.

author: aghp
author_rank: Regular Contributor
timestamp: January 03, 2013, 12:35:23 pm
content: Quote from: EEVblog on January 03, 2013, 11:45:16 amdBm is related to milliwatt. All (I hope) know that 0dBm = 1mW (decibel related to one milliwatt power). Independent of system impedance. But, as stated mostly today in RF systems impedance is (nominal) 50ohm.If we talk about dBm it is cood practice to also tell impedance if it is not just clear. Specially if we talk also example voltages in system.Also in test gears, 50ohm is not only impedance but very common today. There is lot of stuffs also with example 75ohm but still there can use (of course) dBm. Just for one example, my one Wandel&Golterman measuring receiver (selective RF volt meter - receiver) have Zi/ohm settings 50,60,65,75, 124, 135 and 150 ohm and its measuring range is -100dBm to +24dBm.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 03, 2013, 01:09:52 pm
content: Quotei imagine there is usually one 50ohm resistor at the source output (FG), and then another 50ohm to ground as destination output (SA), so the source (FG) is driving 100ohm load (ie to ground isnt it?). so why still use 50ohm as effective load for power calculation? this is quite confusing.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 03, 2013, 01:19:10 pm
content: ok ok i get it now... the FG output voltage is measured after the 50ohm source impedance, iehalved of what the FG's internal is outputting, so what matters is the power consumed in the 50ohm termination at the destination end. yes it should be 50 ohm. so -3dBm = 10 * log(V^2/50), hence V = 158mVrms or 224mVmax or 448mVpp. thanks for correction!

author: Sm58
author_rank: Contributor
timestamp: January 03, 2013, 03:04:41 pm
content: Quote from: EEVblog on January 03, 2013, 11:45:16 amYou are right.