EEVblog #49 – Decibels (dB’s) for Engineers – A TutorialPosted on December 13th, 2009 50 comments
Are you a pessimist or an optimist? Dave explains dB’s and how they are useful for us engineering types.
What happened ?
I watched #49 and a few hours later it was gone. Now it’s back again. Was I hallucinating ?
Oh and here’s a blog post at “Make” about the engineer who developed the NE555 timer chip.
Where I come from “jaja…” is easily interpreted as an equivalent to Bender’s (futurama) favourite line:
“Bite my shiny metal A..”
When you talk about dBm’s you say that the reference is one milliwatt, so -3dBm are 0.5 times a milliwatt, but in your video you write two times that
-3dBm = 0.5 W
Keep up the good work!
Nice explanation of the decibel measure. I remember this from my study for my amateur radio license.
That explains why my old stereo amp seemed to have its volume scale drawn “backwards”, with the numbers getting “smaller” as the volume went up… compared to normal amps where the volume starts at “0″ and goes up to “11″.
The scale on the volume knob was showing me the number of dBs below the maximum power output of the amp, not how “loud” it is compared to being switched off.
you got all your dBm’s wrong, by no less then three orders of magnitude Which is, by the way, on of the great dangers of dB. If you get them wrong, it is usually wrong big time.
As you said, dBm is with reference to 1 mW. But then you equate -3 dBm with 0.5 W. 0.5 W would be 27 dBm, not -3 dBm. -3 dBm are 500
great tutorial. All of these rule of thumb values are rounded by the way…6db would be a ratio of 1.99526 (also rounded). Not trying to be a nitpicker.
Greetings from Germany!
No no no Dave. When you’re an Engineer the glass is neither half empty or half full. The glass is twice as large as it needs to be
Good blog. The only thing I have to add is that dBm is actually referenced to 1mW @ 600Ohms. I’m not aware that it’s ever used at different impedances, but that’s something to keep in mind.
Also, most multimeters that I’ve used only work in dBms. I’ve often wondered why this is, because I’ve more often found dBVs a more convenient expression.
The Agilent (or HP) 34401A can work in dBs, but you have to set the reference point first, it doesn’t to my knowledge use 1V as its reference. You can use it to do roll off measurements, but you have to take a reference at your centre frequency first. The only dB measurement the 34401A is capable of directly measuring in is dBm, and you can actually set different impedances as well. 600Ohm is default.
I have often found with other multimeters that they only take dBm measurements. Bizarre if you ask me, because I’d prefer dBV measurements most of the time!
Once again, good blog
Andrew / Dave,
Hmmmm okay. Well, my understanding of dBm seems to be correct because I wasn’t actually saying that it’s always referenced to 600 Ohms, I just meant that it’s *usually* referenced to 600 Ohms (and certainly almost always is with multimeters).
However, I do appear to be wrong in my interpretation of dBu. I thought dBu was a measure of magnitude, the same as dBV, except with a reference of 0.774V instead of 1V.
If I understand you correctly (Dave), you’re saying that dBu is in fact a power measurement, referenced permanently to 600Ohms?
I’m going to have to look this up now. I’ll check your link, thanks for that!
Super GREAT video! I just had to watch it twice it was so good. I’m going to post a link to this one from my blog.
Only a master could explaining such Mathematics in such an easy to understand way. Normally people that try to explain things they themselves have not yet mastered tend to explain it in the most complicated way possible. Maybe they do this to make themselves seem or feel more intelligent. Your easy to understand video on dBs demonstrates you are a master in Electronics and Mathematics .
For what its worth, we actually hear audio close to what you have shown with the gain vs frequency plot. It just works out that we tend to perceive a factor of ten in audio power as sounding about twice as loud. Thats why they call it a “bell”. A fft on a log frequency scale shows the enormous dynamic range our ears have over magnitudes of power and frequency discrimination.
As one of those beginner-wieners that this segment was directed towards, I sincerely thank you for all of your efforts. Your instructional videos have dramatically increased my understanding of electronics and electronics engineering as a profession. I consider my life enriched by the knowledge that you choose to share via this medium.
I feel compelled to write this, because of some of the negative comments left in this forum– much of which I believe is picayune-bullshit. I _am_ one of those guys that asked for a piece on dB’s and what they mean, and you delivered! For some dickhead to say that you’re “wrong” for using dB instead of ‘L’ is utter and complete nonsense. The tone of many of the professionals that participate here, have the effect of completely ‘turning-off’ people like me– perhaps those most representative of the audience at large. If the intent of these individuals are to force strict adherence to the raw standards and specific method of communication (and teaching) of the EE disciplines– may I suggest that they go back to university. It is not what we expect from watching videos on YouTube. I too would like to see the critics out there, produce a video explaining this better than Dave. Let’s SEE how many in the audience find the content informative AND entertaining enough to watch 20+ minutes– Dave pulls this off like a champ.
I was a bit disappointed why you did not describe *why* for a voltage it is 20 x log10(ratio).
As you stated, dB is used to refer to power ratios.
Now to get power from a voltage, you need to square the voltage, and divide by the impedance.
So in reality it is 10 x log10 (VxV / reference).
It just so happens that logarithms make it real easy to skip the squaring part, simply by using 2 x 10 x log10 (V/reference).
As for dBm – I hardly every use 600ohms, 50ohms for RF thank you very much
0dBm however *always* is 1mW into the nominal impedance – that’s where the ‘m’ comes from.
However, the voltage observed will vary depending upon the impedance of the circuit under test. For RF this is 50 ohms, 600 for audio (phone lines)
A multimeter may have a dBm scale, but you will find it is assuming a 600ohm circuit impedance (and this was shown on the old moving coil meters)
I work on weather radars, and they have a huge range of dBm – some are +88dBm (50ohm) Transmitter power, and have a receiver sensitivity of -112dBm (50 ohm)
200dB difference in power or 10^20!!!
It has been my experience that pretty much nobody has an application for 600Ohms these days. Even in audio circles, 600Ohms seems to be a little used impedance. I do sometimes wonder where this figure came from in the first place! Maybe it’s to do with some legacy Electronics from years ago that did make use of 600Ohms impedance (like your telephone line example).
On that subject, it has also been my experience that there is nothing particularly special about 50Ohms in RF circles. I once spent a long time questioning this impedance value, assuming there was some special significance to it, but apparently there is not. 50 Ohms is still widely used and accepted in RF circles though, unlike the 600Ohms which I hardly ever see used in modern audio designs.
By the way, I do understand the need to use some accepted impedance value in RF designs (so that lines can be matched to limit reflections etc) but I never understood why it was chosen to be 50 Ohms and always assumed there must have been some special significance for it, which I now believe there is not.
Choice of 50 ohm for RF is historically related to the properties of air dielectric coaxial cable.
Lowest attenuation occurs for 77 ohm cable, maximum power handling for 30 ohm cable.
50 ohm was chosen as a good compromise.
For more details see:
Thanks Ray, must confess I was left wondering about the difference – your comment explained it perfectly!
Thanks Ray, I was wondering when/if someone would mention this. For me dB is always power. (V^2 = power… the factor of 2)
Dave.. love the video blogs.
YOu make mistake at 19:20 minutes, -3dB is not 0.5, but it’s 1/Sqrt(0.5)
Ups, i didnt see you mention for power, so 10*Log…..
Great, keep doing great job, MAYBE you can “represent” PIC, ARM….
Nice tutorial wonderful explanations about how much handy dBs are. Congratulations on your video blog.
Thanks Dave. I’m a Radio Amateur, but not an engineer. I love these types of tutorials and have listened to most of your video blogs. This one was particularly great (also liked the one on the Arduino).
Keep up the good work.
Oh…and from all us hams out there…yeah, 50 ohms is “it.”
The 600 ohm level comes from the telephone technologies.
HP had build a signal generator that the tech’s used to check the lines and frequency response.
The 0dB level choosen was simply the max output voltage of the generator. It is arbitrary but became a standard because the tech’s requested that newer Signal Generators had the same output so they could compare old and new measurements.
I read this history somewhere on the web.
The 600 ohms seem strange for audio but if you are dealing with long transmission lines this becomes important.
hi. i’m total blind of this term db. but from what i got in the video stated db is a comparison to something else, ie its relative. and i think mathematic is not about comparison or relative, math is about absolution, 1+1=2, thats it. so this db thing, is actually a concept, not mathematic. its like the concept of length, some people use cm, some ft, inches or other. so if you said it should be compared to 1mA, 1mW, 600 ohm, 0.774V or some shit like that, i think that is your own preferences, it is specific to some particular community or field. now i think i can apply the db concept in mass, length, force etc in my mechanical eng knowledge, cant i? thanks alot for the info dave.
for those who want the super duper precise information on mathematical theoritical of db, i suggest you go to uni or buy some super duper electronic books. dave have mentioned/advised the art of electronic, GET ONE! and if you have any complain in it, go complain the author or any proffesor around!
i think this blog is more on practical, not theoritical or academic sh*t. i have 1-2 foot thick EE books and i never successfully finish them for soooo many years. simply by watching eevblog, releasing some tension in my synaptic networks, very good blog!
Awesome post! I think it would be great, and beneficial to most, if you could do a post on a list of things every Electrical/Electronics Engineer should know!
Dave, I think the frenetic pace of your blog left a few people bobbing in your wake!
I will try to toss them a lifebuoy (Or an anchor).
A 1kHz tone is applied from a 600 ohm source , at a level of 0dBm,to an
audio line amplifier,with 600 ohm input & output impedances.( I will use the letter “ Z” from now on)
The amplifier is terminated in 600ohms.
If a level meter set to high Z input is bridged across the input, it reads 0 on the dBm scale,& approx
774.6 mV on the voltage scale.
Using the power formula :- P=V^2/R this gives (774.6 x 10^-3)^2/600=10^3watts,or 1mW.
The meter is now bridged across the output, reading: +30dBm,& 24.495volts approx.
Again applying the power formula, output power is 1watt.
Using both dB formulas,
10 log 1/10^-3 = +30dB,
20 log24.495/774.6 x 10^-3 = +30dB
OK, at this point we decide we really want our amplifier to drive an 8ohm speaker instead, so we connect a (perfect) 600 ohm to 8 ohm transformer across the output.
The voltage across the 8 ohm termination reads approx 2.828 volts.
Using these values in the power formula still gives Pout=1w,so 10 log p1/p2 gives 30 dB of gain.
Let’s try 20 log v1/v2 —– 20 log 2.828/ 774.6^-3=11.248 dB???????
OK, we’ve learnt something— the voltage dB formula only works for equal impedances!
Some folks are a bit concerned that 600 ohms are not commonly used in modern audio equipment, & someone said, “What’s this about 774.6 mV”??.
Modern amplifiers have large reserves of gain,so do not have to be impedance matched for maximum power transfer,so designers found it was easier to make amplifiers with low Z balanced outputs,& simply bridge the existing 600 ohm loads across them,than to make amplifiers with 600ohm outputs.This maintained compatability, because as long as the following device saw 774.6mV across the input,it was quite happy.Terminated dB meters still read the level as dBm.
RF equipment commonly uses either 0dBm= 1mW in 50ohms,or 1mW in75 ohms.The cheap trick used with audio isn’t used with RF as there are other reasons for Z matching.
I learned a few things here, and for that I thank you. It’s wonderful that you take the time – so much of your own time – to teach unseen stranger like me the “facts of life” as it pertains to electronics. Interesting, fun and informative.
Things are becoming clearer with every episode you make.
At 14 min 12 seconds into the video you say that it’s 25db per decade.. but I don’t follow..
Isn’t the signal dropping 2.5 db every decade (10 hz)?
Or perhaps the screenshot doesn’t show clearly on the video?
Nice explication… I like it
This is one of the best explanations of dB’s I have ever seen… or as I like to think of it, the power of logarithmic scaling
I wish I had this video when I was learning this over 10 years ago Not only did you explain it quite well but you backed it up with a solid explanation and illlustration of the benefits.
In other words you didn’t just say
“learn this, you will need to know it”
“learn this, you will need to know it, here is why you will WANT to know it”
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