Mr RB, If you don't want to use dB, don't. But just because in your experience you don't see much reason to use dB doesn't mean that it's not useful or is obsolete.
We were told at college almost 40 years that "only people who don't know what they are doing use dB's"
While probably not 100% true, it's not too far off the mark - it's trying to simplify things for people who don't understand.
We were told at college almost 40 years that "only people who don't know what they are doing use dB's"
So please tell me how do you treat with it while see dB value for something (say for a 40dB high pass filter)? somebody tells you if a 40dB high pass filter is good enough for your job, then what would be your response? And do you try to change such a value to its non dB equivalent scale (decimal)?
So please tell me how do you treat with it while see dB value for something (say for a 40dB high pass filter)?
Don't understand why you were told that. I believe whoever said that never worked as a engineer.We were told at college almost 40 years that "only people who don't know what they are doing use dB's"
While probably not 100% true, it's not too far off the mark - it's trying to simplify things for people who don't understand.
Now I'm confused.
I don't want to muck up this thread too much, but if one calculates the voltage gain, say, of an amplifier, in dB, how is that a power ratio?
And I thought dBs could be used to express any ratio that lends itself to logarithmicity (to coin an unwieldy word).
Anyhow, it's the power thing that's throwing me now.
Don't understand why you were told that. I believe whoever said that never worked as a engineer.
That's probably true.I think that decibels can be summed up as:
For those who get it, no explanation is necessary
For those who dont get it, no explanation is possible
No, you are correct. Technically it is incorrect to define amplifier voltage gain in dB if the input and output impedances are different. But it is common practice, and those that work with amplifiers understand this useage, so it's not likely to go away......It seems the place where the use of dBs is the most confusing, and perhaps inappropriate, is measuring gain where the input and output impedances are different (comparing what to what?). Am I wrong about this?
hi,
Look at these links, watch the video for audio explanation, the important point to remember its a logarithmic scale.
For a HPF of 40dB I would use the VOLTAGE ie: 20log(v2/v1) and assume the impedances are the same.
For a Power ratio I would use 10Log[ P2/P1] , I could generalise and assume equal impedances but if I wanted an accurate value I would use the input and output impedances to calculate the P's in the equation.
http://www.youtube.com/watch?v=CAFu7i7nZ18
http://en.wikipedia.org/wiki/Decade_%28log_scale%29
http://www.jimprice.com/prosound/db.htm
http://en.wikipedia.org/wiki/Decade_(log_scale)
So please tell me how do you treat with it while see dB value for something (say for a 40dB high pass filter)? somebody tells you if a 40dB high pass filter is good enough for your job, then what would be your response? And do you try to change such a value to its non dB equivalent scale (decimal)?
Crutschow said:...
It makes it easy to determine the total gain of a string of amplifiers and interconnect lines, since all the dBs simply add. ...
Externet said:...
Because of the 'law of the least effort' and because in the 1940s when the decibel was implemented, nobody had calculators (perhaps some slide type) nor US was considering anything metric. But adding and substracting was a piece of cake. ...
The point is that the dB is a power ratio so to use it for a voltage ratio, such as voltage gain, the impedance of the input and output must be the same for the power ratio to be correct. But as I noted, it is common practice, even though it's not technically correct, to use dB for amplifier voltage gain even when the input and output impedances are not the same.....What is the point behind of assuming the same impedances?!....
So for example, with two amps and an attenuator, multiplying 5.62 x 12.59 x .662 is as easy to to do in your head as adding 15dB +22db - 5db? If so you are much better at mental math than I.I understand your argument there but I don't think it really holds up. As an example if you have a 3 stage amp and you work in dB its A+B+C=gain. But in linear gain you can just do A*B*C which should still be almost as easy in your head. So there's little benefit to dB.
Obviously you haven't worked in the real world of RF and microwave. All spectrum and signal analyzers are calibrated to read directly in dB. For example, displaying a signal and its spurious harmonic which might be 60dB down is impossible to see on a linear scale but is readily displayed on a dB scale.BUT in the real world if you now want to see if you are getting the correct amplification you connect a scope. With linear gain you look at Vout/Vin and you have everything effortlessly, probably still in your head. But if you were working in dB, you are reaching for the scientific calc again and doing a silly log calc that was just not necessary.
Please don't hold you breath waiting for that to happen.dB is an obsolete system that hasn't quite died its natural death yet.
So people who use dBs are idiots, eh? I resemble that. But I have more company than you and Externet.What really ticks me off is small ish gain value; when a filter or amp etc has a perfectly usable linear gain of 7.3 and some idiot has expressed that in dB. The odds of that dB value making my life easier is about 0 and the odds that a "gain of 7.3" would have made my life easier appoaches 100%.
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