Yes. so 0dBm is 1 mW.
Yes too.
and -30dBm is 1uW. And it would be 60dB less ( 1 million times ) than +30dBm. I.e. the dB difference between the numbers yields the ratio.
How is that *not* a ratiometric measurement ? What is your point ?
Graham
Perhaps we can help Guy. If we drive a 600-ohm destination directly from an opamp with an output of 6 volts (rms), we'll see 10mA current and the load will receive 10mA*6 = 60 milliwatts. But if we add 600 ohms to the opamp output so as to "impedance match" the system, the 600-ohm destination sees 3V, as expected, and it gets 1/4 the power, or 15mW. Both of these amount to -6dB.
Hmm, if the load gets 1/4 the power, or 15mW, where'd the other 3/4 of the power, 45mW, go? If the opamps's source resistor dissipates the same power as the load (matched impedances, remember), namely 15mW, where'd the rest of our 60mW go? Aha! Don't forgot that the opamp's new load is 1200 ohms rather than 600, so it now delivers 1/2 of the former power, or 30mW instead of 60mW. Guy argues that the load gets 1/2 the delivered power or -3dB, which is only part of the picture. The other part is only 1/2 as much power is sourced by an impedance- matched amplifier. The bottom line is that matched-impedance systems deliver -6dB (not -3dB), whether we're considering voltage or power.
Guy, your statement a few days ago, which precipitated the argument, "Back to the topic, 3DB is also the amount of drop you get when you have the output impedence and the input impedence matched, which was common in the days of transformer-balanced audio lines," was wrong. BTW, that's impedance, not impedence, and dB, not DB. :>)
--
Thanks,
- Win
"I was judging everything and it seemed only natural that I should. After all, who else could do it? Heaven became a courtroom where I not only judged myself and the Mother, but later on I was even asked to settle little differences among the spirits. Even though I realized this wasn't going anywhere, I felt locked into the role of judge.
"It seemed the best I could do was to be consistent based on my previous judgments. It wasn't until I questioned the whole purpose of judging that I realized I was going about it in the wrong way."
Source: [
Perhaps in your haste to judge you missed the fact that I have repeatedly specified that 0.5 x *WATTS* is -3dB power. Go look it up. That's the specific statement that Poop Bear disagreed with. Do you really believe that I would have gotten as far as I have without knowing that 0.5 x Volts is -6dB power?
"The punishment and obvious result of negative judgments is guilt. Guilt erodes the sense of self worth and makes it very difficult to feel feelings that have been infected with it. Depression is often the consequence of guilt-bound feelings that cannot move and are therefore denied.
"Since you are fundamentally innocent, guilt is not in its right place in you. Guilt is held in place by judgments. Release the judgments, and the guilt goes too."
"You can very quickly and dramatically change your life for the better when you release the judgments that have held you captive and left you cut off from love and from your true emotional strength and power. This is much easier to do than it's seemed.
"Because judgments are mental decisions, they are easy to change. The secret is simply to decide again. Take back your original judgment, change your mind, undecide, unjudge."
Source: [
Here are exact quotes from my previous posts. Please explain what part of them in "plain wrong."
"half the voltage equals -6dB but half the power equals -3dB."
"Half the voltage equals -6dB. Half the power equals -3dB. Double the voltage equals +6dB. Double the power equals +3dB."
I see nothing about what I actually wrote that is incorrect, but I welcome you showing me what part of the above is wrong[1].
([1]: I *will* admit to rounding -3.01029995... to -3.)
Complete nonsense.
I would suggest that you increase you electronic knowledge by about
20db.Kevin Aylward snipped-for-privacy@anasoft.co.uk
As is 0dBV
So how come you think you can't use dBs to denote voltage ratios ? I do this 'everyday' in audio !
No hang on - that was Adrian who said you can't use dBs for volts. Why are you replying to my comment about his nonsence ?
Graham
Graham
Somoene else finally brings some sense to this thread !
Graham
But can I give the loons a good spanking first please ?
Graham
Hahahahahahaha ! You got caught with your pants down and instead of blushing and going away quietly you simply continued broadcasting your insanity all over the place !
Aren't you the guy who took over *moderating* an ng recently ?
God help whoever posts there !
Graham
Guy Macon wrote: < snip >
No it damn well *isn't* !
You said terminated ( 600 ohm ) lines 'lost' 3dB ( half power ).
They don't - they 'lose' 6dB - quarter power or *half the damn volts*.
Now troll off ! Go back to moderating you personal little cave of idiocy.
Graham
That's an awfully big shovel you've got there !
Graham
Use voltage ratios with my blessing. I use the frequently to good advantage. Obviously, it wasn't me who said you couldn't.
After you carve up a quote, how the hell can anyone know who posted it?
You can *measure* the voltage but the calculations are based on power.
You can't have different definitions of a decibel, depending on the units you use to measure it. That's like saying you have different definitions for Ohms Law, depending on whether you are measuring volts and amps or deriving it from Watts. The formulae work all right, but the definition of an Ohm does not include Watts.
The other formulae are useful derivatives (under certain conditions which should be specifically stated), but they are not the definition. (They can also be very confusing to a beginner who thought he had just got a grasp on what a decibel was.)
If you add a suffix '$' to denote some reference level, then dB$ becomes an absolute unit.
Hope that makes sense.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
Incorrect. Please see below about this.
True, it is not. See below.
That is not the first indication. See below.
The first indication was before you could possibly have killfiled him, seeing as how you responded to his innocuous correction in your post of June 22, 1:33 AM Message-ID: , which I quote (with '|' for clarity) following: | Pooh Bear wrote: | >
| >Guy Macon wrote: | >>
| >> Back to the topic, 3dB is also the amount of drop you get when you | >> have the output impedance and the input impedance matched, which was | >> common in the days of transformer-balanced audio lines. | >
| >Nooooo ! That's a 6dB drop. | | Yeeeeees! That's a 3dB drop. :)
Graham's above quoted post ("... Nooooo! ..."), "referenced" (quoted, actually) your mistake in post news: snipped-for-privacy@hotmail.com . If you had been able to recognize your mistake then, instead of what you did, (see above), this thread would have been somewhat shorter.
That's not how I read the thread. As you can see, either from my above excerpts or by reviewing the thread yourself, at first you affirmed your mistake. It was only later that you began stating the common knowledge of what "dB" means, and nobody has been disagreeing with that.
Mr. Hill's remark concerning spelling was, verbatum: "BTW, that's impedance, not impedence, and dB, not DB. :>)" Nobody in their right mind can honestly characterize that as a flame. I urge you to be more careful when characterizing unquoted material.
-- --Larry Brasfield email: donotspam_larry_brasfield@hotmail.com Above views may belong only to me.
What's staggering Win, is that we have to explain it to him, the guy who thought that it was me who was the ineducable one earlier in this thread ! I did mention he might like to calculate the power delivered by and dissipated in the source but he missed the hint.
A lesson learnt. I'll know not to pay too much attention in future to any 'advice' he gives.
Cheers, Graham
First indication that the above is what Pooh Bear was disagreeing with, and it comes long after I killfiled the little pest.
You are correct. Too bad it took so long for anyone to reference the actual incorrect statement. Instead I kept posting things like
"Half the voltage equals -6dB. Half the power equals -3dB. Double the voltage equals +6dB. Double the power equals +3dB."
and I kept getting strong disagreement and abuse when I did so. A simple "yes, that is correct, but what you said *before* was in error" would have avoided a lot of trouble, but instead I got a load of "You are wrong" replies to the correct statement above.
(Typo flame deleted)
I should also point out that, when talking about power as opposed to voltage, the incorrect statement...
"Back to the topic, 3dB is also the amount of drop you get when you have the output impedance and the input impedance matched, which was common in the days of transformer-balanced audio lines,"
...is under some conditions, *still* incorrect if you just change the
3dB to 6dB. The obvious question is "drop from what starting condition to what ending condition?" Assume the ending condition is matched Z. If the starting condition is a near-infinite-Z load, close to zero power is being transfered as the starting condition, so changing to a matched Z system is a large gain in power transfer, not a drop.What I was thinking of when I wrote the above was the case of the starting condition being a near-zero-Z source. I was thinking of the way Joule misapplied Jacobi's theorem and concluded that an electric motor driven by a battery or dynamo could not be more than
50% efficient. Edison & Upton showed this to be a bad assumption when they made a 90% efficient system by making the source Z very low.-- Guy Macon
to
you
That's why trimming and snipping is good but needs to be done carefully.
Cheers, Graham
You're as pig-ignorant as Guy! Decibels are a log ratio of *anything*. Crap, what are they teaching engineers these days?
-- Keith
...and it's full of dung. I guess it's too dark there for him to admit we is wrong now.
What a maroon!
-- Keith
Harvard teaches it in their very popular Physics 123 course, using the book Paul and I wrote. They also teach it in the summer, when most of the students are from elsewhere and come to Cambridge just to take the course.
And many other schools use our book to teach how an engineer thinks about electronics circuit design.
--
Thanks,
- Win
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