# 3 dB bandwidth

Discussion in 'Electronic Design' started by [email protected], Jun 21, 2005.

1. ### Pooh BearGuest

The potential divider is formed by the source Z ( 600 ohms in this case ) and the
load Z ( also 600 ohms ).

Nother more complicated than high school math / science should illustrate that forms
a potential divider with a value of 0.5.

No-one works that way any more actually but I've been there, done that, got the
diploma. Old ATRS ( analogue tape recorders ) that we used at Sound Developments for
example ( toob based Ampex 350Cs and 351s ) had a selector switch that chose between
10k ( bridging ) load and 600 ohm ( terminated ) load. When on a 600 ohm circuit,
select terminated and the level drops by 6dB relative to bridging assuming nothing
else is on the circuit. You can even see it on their VU meters !

Btw - try working out the power supplied by the signal *source*.

You may get a surprise ! And I mean *work it out* not believe what the faeries at
the end of the garden say !

Graham

2. ### Guy MaconGuest

The 0.5 appears to be correct, but the -6dB he pulled out of his arse.
If he had bothered to read the references I provided, or even done a
basic web search, he would have found out that half the voltage equals
-6dB but half the power equals -3dB. Alas, he is ineducable and thinks
that aserting "0.5 equals -6dB" over and over again does not change the
reality of 0.5 equaling -3dB when measuring power.

He is also too dim to realize how silly he looks flaming away at someone
who has killfiled him and thus cannot see his posts.

3. ### Keith WilliamsGuest

Appears to be? Do you have a problem with *voltage* dividers? Do you
have a problem calculating 20log(.5)?
Umm, isn't 1/2 voltage = 1/4 power? ...thus 6bB, either way (funny how
the arithmetic works out).
LOL! You're priceless.

4. ### Guy MaconGuest

Given the repeated basic errors that the poster in question has made,
I was not sure whether he meant voltage when he wrote "potential"
Thus my usage of "appears to be." One can't be too careful when dealing
with the ineducable; sometimes they appear to make sense when actually
they are simply lacking the vocabulary to express their ignorance.
Nope. Do *you* have a problem calculating 10log(.5)?

Do you have a problem with the established fact that decibels are,
by definition, a ratio of *power* levels?

"bel (B): A unit of measure of ratios of power levels, i.e., relative
power levels. Note 1: The number of bels for a given ratio of power
levels is calculated by taking the logarithm, to the base 10, of
the ratio. Mathematically, the number of bels is calculated as
B = log10(P 1/P 2) where P 1 and P 2 are power levels. Note 2: The
dB, equal to 0.1 B, is a more commonly used unit."
Source: http://www.atis.org/tg2k/_bel.html
If you had bothered to read the references I provided, or even done a
basic web search, you would have found out that half the voltage equals
-6dB but half the power equals -3dB.

Do A Google web search on [ decibels dB power -3db half voltage -6db ]

Key quotes from the first ten pages returned by the above search:

"The -3 dB point is often referred to as a "half-power point"

"A level difference of 3 dB is roughly double/half power"

"notice at each level the voltage or current dB value is twice the
power dB value"

"
Decibels Power Voltage
-----------------------
3 2 1.4
6 6 2.0
"

"the formula for dB in power is 10 times the logarithm of p1/p2 but
the formula for dB in voltage is 20 times the logarithm of v1/v2."

"Look at the decibel equivalents for a power change of 2.0 and for 0.5.
If you double or halve the power then this gives very close to 3 dBs.
This is only coincidence, but it explains the frequent references you
see to 'the -3 dB point': it really means half the power."

Half the voltage equals -6dB. Half the power equals -3dB.
Double the voltage equals +6dB. Double the power equals +3dB.
Anyone who disagrees with this basic fact after having the
references proving it provided to him is willfully ignorant.

5. ### Keith WilliamsGuest

What does "impedance matching" or "maximum power transfer" mean to you?
Hint: It's a R/R+R voltage divider.
Well, a *voltage* divider divides, um, voltage. Thus 20log(.5) or 6dB.
....and an R/R+R voltage divider does what to the power level. Another
hint: It drops it by 6db. P=V^2/r sorta thing, dontcha know.
Wow, you know how to use Google. Now try learning something about
electronics.
Man you are dense. If you drop the voltage to half, the power is
quartered; 6db. The arithmetic works again.
You are amazing!
That's well known in these parts. Now what happens to the power when
the voltage is halved (we are talking about a R/R+R voltage divider
here). Is it really your position that it is cut in half?

<snipped obvious C&P stuff from Google and just as obviously not
understood by GM>
Let me try translating into idioteese; YOU DON'T HAVE HALF THE POWER.
YOU HAVE ONE QUARTER THE POWER. ONE QUARTER POWER = 6dB, AND AGAIN,
THE MATH WORKS).

Amazing. Simply amazing.

6. ### Guy MaconGuest

I would agree if the statement was made by a "normal" person, but I
have found that the ineducable can, by random chance, stumble on to
the correct terminology without having any real understanding. Thus
my usage of "appears to be."
Decibels are units of power, not voltage.
The children who played with Mattel's best selling new toy for the
year 2000 and the pilots who fly airlanes tested with my AV-8B Harrier
actuator test system seem to be satisfied with my abilities.

I deleted the rest of whatever you had to say unread. My time is too
valuable to waste on someone who thinks that personal attacks are a
substitute for a civil discussion of the topic at hand.

*plonk*

That is the formula for calculating decibels, decibels are defined with
respect to power.

[...]

There is no such thing as a 'voltage decibel' so please avoid using the
expression, it can only lead to confusion and sloppy thinking.

8. ### Guy MaconGuest

I agree 100%. I was trying to accomodate the existing sloppy
thinking of certain ignorant induhviduals, but I see now that doing
so was a mistake; they just run in cyircles shouting "0.5 is -6dB!
0.5 is -6dB!" and flaming anyone who tries to educate them.

Thank you for the excellent advice.

9. ### Keith WilliamsGuest

Do you often talk about yourself?
Nonsense! Decibels are dimensionless. Watts/watts is dimensionless,
as is Volts/volts, or SPL/SPL. Decibels is simply a log-scaled ratio
of two quantities. It's useful because many things in nature are log-
scaled.
That is scary.
I see. You're too stupid to be educated. ...too stubborn to read.
Oh no! I've been plonked! What will I do? ...better plonk all the
rest of the engineers here too.

BTW, your followup-to alt.dev.null didn't work. What a candy-assed
loser!

Decibels are, as you say, dimensionless; but they are specifically a way
of expressing the ratio of two power levels - only power levels, not
voltages or currents or anything else.

11. ### Don BoweyGuest

Obviously, some of you have never had occasion to deal with dBv (that¹s
Volts), but your ignorance does not make the measurements and calculations
non-existant.

According to my Bell "Green Brain," we even have dB=20 log I1/I2 (that's I
as in current).

I bet when I suggested to Kevin that he put 3 dB of the total weight in each
arm he even knew what I meant.

Don

12. ### Pooh BearGuest

No no no no *NO* ! dBs are simply ratios. They can however be referred to
units such as dBV ( decibel Volt ) , dBm ( decibel miliwatt ) and so on and
are *widely* used in this way throughout the electronics industry !

What the heck has got into this thread ? God Almighty !

Graham

13. ### Don BoweyGuest

A little problem with dBm; it isn't a ratio. dBm expresses a reference to
1 mW. 30 dBm= 1W.

Don

14. ### Pooh BearGuest

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

Wrong. Decibels are an expression of the RATIO of two powers,
OR two voltages, and, in most applications, the method being
used (power or voltage ratios) is usually specified, except
in some audiophool crap as marketing hype. "Peak Envelope
Power", indeed.
Oh, go plonk your own self.

Good Luck!
Rich
[aside - while seeking out Guy's personal NG to crosspost this to -
he had followups set to alt.null, as per usual - I found out that
couldn't find enough people on Planet Earth to support a Guy
Macon USENET blog. Oh, well.]

16. ### Richard HenryGuest

It's a ratio to 1 milliwatt.

17. ### Rich The PhilosophizerGuest

Precisely.

But he's promised to change:
http://www.godchannel.com/ahriman.html

But we each need to let go of the unlovingness within ourselves, to
really heal.

18. ### Rich GriseGuest

I think his point was that he wanted to say something that was right.

A 'dB' is a certain ratio of powers. A 'dBm' is an expression of the
ratio of the expressed power to a milliwatt, therefore, it is a
dimension, if you will. It's a number of milliwatts, expressed in
a log scale. A dBV, like he said, is the ratio of the applied voltage
to one volt, expressed as a log, but because watts are proportional
to volts squared, dB Volts are 20 log, and dB milliwatts are 10 log.

Hope This Helps!
Rich

19. ### Rich GriseGuest

Guy Macon, you're being a twit.

0.5 x Volts _IS_ -6dB Power, because power is proportional to the square
of the voltage. Remember, P = E^2 / R ?

Please go re-take high school logarithms and algebra. And maybe Ohm's
Law, or whatever that next one is, P = E * I.

Thanks,
Rich

20. ### Winfield HillGuest

Guy Macon wrote...
-6dB was and is the correct answer, you were, are and continue
to be plain wrong. Read it and weep, sorry.