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3 dB bandwidth

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

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  1. Pooh Bear

    Pooh Bear Guest

    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 Macon

    Guy Macon Guest

    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. 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 Macon

    Guy Macon Guest

    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 ]
    ( http://www.google.com/search?&q=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. 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 Macon

    Guy Macon Guest

    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*
     
  7. 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 Macon

    Guy Macon Guest

    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. 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!
     
  10. 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 Bowey

    Don Bowey Guest

    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 Bear

    Pooh Bear Guest

    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 Bowey

    Don Bowey Guest

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



    Don
     
  14. Pooh Bear

    Pooh Bear Guest

    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
     
  15. Pig Bladder

    Pig Bladder Guest

    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
    there isn't any such thing. It's dead already. Apparently, he
    couldn't find enough people on Planet Earth to support a Guy
    Macon USENET blog. Oh, well.]
     
  16. It's a ratio to 1 milliwatt.
     
  17. 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 Grise

    Rich Grise Guest

    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 Grise

    Rich Grise Guest

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