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3 dB bandwidth

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

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  1. Don Bowey

    Don Bowey Guest

    What's with the "yes's?" I didn't ask if I was correct.

    My point is, dB expresses a ratio, for example, of two powers. A 10dB
    increase in power my be from 0.1 Watt, to 10 Watts; or 10 Watts to 100
    Watts. It conveys no absolute info without being given a reference.

    0 dBm is a specific amplitude.

    BIG difference.
     
  2. Guy Macon

    Guy Macon Guest

    "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: [ http://www.godchannel.com/judgment.html ].
    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: [ http://www.godchannel.com/judgment.html ].
     
  3. Guy Macon

    Guy Macon Guest

    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.)
     
  4. Complete nonsense.

    I would suggest that you increase you electronic knowledge by about
    20db.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  5. Pooh Bear

    Pooh Bear Guest

    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
     
  6. Pooh Bear

    Pooh Bear Guest

    Somoene else finally brings some sense to this thread !

    Graham
     
  7. Pooh Bear

    Pooh Bear Guest

    But can I give the loons a good spanking first please ?

    Graham
     
  8. Pooh Bear

    Pooh Bear Guest

    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
     
  9. Pooh Bear

    Pooh Bear Guest

    Guy Macon wrote:
    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
     
  10. Pooh Bear

    Pooh Bear Guest

    That's an awfully big shovel you've got there !

    Graham
     
  11. 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.
     
  12. Pooh Bear wrote...
    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. :>)
     
  13. Pooh Bear

    Pooh Bear Guest

    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
     
  14. Guy Macon

    Guy Macon Guest

    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.
     
  15. Don Bowey

    Don Bowey Guest

    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?
     
  16. Pooh Bear

    Pooh Bear Guest

    That's why trimming and snipping is good but needs to be done carefully.

    Cheers, Graham
     
  17. 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 .
    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.
     
  18. keith

    keith Guest

    You're as pig-ignorant as Guy! Decibels are a log ratio of *anything*.
    Crap, what are they teaching engineers these days?
     
  19. keith

    keith Guest

    ....and it's full of dung. I guess it's too dark there for him to admit we
    is wrong now.

    What a maroon!
     
  20. The Decibel is a Unit of Attenuation, defined in
    terms of the ratio of power levels only.

    Attenuation = 10 * log10 (Pin/Pout) in Decibels.

    If (and only if) the input/output impedances are identical,
    then the Attenuation (in Decibels) can be calculated as the
    log10 ratio of the *square* of the currents or voltages.

    2 2
    Attenuation = 10 * log10 (Vin/Vout) in Decibels.

    Or, Attenuation = 20 * log10 (Vin/Vout) in Decibels.
    /|\
    |____ Notice where the squares went to?

    That calculation using (Vin/Vout) produces units of dB
    only when the input/output impedances are identical.

    For example. An audio power amplifier requires an input
    of 1mW into 600 ohms, for an output of 1W into 15 ohms.
    The voltage/current ratios are 0.77/3.87 and 1.29/258.

    10 * log10 (1mW/1W) is -30, Decibels of Attenuation.

    20 * log10 (0.77/3.87) is -14, Units of Nothing.

    20 * log10 (1.29/258) is -46, Units of Nothing.

    Only the power ratio produces legitimate units of dB.
     
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