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Exact meaning of 3db bandwidth?

Discussion in 'Electronic Design' started by Mike Noone, Jan 4, 2007.

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  1. Mike Noone

    Mike Noone Guest

    Hi - I recently came across the term "3db bandwidth" used in a way
    which I was not entirely familiar. I am most familiar with the term
    when used with amplifiers. For example, say in a FET amplifier where
    you know that the midband gain [in db] = 20 * log(midband gain [in
    V/V]), and the 3db bandwidth is the frequency which gives you a gain of
    3db less than the midband gain.

    Where I'm seeing it is in the datasheet for a gyro - the Analog

    Can somebody please explain what exactly it means in this context?

  2. PeteS

    PeteS Guest

    Looking at the datasheet, it states:

    External capacitors CMID and COUT are used in combination with on-chip
    resistors to create two low-pass filters to limit the bandwidth of the
    ADXRS401’s rate response. The –3 dB frequency set by ROUT and COUT is:

    fOUT = 1/(2 x pi x Rout x Cout)

    This is the standard -3dB point for a passive RC filter.

    In this particular case, it refers to the rate of change OF the yaw rate
    (2nd order differential) output. The rate output is specified as v being
    proportional to rate (in degrees / sec) - i.e. a DC level for a constant
    yaw rate. (My take, anyway)



  3. Guest

    3dB point of the low pass filter on page 9? come on dude. I suggest
    reading some filter design books.
  4. Guest

    The best way to understand RC filters is use ltspice. try this, save
    as rc.asc or whatever.asc
    and run in ltspice.

    Version 4
    SHEET 1 1072 680
    WIRE -48 192 0 192
    WIRE 0 192 0 208
    WIRE -128 192 -208 192
    WIRE 0 272 0 320
    WIRE 0 192 64 192
    WIRE -416 208 -416 176
    WIRE -416 288 -416 304
    WIRE 416 192 464 192
    WIRE 464 192 464 208
    WIRE 336 192 288 192
    WIRE 464 272 464 320
    WIRE 464 192 512 192
    WIRE 832 192 880 192
    WIRE 880 192 880 208
    WIRE 752 192 704 192
    WIRE 880 272 880 320
    WIRE 880 192 960 192
    FLAG 0 320 0
    FLAG 64 192 Output1
    FLAG -416 304 0
    FLAG 464 320 0
    FLAG 512 192 Output2
    FLAG 880 320 0
    FLAG 960 192 Output3
    FLAG -416 176 ACin
    FLAG -208 192 ACin
    FLAG 288 192 ACin
    FLAG 704 192 ACin
    SYMBOL res -144 208 R270
    WINDOW 0 32 56 VTop 0
    WINDOW 3 0 56 VBottom 0
    SYMATTR InstName R1
    SYMATTR Value 1600
    SYMBOL cap -16 208 R0
    SYMATTR InstName C1
    SYMATTR Value 10pF
    SYMBOL voltage -416 192 R0
    WINDOW 123 24 132 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value ""
    SYMATTR Value2 AC 1
    SYMBOL res 320 208 R270
    WINDOW 0 32 56 VTop 0
    WINDOW 3 0 56 VBottom 0
    SYMATTR InstName R2
    SYMATTR Value 806
    SYMBOL cap 448 208 R0
    SYMATTR InstName C2
    SYMATTR Value 10pF
    SYMBOL res 736 208 R270
    WINDOW 0 32 56 VTop 0
    WINDOW 3 0 56 VBottom 0
    SYMATTR InstName R3
    SYMATTR Value 1000
    SYMBOL cap 864 208 R0
    SYMATTR InstName C3
    SYMATTR Value 10pF
    TEXT -296 40 Left 0 !.ac dec 100 1000 10000e6
  5. PeteS

    PeteS Guest

    --------8<----------------- Snip

    I think Mike understands filters, it's just the *way* the term was used
    in this context was a bit different (which it is).


  6. Guest

    Lord, I appologize for that...that ain't right...and be with the
    starvin' pygmies in New Guinea. Amen!"
  7. Tim Wescott

    Tim Wescott Guest

    Just the same as an amplifier.

    The ideal gyro would give you an exact output (volts?) vs. rotation rate
    input, no matter how fast the rotation rate was changing. Your gyro
    will not do this. If you made a rate table that could impose a
    sinusoidal rotation on the gyro at any frequency, you would find a point
    where the output-to-input ratio was 0.707 the value that it is for a
    constant rotation -- that's the 3dB point.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at
  8. Ray

    Ray Guest

    Firstly, is the output measurement as a power or simple voltage?

    If it is power (V^2), certainly 0.707 is the figure.
    A simple voltage will however be 0.5.

    I am certainly no expert on gyros, but I suspect in this instance we may
    be dealing with a simple voltage, feedback and all that......

    As the test apparatus is probably quite involved mechanically it may be
    a tricky thing to verify I suspect.

    a dB is actually defined as 10 log(test/ref) and is dimensionless, so it
    can apply to any sort of measurement you care when comparing against a
    known reference.

    20 log (test/ref) really is a convenient way to perform POWER
    measurements from a voltage reading, the V^2 is done in the log domain
    (20 instead of 10).

    Perhaps if I am only half correct in my reply, I'm 3dB out!

    By all means, happy to be corrected if need be :)

    Cheers, Ray
  9. PeteS

    PeteS Guest

    The -3dB point is 0.5 Po/Pi, but 0.707 Vo/Vi for this sort of measurement.

    0.5 Voltage (in dB) is -6dB


  10. Mike Noone

    Mike Noone Guest

    Hi PeteS - that clears things up quite a bit. Thanks!

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