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LTSpice Question

Discussion in 'Electronic Design' started by Gary Richardson, Jan 15, 2004.

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  1. I've created a LTSpice simulation of the bandpass filter of Figure 5.19 on
    page 278 of A of E. My circuit is shown below; the center frequency is
    60kHz.

    In the LTSpice simulation the gain at the center frequency is +9 dB and the
    Q is about 60; the expected values are 0 dB and 20. However, the simulation
    using Ansoft Serenade (Student Version) does produce the expected values.

    I'm an LTSpice newbie so I probably have something screwed up in my
    simulation but I have no idea what it might be. Any suggestions or comments
    appreciated.

    Gary Richardson

    ----------------------------------------------------------
    State-variable bandpass filter. A of E, page 278
    R1
    +--------------/\/\-----------------+
    | 100k |
    | |
    | Rq R C |
    +-/\/\-+ +--/\/\-+ +--||--|
    | 5k | | 10.0k | |1.0nF |
    | | | | | |
    Rg | |\ | R | |\ | Rf | |\ |
    In -/\/\-+-|-\ +-/\/\-+--|-\ +-/\/\-+-|-\ | Out
    100k | >-+10.0k | | >-+ 2.65k | >-+---O
    +-|+/ |+-|+/ +-|+/ |
    | |/Q1 || |/Q2 | |/ |
    V |V V Q3 |
    | C |
    | +--||-+ |
    f = 1/2 Pi Rf C \ |1.0nF| |
    R/10.0k | | Rf |
    Q = R1 / Rq \ | /| | |
    | | / |-+-/\/\---+
    G = R1 / Rg +------+< | 2.65k
    \+|-+
    Q1, Q2 - LT1498 \| |
    Q4 V
     
  2. Gary,
    The unexpected peaking is due the the 1498's not having
    infinite GBW. There's a opamp guideline of 3*Q minimum
    gain at resonant freq. for statevariables in D.L.'s _Active-
    Filter CookBook_, but this version of the circuit seems to
    need more a times times more than that to accurately
    duplicate the desired gain and Q. Since 3*Q is 35dB but
    the 1498 model has 44dB at 60KHz.
    That would have been with a different opamp or model.

    --Mike
     
  3. [snip]
    Thanks for the info. I ordered a copy of the "Cookbook" today.
     
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