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LTSpice: How to write a behaviorial model?

Discussion in 'CAD' started by Mike Rocket J. Squirrel Elliott, Nov 6, 2003.

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  1. I need to drive an amplifier circuit from a pre-emphasized voltage
    source. The goal being to confirm that the complementary de-emphasis
    portion of the circuit results in overall flat frequency response.

    Generally speaking, when doing an .ac analysis, the signal source is
    held at a constant amplitude and swept from the start to the stop
    frequency. In my application, I need to vary the amplitude as a function
    of frequency as it sweeps.

    This appears to be a job for the "B" function. Alas, my math skills are
    too puny to attack the problem.

    Beige Bag SPICE shows how to do exactly what I want to do for their
    software at

    http://www.beigebag.com/case_riaa_2.htm ,

    It is the Inverse RIAA Parameters that I am wanting to stir into the B
    model. I'm not seeing how to write a B behavioral model of the inverse
    RIAA equalization transfer function in LTSpice from their example.
     
  2. Mike,
    Basically, you change the word "gain" to "Laplace". Below is
    a schematic that uses the equation from that site. Note I used
    the reciprocal so that the response was more easily recognizable
    as RIAA equalization. Just change the value of E1 to
    Laplace=(1 + 3.18e-3*s)*(1 + 7.5e-5*s)/(1 + 3.18e-4*s) to
    get the inverse. Note that LTspice will also execute the
    PSpice syntax for these things, which while a bit more cumbersome,
    is the most widely used standard I know of.

    --Mike

    --- RIAA.asc ---
    Version 4
    SHEET 1 880 680
    WIRE 32 64 32 96
    WIRE 32 176 32 192
    WIRE -144 240 -144 208
    WIRE -144 128 -144 112
    WIRE -144 112 -16 112
    WIRE -16 160 -32 160
    FLAG 32 192 0
    FLAG -32 160 0
    FLAG -144 240 0
    SYMBOL e 32 80 R0
    WINDOW 3 28 94 Left 0
    SYMATTR Value Laplace=1/((1+3.18e-3*s)*(1+7.5e-5*s)/(1+3.18e-4*s))
    SYMATTR InstName E1
    SYMBOL voltage -144 112 R0
    SYMATTR InstName V1
    SYMATTR Value ac 1
    TEXT -56 248 Left 0 !.ac oct 100 1 100K
     
  3. Mike,

    That's brilliant -- thank you!

    Do you ever sleep?
     
  4. Uwe Bonnes

    Uwe Bonnes Guest

    : Basically, you change the word "gain" to "Laplace". Below is
    : a schematic that uses the equation from that site. Note I used
    : the reciprocal so that the response was more easily recognizable
    : as RIAA equalization. Just change the value of E1 to
    : Laplace=(1 + 3.18e-3*s)*(1 + 7.5e-5*s)/(1 + 3.18e-4*s) to
    : get the inverse. Note that LTspice will also execute the
    : PSpice syntax for these things, which while a bit more cumbersome,
    : is the most widely used standard I know of.

    : --Mike

    : --- RIAA.asc ---
    <snipped>

    Mike,
    please consider including the examples from the last postings into the
    release.

    Thanks
     
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