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LTspice - fast NRZ pulses how?

Discussion in 'CAD' started by [email protected], Aug 29, 2008.

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  1. Guest

    Does anyone know a trick to make fast random NRZ pulses in LTspice?

    I don't want to use the read-from-file option, but would like to
    generate them procedurally.

    This is for speed and convenience.

    The pulses should be rectangular, be 1s (or whatever 1/datarate is)
    and be either +1 or -1, with a random probability distribution.
     
  2. batguy

    batguy Guest

    You could probably use two independent sources to make a
    multiplicative congruental generator.

    Something like Va = fraction ( pi * Vb ) and Vb = Va.

    You'll have to set up Va to be nonzero when the simulation starts, and
    I'm not sure how you would put in a proper time delay between the two
    sources.

    Then you have a pseudorandom number, to make data, you'll probably
    have to sgn() it etc. Probably need a sample & hold to to hold it
    still for one bit period.
     
  3. Guest

    Thanks! I tried this:

    * C:\Program Files\LTC\SwCADIII\nrz_sim\random_nrz_bits.asc
    B1 a 0 V = if ( rand( time ) , 1, -1 )
    ..tran 1000
    ..backanno
    ..end

    and it seems to work! It's fast too - around 4000 seconds per
    second.

    How about some other tricks to accomplish the same?
     
  4. batguy

    batguy Guest

    That's a cool trick.

    For grins, I tried B1 a 0 V= pi*v(a) - floor( pi*v(a) ) combined
    with a sample & hold.

    That worked too, but it's a lot slower.
     
  5. Hello,

    I recommend a B-source.

    V={ampl}*(rand(time*{br}) >=.5)


    ..param br=533meg
    ..param ampl=2


    The option below enables the eye diagram.

    ..options baudrate={br} delay=.1n

    Setup for a transient simulation.

    ..tran 0 1u 2n 10p
    ..options plotwinsize=0


    Best regards,
    Helmut
     
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