Request Help -- very odd error with SIN

  • Amal Banerjee

    Amal Banerjee - 2012-11-24

    Could some ngspice guru please help ?
    I am running ngspice - 24 on a Fedora 15 machine.
    I have the following two statements in a netlist:
    VG2 2 0 DC 0.0 SIN(0 10 200K 0 0)
    VG1 4 0 DC 0.0 SIN(0 5 1000 0 0)

    The second SIN produces a perfect sine wave.
    The first produces a set of spikes at start and end,
    .TRAN 50.0us 7500.0us 10.0us UIC

    May O know what the problem might be ? I have
    checked by decoupling the circuit that is being
    fed with these signals, from the signal sources
    and the result is the same. The ngspice manual
    states: SIN(Vo VA FREQ TD ThETA)

    Any hints, suggestions would be helpful.

  • Robert Larice

    Robert Larice - 2012-11-24


      your timestep specified in the .tran step to be 50us
        is much too large to sample a 200kHz sine wave.

      reduce it to something considerably smaller than 5us

      in other words, you have down sampled 200kHz
       with a 1/50us = 20kHz sampling frequency
    to Zero, that is to DC

      if ngspice would have exactly sampled at
        k * 50us
      you would have seen exclusively zero samples.
      but ngspice starts with smaller timestamps,
        thus your samples are not all at k*50us
        and thus some of your samples dont happen
        to yield zero voltage.

      try print allv to see the exact sample points,
        and how they differ from k * 50us



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