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Revision as of 06:24, 7 August 2009

B-sources

This is supposed to be like the corresponding Help file topic, but with more complete information. (Note: some of information is somewhat out-of-date, but is still generally correct.)

B. Arbitrary behavioral voltage or current sources.

Symbol names: BV, BI, *BR (arbitrary resistor)

Syntax (* denotes undocumented features):

   Bxxx n1 n2 V=<expression> OR I=<expression> [Rpar=<value>]
   + [[ic=<value>] tripdv=<value>] [tripdt=<value>]
   + [Laplace=<func(s)> [window=

The first syntax specifies a behavioral voltage source and the next is a behavioral current source. For the current source, a parallel resistance may be specified with the Rpar instance parameter.

Tripdv and tripdt control step rejection. If the voltage across a source changes by more than tripdv volts in tripdt seconds, that simulation time step is rejected.

The Laplace transform is applied to the result of the behavioral current or voltage signal. The Laplace transform must be a function of s. The frequency response at frequency f is found by substituting s with sqrt(-1)*2*pi*f. The time domain behavior is found from the impulse response obtained from the Fourier transform of the frequency domain response. LTspice must guess an appropriate frequency range and resolution. The response must drop at high frequencies or an error is reported. It is recommended that the LTspice first be allowed to make a guess at this and then check the accuracy by reducing reltol and/or mtol (*default=1) or explicitly setting nfft and the window. The reciprocal of the val-ue of the window is the frequency resolution. The value of nfft times this resolution is the highest fre-quency considered. For Laplace expressions, ^ signifies exponentiation.

* The transfer function of the Freq circuit element is specified by an ordered list of points of freq(Hz), mag(dB) and phase(deg) as follows: <(f1,m1,p1)[(f2,m2,p2)...]> where f1<f2<f3, etc. The following units specifiers may optionally precede the Freq keyword: “rad”=radians, “mag”=non dB, (“dB” and “deg” return the defaults), “r_i”=real and imaginary in place of magnitude and phase. If a delay value is called out, the phases of the table values are modified to reflect the delay (delay is automatically adjusted to maintain causality in any case).

Expressions can contain the following:

  • Node voltages and differences, e.g. V(n1) and V(n1,n2).
  • Circuit element currents, e.g. I(S1), the current through switch S1 or Ib(Q1), the base current of Q1. However, it is assumed that the circuit element current is varying quasi-statically, that is, there is no instantaneous feedback between the current through the referenced device and the behavioral source output.
  • The following operations, grouped in order of precedence of evaluation (* denotes undocu-mented features):
 Symbol | Operation
--------+--------------------------------------------------
    !   | convert succeeding expression to Boolean then
* or ~  |   invert
   **   | floating point exponentiation
    ^   | floating point exponentiation (Laplace only)
--------+--------------------------------------------------
    /   | floating point division
    *   | floating point multiplication
--------+--------------------------------------------------
    -   | floating point subtraction
    +   | floating point addition
--------+--------------------------------------------------
*  ==   | true if preceding expression is equal to
        |   succeeding expression, otherwise false
   >=   | true if preceding expression is greater than or 
        |   equal to succeeding expression, otherwise false
   <=   | true if preceding expression is less than or 
        |   equal to succeeding expression, otherwise false
    >   | true if preceding expression is greater than 
        |   succeeding expression, otherwise false
    <   | true if preceding expression is less than 
        |   succeeding expression, otherwise false
--------+--------------------------------------------------
    ^   | convert adjacent expressions to Boolean then XOR
    |   | convert adjacent expressions to Boolean then OR
    &   | convert adjacent expressions to Boolean then AND

For Boolean operations True is 1 and False is 0. Boolean conversions return True if <expression> eva-luates to greater than .5, else False.

  • The following keywords (global variables and constants):
   Name   |     Value      | Description
----------+----------------+--------------------------------
   time   |    variable    | time in seconds
    pi    |  3.14159265359 | 
*  boltz  |  1.38062  e-23 | Boltzmann constant
* planck  |  6.62620  e-34 | Planck's constant
* echarge |  1.6021765e-19 | charge of an electron
* kelvin  | -2.73150  e+02 | absolute zero in degrees C
  • Any user defined parameters or functions. Note that the parameter substitution scheme is gen-erally symbolic, but that when curly braces are encountered, the enclosed expression is evaluated immediately. With functions all parameter substitution evaluation is always done before the simulation begins. For details, refer to the .param and the .func simulator directives defined in Help under the subchapter on Dot Commands.
  • The following functions (* denotes undocumented functions):
*Status      Name  | Function
-------------------+---------------------------------------
            sin(x) | sine
            cos(x) | cosine
            tan(x) | tangent
           asin(x) | arc sine
           acos(x) | arc cosine
           atan(x) | arc tangent
        atan2(y,x) | arc tangent of y/x (four quadrant)
        hypot(y,x) | hypotenuse: sqrt(x*x+y*y)
           sinh(x) | hyperbolic sine
           cosh(x) | hyperbolic cosine
           tanh(x) | hyperbolic tangent
          asinh(x) | arc hyperbolic sine
          acosh(x) | arc hyperbolic cosine
          atanh(x) | arc hyperbolic tangent
            exp(x) | exponential
   ln(x) or log(x) | natural logarithm
          log10(x) | base 10 logarithm
            sgn(x) | sign (0 if x = 0)
            abs(x) | absolute value
           sqrt(x) | square root
*        square(x) | x**2
*         pow(x,y) | x**y
*         pwr(x,y) | abs(x)**y
*        pwrs(x,y) | sgn(x)*abs(x)**y
*         round(x) | round to nearest integer
            int(x) | truncate to integer part of x
          floor(x) | integer equal or less than x
           ceil(x) | integer equal or greater than x
          min(x,y) | the lesser of x or y
          max(x,y) | the greater of x or y
      limit(x,y,z) | equivalent to min(max(x,y),z)
         if(x,y,z) | if x > .5 then y else z
 table(x,x1,y1...) | interpolate y(x) per a lookup table
 or *tbl: x1<x2... |   of x-ordered point pairs
          uramp(x) | x if x > 0, else 0.
   *stp(x) or u(x) | unit step, 1 if x > 0, else 0
            buf(x) | 1 if x > .5, else 0
    !(x) or inv(x) | 0 if x > .5, else 1
           rand(x) | 0 < random num < 1 at x sharp steps/sec
         random(x) | 0 < random num < 1 at x soft steps/sec
*         white(x) | -.5 < ran num < .5 at x smooth steps/sec
*           fra(x) | white(x), but 0 if not SMPS steady state
*           ddt(x) v time derivative (v = Verilog-A compatible)
* idt(x) or sdt(x) v time integral: idt(x[,ic[,assert]])
*                  v   ic=initial constant, assert<>0 resets idt
*        idtmod(x) v wrapping idt: idtmod(x[,ic[,mod[,offset]]])
*                  v   offset < idtmod(x) < offset+mod
*       delay(x,y) v delay of x by y seconds
*absdelay(x,y[,z]) v delay of x by min(y,z) seconds