Basics of Analog Simulation

   1.  LOG and AnaLOG are simulating your circuit constantly, even
       as you edit.  But unlike the digital simulator, AnaLOG requires
       your circuit to be "complete" before it can simulate.  That is,
       every pin must be connected to another pin, not left dangling.
       If the circuit is incomplete, i.e., there are still unconnected pins
       on some of the gates, AnaLOG will display a message to that effect
       on its scoreboard.  As soon as the circuit is complete, the
       scoreboard will switch to "Simulation in Progress".

   2.  The gate that looks like a box with an arrow on it is the METER.
       To monitor your simulation, sprinkle meters liberally around on
       all of the interesting voltages.  There is no limit to the
       number of meters you can have.  Remember that the meter's red
       dot is at the tip of the arrow, so the tip must be exactly
       touching the wire or pin of interest.  If the meter is
       unconnected or unhappy, it will display a "~" sign.

   3.  The "clock icon" is the round thing in the upper-right corner
       that looks vaguely like a clock.  Its importance is in the two
       numbers directly below it.  These are the current time and
       current timestep, respectively.  At the beginning, these are
       both zero.  As soon as the simulation begins, the current
       time will start advancing.  How fast it is advancing is shown
       as the current time-step, and may vary depending on how fast
       things are happening in the circuit.

   4.  A common error is to watch, say, a capacitor charging, then
       go in and double the capacitance, and complain that it still
       seems to be charging "at the same rate."  If you look at the
       clock icon, though, you will find that the timestep is twice
       what it used to be:  AnaLOG can simulate the ramp function
       just as quickly in "real time," but the circuit is (correctly)
       half as fast in "simulated time."  Make a habit of paying
       attention to the clock so that your observations have the
       correct frame of reference in time.

   5.  Although simulation happens continuously, it is sometimes
       necessary to "reset" the circuit back to its starting conditions.
       For example, suppose you remove a wire from a running circuit and
       the voltages that were in the circuit at the time are "impossible"
       for the new circuit.  The simulator will work as hard as it can
       to figure out what to do, but may eventually give up and print a
       message.  Another example is that you may want to reset various
       nodes to the initial voltages that you have programmed for them,
       as described in the next paragraph.  In any case, all you have to
       do is select RESET in the Misc menu, or press the "R" key.  This
       sets the simulation clock back to 0 seconds, and returns the
       voltages to their initial values.

   6.  If you open any AnaLOG gate for configuration, one thing you will
       find is a set of three attributes for every pin on the gate.
       These attributes are present voltage, reset voltage, and parasitic
       capacitance, respectively.  Every pin has a slight parasitic
       capacitance to ground, initially 10fF.  This capacitance must
       exist for the simulation to run, but you can set it lower if
       you find that it's in your way.  The "present voltage" is an
       actual indicator of the voltage currently on that pin, and it
       changes while you watch during simulation.  You can enter new
       values here if you wish, but the simulator may not like you if you
       go around making instantaneous changes in the circuit's voltages.
       The "reset voltage" is used by the RESET command.  If you leave
       it blank, AnaLOG will solve for an appropriate initial voltage
       at RESET time.  Notice that Present and Reset voltages are for
       nodes, not gates: if you change a pin's reset voltage, then
       open up another gate connected to that same node, the reset
       voltage shows up there, too.

   7.  When you reset, you will notice that the timestep jumps down,
       usually to about 1E-18 seconds, then starts increasing.  This
       is partly because AnaLOG is cautiously trying to discover how
       fast it can safely proceed in the simulation, and partly
       because it has to solve for unspecified initial voltages by
       simulating the brief charge-up that would occur when you
       powered up a real circuit from scratch.  If you push RESET
       and the voltages seem to be frozen, don't depair -- wait for
       the timestep to get out of the femtosecond range!

   8.  The AnaLOG scoreboard has some words on its left and right
       sides.  On the left is Memory, with indicators Set and Erase.
       If you tap the word "Erase," all of your "reset voltages" will
       be erased.  You might do this if editing has caused your reset
       voltages to be no longer appropriate, and you want AnaLOG to
       solve the circuit from scratch.  If you tap the word "Set,"
       all of the present voltages in the circuit are copied into the
       reset voltages.  For example, you might turn your input
       waveforms off, let the circuit settle to a stable state, then
       push Set so that next time you RESET, the circuit will start
       out at that stable state and you won't have to wait again.

   9.  AnaLOG is normally very cautious about voltage changes.  If it
       is going along at a particular timestep and some node suddenly
       starts changing more than about .1V per timestep, AnaLOG will
       reduce the timestep and simulate in smaller chunks.  This is
       because trying to simulate fast voltage changes all at once
       results in ugly and inaccurate waveforms.

  10.  Sometimes all you are looking for is a DC point anyway, and
       you don't care how ugly your transient waveforms are.  In this
       case, you can touch the word "Relaxed" on the scoreboard.  This
       tells AnaLOG to take timesteps as large as it can regardless of
       how fast the voltages are changing.  Touch "Exact" to return to
       an accurate simulation.  Often what people do is touch Relaxed
       right after a RESET so that the initial charge-up proceeds
       quickly, then touch Exact when the timestep gets up to the
       picosecond-nanosecond range.

john [dot] lazzaro [at] gmail [dot] com