MiniMoog VCO

[jhaible]

> Oscillators always seem to be a bit of a problem. Iterative loop errors
> and all that. And getting the initial states right can save a lot of
> simulation time. And fast rising edges...

A few hints for VCO simulation:

(1) Connect a switch across the integration cap which will open at t=0.
Keep the Ron and Roff of the switch within reasonable ratio. Infinite (off)
and zero (on) is the worst you can do. You need a high resistance for
off, though, so start with 10GOhm (off) and 1kOhm (on).

(2) While the variable timestep was one of the great advantages that made
Spice the standard it is, there is only one choice for accurate VCO
simulation:
Turn it off ! That is, set a maximum timestep, like 1us if you're simulating
a VCO at 1kHz. This will slow the simulation down, but it's the only cure
against the simulator running over an immediate incident like a sawtooth
capacitor discharge, and then hardly finding back to a stable point again.

(3) Separate the problem.
Waveshapers can be optimized in Spice to 99% with DC voltage sweeps.
If you have AC coupling in your waveshapers, get the DC operating point
and then replace the capacitors with voltage sources.
Simulating core and waveshapers separately will help you save the time
which you have to spend for fixed timestep method as in (2).

JH.