oscillatorpspicesimulation
I'm just curious about why a ideal LC oscillator (See Figure) has a damped response in Spectre Simulator but not in Ngpsice. In both cases I'm using 5V of initial condition. I'm also using gmin=0 and gmin_in Spectre in order to avoid some energy losses. In Ngspice, I'm using default convergence options.
simulate this circuit – Schematic created using CircuitLab
The Spectre results gives me something like:
Does anyone has a hint about it?
Thanks
Although this is an old question. It would be worth noting that there are 3 things going wrong here. Firstly, ITOL should be lowered to be 1/10 of your lowest current, as well as IREL. This makes things take longer, but you need to have enough resolution for convergence. The biggest change should be to set the simulator to use the "Gear" method for convergence instead of trapezoid. What I believe that you have is something that is not converging with trapezoid due to resolution.
I tried your circuit(s) and I realized you're modeling the PWL source normally, but then you're inverting it with the first opamp. So I added a minus sign to the behavioural source, and tweaked the parameters a bit. Here's what came up:
I simplified a bit the circuit, just for the purpose of making and simulating it, but it's equivalent with your values. The behavioural source also has shrunk a bit, and I made the inflexion points as defined variables, for easier modification. I modified the threshold voltage and the last (highest) inflexion point to be lower. What you see is the result. It does seem to show some randomness. Maybe it's all in the settings.
Best Answer
I'm going to guess that it's the integration method used. Trapazoidal integration is good for oscillators, but Gear integration introduces numerical damping that tend to make oscillators damp out rather than oscillate.
Most simulators provide an option to select the integration method. Try the same method in both simulators to see if you get similar results.
See here:
"The one area where you should definitely avoid Gear is in LC oscillators. A pure LC tank circuit with an initial voltage on the cap will oscillate forever with trapezoidal integration, but will rapidly decay with Gear integration."