I'm running Cadence Virtuoso 6.1.6 through a remote desktop to computers at my school. These computers run Fedora 22. I'm simulating a ring oscillator VCO on Cadence, but it is taking literally FOREVER to complete the simulation. Here is the schematic of my VCO –
I have a vpulse generator which is attached to the feedback loop (as can be clearly seen in the above figure). I have a feeling that this pulse generator is the main source of the problem because the really slow simulations started after I added this component into my schematic.
Here are the parameters I have set for the pulse generator, just so you can get an idea –
I started simulation and it takes about 20 minutes to get to 12%. When I click the "Plot Outputs" button in ADE L, it shows that the simulation hasn't even started oscillating yet…and the estimated time of completion ranges from 1Hr16Min to 3Hrs+!
Has anyone else encountered this problem? If so, how can this be solved?
EDIT: I used the Ultrasim simulator on Cadence which simulates thousands of times faster than spectre, which was the simulator I was using before.
Best Answer
You are right. The problem is the voltage source you have in the feedback loop.
You're trying to build a VCO, the frequency of which i believe you're controlling with the current level of the current starved inverters you have in the middle cell. Your frequency of oscillation will be determined by the propagation delay of the inverters in your chain, and trying to force it to another value with a pulse generator in the feedback loop will simply make the simulator go bonkers, as you're trying to drive a fixed frequency to a positive feedback loop that is trying to oscillate itself at a frequency determined by circuit components.
Remove the pulse generator, run a transient simulation to determine your oscillation frequency. You can guesstimate it by adding the propagation delays of the inverters you have in the chain.