Why does a disconnected circuit start to oscillate?

[mplucinski]

Hi all 👋

I've build a circuit that's meant to divide input frequency by two. Then I duplicated it. Then I noticed that the second circuit started to oscillate together with the first one, even though they're not yet connected by any wire. There's also no clock source in the second circuit.

I assume voltage sources and grounds are meant to be ideal, so shouldn't conduct any noise between the circuits.
So I'm wondering whether what I encountered can be considered a bug, or rather it's some kind of fundamental limitation of the simulation.

See it here - in the right side circuit, voltages switch without apparent reason, but in phase sync with switching in the left side circuit. Disconnecting the clock from the left circuit also stops oscillations in the right circuit.

https://tinyurl.com/27platgw

Bonus question: does this indicate a design flaw in the circuit? I'm rather new to electronics, still only partially understanding of what I'm doing at all :D In this case, I took a schematic found in the internet, not fully understanding how it works.

[pfalstad]

I think it's a simulator bug caused by roundoff error when solving the circuit. Everything gets loaded into one matrix, and when solving it, some unrelated nodes can become connected.

[mplucinski]

Hm, makes sense 🤔 Is there perhaps a general way to deal with such issues? It doesn't happen for the most of circuits after all. I'm also wondering whether it may be an indicator of a poor design of a circuit - if a rounding error can trigger a switch to an opposite state, it probably means the circuit is rather instable, right?

Btw, thanks for your great simulator, Paul. I'd probably never be able to get into electronics without such an understandable visualisation :)

[pfalstad]

I don't think so. There are a lot of commonly used circuits that are tricky to simulate. This particular circuit looks pretty common. It looks like one of the transistor examples in the menus, Circuits->Transistors->Multivibrators->Bistable.