Simulating CAN bus wiring with LTspice and iCircuit

The primary motivation was to explain the overshooting and ringing I see from time to time when troubleshooting CAN bus networks

To anticipate it, the preferred tool is LTspice due to significantly better accuracy.

Simulation with LTspice

Basic Setup

Assumptions:

• 250 kBaud
• 120 Ohms impedance
• 2x 120 Ohms termination resistors
• 2x20m CAN bus wires going from the transceiver to each terminating resistor
• Signal speed 200'000 km/s -> 5ns/m
  • Signal delay time 0.1μs for 20m (used in tline)

The CAN transceiver is modelled as two square wave generators. One for CAN high and one for CAN low

"Good" CAN Bus

A few disturbances

Strong disturbances

The appropriate FFT

Simulation with iCircuit

The Tool iCircuit is available for Mac/iOS, Windows and Android

This simulation should be taken with a grain of salt as the accuracy of iCircuit seems to be very limited at higher frequencies!

I was also not able to figure out how to improve the simulation accuracy or decrease Tstep