Spice is a powerful tool for circuit analysis. But it often surprises users with
. holes in its abilities and strange definitions
. lack of industry-wide standards
. the need for the user to model the physics of their circuit, not just draw the schematic

I hope this blog will educate users and promote discussion in these areas.

Sine wave Oscillator - no output

Problem
Spice finds the correct DC operating point for the oscillator but there is no oscillation in Transient analysis.

Discussion
AC coupled sine wave oscillators can fail to start oscillating in the real world. They often fail to start in Spice.

Fix
Use a tiny coupling capacitor, perhaps 1pF, to couple a voltage source with a Pulse waveform into the oscillator. Set the Pulse initial level to 0V and the pulsed level to 1V. In 5Spice use the Step waveform.

This injects a disturbance into the circuit to start it oscillating.

Crystal Oscillators - slow to stabilize
The extremely high Q of a crystal oscillator means it takes thousands of sine wave cycles to reach steady state.


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