Hi *,
I am simulating a 32kHz high Q crystal oscillator with a pulse shaping circuit. I set up a PSS analysis using the Shooting Newton engine. I set a beat frequency of 32k and used the crystal output and ground as reference nodes. After the initial transient the amplitude growth was already pretty much settled such that the shooting iterations could continue the job.
My problem is: In 5...10% of my PVT runs the simulator detects a frequency divider in the initial transient simulation. The output log says:
Frequency divided by 3 at node <xxx>
The Estimated oscillating frequency from Tstab Tran is = 11.0193 kHz .
However, the mentioned node is only part of the control logic and is always constant (but it has some ripples and glitches which are all less than 30uV). These glitches spoil my fundamental frequency (11kHz instead of 32kHz). Sometimes the simulator detects a frequency division by 2 or 3 and the mentioned node <xxx> is different depending on PVT - but the node is always a genuine high or low signal inside my control logic.
How can I tell the simulator that there is no frequency divider and it should only observe the given node pair in the PSS analysis setup to estimate the fundamental frequency? I have tried the following workarounds but none of them worked reliably:
- extended/reduced the initial transient simulation time
- decreased accuracy
- preset override with Euler integration method for the initial transient to damp glitches
- tried different initial conditions
- specified various oscillator nodes in the analysis setup form
By the way, I am using Spectre X (version 21.1.0.389.ISR8) with CX accuracy.
Thanks for your support and best regards
Stephan