Issue with EMX Oscillation Results for VCO and Coil

Hi,

I used EMX to simulate my coil and connected it to the VCO core at the schematic level. The circuit oscillated up to 255 GHz (a semi-schematic level simulation). Next, I completed the layout for my VCO core, connected the VCO core layout to the coil, and ran EMX for the entire structure using the black box approach. The transient simulation showed oscillation at 204 GHz.

Subsequently, I ran EMX for the VCO core layout alone using the black box and connected it separately to the EMX coil in the schematic. This configuration resulted in two EMX-generated symbols: one for the EMX VCO core and another for the EMX coil. However, in this setup, the circuit did not oscillate, which is unexpected since the layouts are identical in both cases. Additionally, the transient simulation time was set to 3 ns, but it could not progress further ( as shown in the figure). I checked the real (y11) of collectors of VCO and it is negative in the desired frequency range.

Running EMX for the entire circuit is very time-consuming. Therefore, I prefer to focus on the layout of the VCO core to reduce the frequency discrepancy between the semi-schematic and post-layout simulations.

Could you please help me understand how to resolve this issue?

Thank you in advance for your support.

Best regards,
Fatemeh