Hi analog_chip,
I can help you out.
This is no where related to your marker positions. This is something you need to take care in "NO OF SAMPLE POINTS"
In the S Parameter menu. You give inputs like, Start Frequency , Stop Frequency, No of points etc"
When you give more no of sampling points. you can place the marker accurately.
It may take some extra time for running simulation process.
If this is S2P file simulation, No Problem.
For Layout Simulation, it may take a quite a long time.
If you need any further clarifications, let me know.
Regards,
Shunmuga Sundaram