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Strange group delay at lower frequencies in Momentum

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Hamid.Kiumarsi

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Hi,

Say I have a simple CPW line and I want to see its group delay up to 40 GHz. I use Keysight's Momentum for my simulations.
My understating is that group delay will be almost flat and eventually will increase past a certain high frequency (say 30 GHz).
But to my surprise, I get an increased group delay at lower end of my simulation bandwidth. it doesn't matter whether I set the lower simulation frequency to 1GHz or 100MHz.
I get nonsense results when I try to go to very low frequencies (compared to the higher end of 40 GHz) like 1MHz or 10 KHz.

Since I don't see any reason for the increased group delay at lower end, I suspect that this might be a shortcoming of the simulator or my setup.

Is it safe to assume that group delay in CPW (which has a good group delay flatness) shouldn't increase at lower end of the desired bandwidth?

Please advise
 

Is the load on the CPW equal to its characteristic impedance?
 

Is the load on the CPW equal to its characteristic impedance?

It's a normal CPW line with a characteristic impedance of 50-ohm terminated on 50-ohm ports at both sides.
 

it should be relatively flat, unless there is dispersion at the higher frequencies.

I am not up on momentum, but if it is a meshing software, maybe your mesh size is whacky and it is predicting poorly.
 

Momentum is 2.5 D electromagnetic solver, that uses meshing to solver structure. You can re-run simulation with dense mesh setting and take more than 50 cells/wavelength.
 

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