Yes, Rvar= k*sqrt(f) is modelling skin effect.Does LTspice compute the skin effect resistance from k in Coilcraft's model?
It's LTspice s-parameter simulation mode (.net command), compensating port matching loss. The setup calculates s21 correctly.Shouldn't your filter have > |-6dB| loss?
I agree it's automatic because the input has a given series value of ohms while the output has an equal value of ohms to ground. Thus the filter taps at the middle value of a resistive divider. My simulated output (post #2) is one-half the amplitude of the input. So that can be seen as equivalent to automatic 6dB loss.Shouldn't your filter have > |-6dB| loss?
Equivalent circuit with skin effect loss term is exactly reproduced in LTspice model. Basic question is why parametric search table gives considerably higher (about doubled) Q numbers.I recommend you to use equivalent circuit of the coils of Coilcraft. They are more accurate.
This example was selected for simplicity, the filters are already synthesized with the ideal inductors.If you want a high-order low-loss Chebychev BPF at 25 MHz you would approach the component choice differently with higher Q's for each LC pole.
You stated a 0.1 dB BW from 24 to 26 dB yet plotted the -6dB normalized attenuation instead of showing the -3dB BW.
What are your passband & stopband design specs?
There is a practical limitation, in case of air core inductors mainly set by DCR + frequency dependant skin effect resistance. No matter if SPICE model Q of about 16 or comparison table Q of 31 gives the right picture, it's most likely not good enough for your purposes. According to comparison table, other (larger) air core types have up to fourfold Q. For range up to a few MHz, closed magnetic path ferrite core inductors with air gap promise Q numbers up to several hundred.is it possible to design a high order band pass filter working in low MHz range with real inductors or is there a fundamental limitation of low Q at those frequencies?
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