THe posivtive S parameters in QUCS!

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noori.re

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Hello Everyone,
does anyone know why my S-parameters in Qucs are positive?
Thanks.

 

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GRT series are not RF Ceramic Capacitors. If you inspect the characteristics of this Automotive Grade Cermaic capacitors, you will see that the Reactance = 0 @4MHz.
So, you have to change this family with an appropriate one.

SimSurfing

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None of the above families are not appropriate capacitors for RF Frequencies.
( I don't know your project but s-parameters are not valid beyond their limits )
 
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    noori.re

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    I didn't know that this serie is not used in HF applications.
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Actually the port in left side is regulator output and the port in right side is the LDO input.
I wanted to see the difference of two structure above.
There is no high frequency signals there.
Would you recommend to I do a transient simulation using spice models instead of frequency sweep?
(Actually I did a transient simulation and it is completely reasonable.)
 

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I found the reason of bad s parameters.
In qucs the s parameters are not in dB.
the dB() function shall be used for this purpose.
 

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Solution
If this system has simply filtering purpose there is no need to use 50 Ohm RF s-parameters simulation. An AC analysis will be sufficient because you don't know the Impedances. s-parameters are valid only and only in case of the characteristic impedance is known. Your circuit is just a filter.
Replace s-parameter defined components with SPICE equivalents, the analysis will be more consistent.
 
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