Filter Simulation LTSpice Impedance - Page 1
Forum section: electronics / beginners

author: OneGeekGuy
author_rank: Regular Contributor
timestamp: October 21, 2020, 08:52:10 am
content: Hi all,I will ask maybe a really simple question, but I am dealing with Impedance simulation in LTSpice, and some doubts came to me. A really simple network, Inductor and Resistor in parallel to this inductor, and I want to know the impedance of this network:We can measure the Impedance creating a plot in Vout, and if we use Cartesian representation we will get the impedance:Since we are using 1A as current source, we will get that in the plot 1kV = 1Kohm Impedance lets say.Till here everything ok right?But now other point..., should I add the load of this network at Vout if I want to get the real impedance? I think no, unless I want to have calculate the impedance of the overall circuity, load included...And to finalize...., why the LTspice is calling to the left axis in the chart "real component" since the continues line should be the X (imaginary value in this case inductance) and the dashed line should be the Real value right?Thanks for your comments! Any other observation will be welcome!

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 21, 2020, 09:17:21 am
content: Hi,Please share your LTspice model by attaching it to your post.There is something wrong with your model, you probably have parallel capacitance in the inductor model. This is the correct answer:This graph shows how the filter impedance changes with frequency.The graph was obtained by plotting LOG on the vertical axis:The corner frequency is 68 MHzThe answer should be:F = R / 2 x Pi x L = 2000/2 x 3.142 x 4.7E-6 = 67.7 MHzI have attached the model.Regards,Jay_Diddy_BFilter Impedance.asc(0.58 kB - downloaded 132 times.)

author: OneGeekGuy
author_rank: Regular Contributor
timestamp: October 21, 2020, 11:12:47 am
content: Hi Jay,The mainly difference is that I am plotting using cartesian system,Is not more clear to plot in Cartesian, since the Impedance Charts are in this axis system?Like:SourceIf I setup the axis in Bode mode, I get more or less your result:Please find attached the .asc file. I am using SDE0403A-4R7M.The reason why I am playing with this and the series resistor, is because I want to check the effect of the placement of this parallel resistor in different values configurations. The idea of this "filters" is to block the AC current trough, and only let the DC go trough.The idea of this R parallel is to increase the Q of the inductor.Thanks for your feedback!

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 21, 2020, 11:45:36 am
content: Hi,The SDE0403A-4R7M model includes the parasitic elements:This gives the right results.Adding the parallel resistor willdecreasethe Q.Jay_Diddy_B

author: OneGeekGuy
author_rank: Regular Contributor
timestamp: October 21, 2020, 12:14:09 pm
content: Yes that it is.At the end we got the same result but in different axis modes.Related to the Q, since the Quality factor formula is: Q = (2 * pi * f *L) / R if we place for example 2K resistor in paralel to the internal AC resistor of the Inductor Real model, that could be for example....12K we are reducing the parallel resistance this means that we will have higher Q since the denominator is lower. Correct me if I am wrong.And other question...It is possible to take in account in this simulations the effect of the load when we have the filter working in the real world?Thanks!!