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mxkdirs

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calculating inductance
Feb 26^th, 2006, 12:05am

Hello,

I need to check the self resonant frequency of an inductor in my design kit. What's the best why to do it. I did an ac analysis with ac voltage source of magnitude 1 and then I did: imag(1/{input current})/(2*pi*freq), meaning I took the imag. part of the input impedance and divided it by omega to get the inductance and then look at what frequency it peaked. I didn't get the right result doing this way. One side of inductor was connected to the ac source and the other was grounded. What's the right why of doing this? Thanks for the help

-m

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ACWWong

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Oxford, UK

Re: calculating inductance
Reply #1 - Feb 27^th, 2006, 4:19am

the self resonant frequency is when the imaginary input impedance is zero (ie the L and C cancel/resonate).
I think what you are seeing is an artifact the inductor you are likely to be looking at is infact a lumped model consisting of series L, series R and shunt C. Also likely to be included at each terminal is an RC network to substrate. You can look at the configuration in your netlist/modelfile.
When you connect one side of the indcutor to gnd, you short one of the RC terminal networks to ground, hence the effective capacitance is changed from if you did the measurement differentially. Now which srf you are interested in (single ended or differential) depends on how you intend to use the inductor in your circuit.

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kokabanga Junior Member Offline Posts: 28	Re: calculating inductance Reply #2 - Mar 15^th, 2006, 7:03pm you need to connect the inductor as a two-port network and then plot the S11. The frequency at which imaginary(S11)=0 will be the self-resonant frequency.
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mg777 Senior Member Offline Posts: 131	Re: calculating inductance Reply #3 - Jan 14^th, 2007, 1:34pm The most tangible signature of resonance is phase changing from +180 deg to -180 deg. This sounds like a scam because +180 and -180 are the same thing, right? But based on the definition of atan in ANSI C it turns out this phase switch is a dramatic manifestation of Imag(s11)=0. M.G.Rajan www.eecalc.com
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