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Coaxial cable for power transmission

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Mad I.D.

Mad I.D.

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Hi.
I'm using coaxial cables to transfer power from the power supply to the device (IC under test). Since there is danger from resonance peaks in the supply's output impedance, I'm trying to model the whole system and find the solution (in form of additional damping).

For e.q, my coax is 50 ohms with 101pF/m. Let's say I'm using 1m of it. From Zo=sqrt(L/C) i calculate that L is 250nH. DC resistance/km is also given. Can I just model the cable as L&R in series, and C at the end to ground?

I'm really not sure if the model is correct and unfortunately didn't manage to google anything useful.

Thank you.
 

The most accurate model would be a lossy transmission line. Otherwise, you could use lumped RLC, but I would suggest you break the length down into a number of smaller segments to get a more accurate picture - say 20 segments should do.

Keith.
 

    Mad I.D.

    Mad I.D.

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Thumb rule: RLC approximation It's 'good enough' to 0.05 of lambda (wavelength), ca 30MHz (including wavelength in the cable).
 

    Mad I.D.

    Mad I.D.

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At frequencies below below e.g. 0.1/λel (20 MHz for 1 m), the single lumped inductor and capacitor should be sufficient model.
Above this frequency, an ideal transmission line model would be my first choice, when using a circuit simulator, because the lossy
character gets's important at much higher frequencies respectively larger cable length only.
 

    Mad I.D.

    Mad I.D.

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