Electronics > Beginners
Series inductors SRF
jmw:
I made some toroid inductors that are about the same value (+/- 10% in inductance and self-resonant frequency). In theory, placing several in series should not change the self-resonant frequency, since \$\omega_0 = 1/\sqrt{(n L) (C/n)} = 1/\sqrt{L C}\$. In practice, I'm seeing the SRF goes down. Individually they are at 55-60 MHz, and by the time I have 4 in series, it's around 35 MHz. It's about the same on different jig set-ups: dead-bug and breadboard, spaced apart or closely. What non-idealities could be affecting the circuit?
MagicSmoker:
You're forgetting the stray inductance of the traces/wires connecting them together.
jmw:
Is straight wire inductance going to be significant against the toroids, which are 12-13 uH each?
MagicSmoker:
--- Quote from: jmw on November 21, 2019, 09:16:20 pm ---Is straight wire inductance going to be significant against the toroids, which are 12-13 uH each?
--- End quote ---
No, and you probably should have mentioned the value of the inductors in the first place. With a self-resonant frequency of 35-55MHz I was thinking the inductors were in the nH-range. My guess is that this is more of a de-Qing type of thing that is causing the SRF to broaden out. How are you actually determining the SRF here?
jmw:
Yeah, oops I left that out originally. I'm measuring SRF by the minimum in S21 when the inductor is connected in series to two lengths of coax+connectors hooked up to a tracking generator + spectrum analyzer. Grounds are tied together with a small sheet of copper.
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