Q factor and SRF - how are these related? - Page 1
Forum section: electronics / projects

author: Plasmateur
author_rank: Regular Contributor
timestamp: January 02, 2017, 06:51:02 pm
content: I'm looking for some inductors with a particular to serve as a notch filter.When looking through Digi-Key or Mouser I see that their are SRF values and Q factor values.When picking out the SRF values, let's say 13MHz, the spec sheets usually list a Q value at a much lower frequency than the SRF frequency.Should I pick out the inductor with a high Q as close the the SRF to achieve what I'm looking for?

author: TimFox
author_rank: Super Contributor
timestamp: January 02, 2017, 08:41:27 pm
content: Are you going to use the naked inductor to give a notch at its SRF (the frequency at which it parallel-resonates without additional capacitance)?The manufacturer's data are measured using standard frequencies (based on traditionalQ-meters,q.v.) which are always below self-resonance, since they must be measured where the reactance is inductive.Normally, you would use an inductor at a frequency below its SRF and add a capacitor (probably adjustable) in parallel to get a useful notch.

author: Plasmateur
author_rank: Regular Contributor
timestamp: January 02, 2017, 09:31:14 pm
content: Quote from: TimFox on January 02, 2017, 08:41:27 pmI've used some before without looking to deep into the Q factor for the purposes of a RF compensated Langmuir Probe and they seemed to have worked for the application. Its time to buy some more and after reading further into why these inductors are used, one of the papers cited a high Q value.Just out of curiosity I hooked my sig-gen up to one of my previously purchased inductors with a set SRF and then into my scope and noticed attenuation close the SRF frequency.Then I did the same with a network analyzer and it appears the attenuation was offset to the SRF by a few MHz.I can try building a notch with an adjustable cap....so long as the adjustable cap is very tiny. I'll have to look into those. Thanks!

author: Plasmateur
author_rank: Regular Contributor
timestamp: January 02, 2017, 09:35:40 pm
content: Quote from: evb149 on January 02, 2017, 09:04:48 pmThere is alot to take in here and I thank you for taking the time to go over this with me. Now I'm rethinking one of my particular applications given the information I've gained in this thread. Thank you very much!

author: G0HZU
author_rank: Super Contributor
timestamp: January 02, 2017, 09:37:22 pm
content: Quote from: evb149 on January 02, 2017, 09:04:48 pmFor a typical (medium to high unloaded Q) wound inductor component with typical self capacitance the measured inductance of the component will tend toincreaseas you go up in frequency towards the SRF. As you get close to SRF the measured inductance will climb rapidly and the impedance will also climb up very rapidly near SRF. Just beyond SRF the inductor will look like a tiny capacitor.

author: G0HZU
author_rank: Super Contributor
timestamp: January 02, 2017, 09:56:50 pm
content: QuoteIf you wound an air cored solenoid type inductor or used a powdered iron (toroid?) core the measured inductance would tend to go UP sharply as you approach SRF. This increase is due to the self capacitance of this type of inductor. These types of inductor are very common in RF bandpass, bandstop, lowpass or highpass filters. Some very lossy inductors using a lossy ferrite core can have an inductance profile that appears to show reduced inductance with increasing frequency but these aren't generally suitable for use in high Q notch filters.

author: alexo
author_rank: Contributor
timestamp: January 02, 2017, 10:23:08 pm
content: Quote from: evb149 on January 02, 2017, 09:04:48 pmI think you might want to re-word this. I can't digest it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2017, 02:35:59 am
content: Note that Q drops when the test frequency is near the SRF, so it's not always a meaningful measurement.At low frequencies, DCR is dominant; at high frequencies, EPR is dominant (mostly core and eddy current losses).Tim