How to determine ferrite slug frequency - Page 1
Forum section: electronics / rf-microwave

author: szoftveres
author_rank: Regular Contributor
timestamp: January 08, 2020, 01:32:35 am
content: Hi All,I bought a bunch of coil forms from aliexpress because they were cheap.Is there an easy way to figure out the suitability of these coils for e.g. VHF without expensive instruments (the only thing I have is a 100MHz scope)?I can wind wire on them and can build oscillators for VHF, just curious what would be an easy method.Any suggestions are welcome.Also, if a ferrite material works on VHF (100MHz), is there anything that would make it unsuitable to use on e.g. low HF (1-2 MHz)?Thanks!

author: unitedatoms
author_rank: Frequent Contributor
timestamp: January 08, 2020, 01:53:01 am
content: You can wind few turns and put in parallel with known capacitor. Then connect to scope (in parallel) and connect to the pulse generator on same scope (should be a tuning pulse post for tuning the probes). The resonance frequency should be visible as period of dissipating transients.So basically one wire and one cap. No need to build an oscillator for simplest estimate.Edit: tried it with random shielded RF coil. It is series L-C from pulse test point to ground probed in between L and C. So not a parallel but series resonance.Edit2: I remeasured with fully screwed in ferrite, also removed a second small tubular cap from bottom of coil, it was a LC tank). The period of transient is 375ns. Coil independently measured with LCR meter is 39uH, cap is 270 pF. So error with frequency is bad. 2.66MHz measured, vs 1.5MHz predicted.I don't know how to explain it. Also something, I noticed first time, the rise time on scope test signal is slow: 1uS but it is perfect trapezoid. I dont know if it is expected by design of the scope.

author: szoftveres
author_rank: Regular Contributor
timestamp: January 08, 2020, 02:17:22 am
content: Quote from: unitedatoms on January 08, 2020, 01:53:01 amWould these transients of a coil on a "non-suitable" core dissipate faster than if they were wound on a suitable one? Or I wouldn't see ringing at all if the core acted as a vhf noise suppressor?

author: David Hess
author_rank: Super Contributor
timestamp: January 08, 2020, 02:54:27 am
content: Quote from: szoftveres on January 08, 2020, 01:32:35 amIn the past some variation of a dip meter (grid dip, tunnel dip, whatever) would be used to easily measure resonate frequency.MFJ sells onebut maybe someone else has a better suggestion.QuoteA material intended for higher frequency operation might require too many coil turns to be used at low frequency. A material intended for low frequency operation would likely be too lossy at high frequency.Quote from: szoftveres on January 08, 2020, 02:17:22 amIncreased losses from either the core material or higher series resistance from too many turns will result in lower Q so faster dissipation of the ringing. One can be calculated from the other.

author: james_s
author_rank: Super Contributor
timestamp: January 08, 2020, 03:33:35 am
content: Seems like the easiest lacking a datasheet is to build a little single transistor oscillator around the desired frequency and evaluate one under real world conditions.You might also be able to take a stab at the core material just by measuring the inductance, shouldn't be too hard to measure the saturation current too.

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: January 08, 2020, 04:17:42 am
content: A said earlier ptu a cap in parallel with coil.Then put a 1k resistor in series with cap and coil.Apply a sine wave generator across resistor open end and joint of cap and coil at other end.Sweep sig gen until a maximum is seen across coil/cap and that tis resonant frequency.I have often used this technique when designing SMPS to determine transformer inductance.The inductance L=1/(4 * pi squared * frequency squared * C)

author: szoftveres
author_rank: Regular Contributor
timestamp: January 08, 2020, 06:33:42 am
content: Quote from: unitedatoms on January 08, 2020, 01:53:01 amI rang the below circuit (the scope calibrator wasn't enough but a 74HC14 oscillator had a steep enough slope), fed the square wave at a tap at 1.5 turn through a 270R resistor.The ringing dies off after 1-1.5 periods with no slug in (roughly 10ns per period). With a nice big VHF slug, it continues for 2-3 periods, but quite surprisingly it does so with a ferrite bead (which was made to suppress noise) of similar size too.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 08, 2020, 07:58:47 am
content: High frequency ferrites have less loss and usually also lower permeability. However there are low permeability ferrites that are not good at high frequency too. So a low permeability alone is not enough to make sure the ferrite is suitable for VHF. With the short slug shape it is not easy to measure permeability.To measure the loss, one needs to have a resonator with little extra damping, so more like weak capacitive coupling and a low loss cap (e.g. good NP0 ceramic or air capacitor. One could observe the ringing if a high Q filter build with such a core. This may need a local amplifier to reduce the loss compared to the scope probe.Another way would be to build a LC oscillator with weak coupling and compare the gain needed to start oscillation to a version with a known good core and maybe a known poor (e.g. ferrite bead).

author: David Hess
author_rank: Super Contributor
timestamp: January 09, 2020, 04:03:35 am
content: Quote from: Kleinstein on January 08, 2020, 07:58:47 amThe simple test circuits I have seen discussed do something like that by measuring the DC power required by the oscillator to produce a constant output.Testing using a dip meter essentially works the same way but makes relative rather than absolute measurements.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: January 10, 2020, 09:03:25 pm
content: The quoted specs on them is 25 to 100 mhz, but in my own testing, I found them to look almost identical to type 6 Iron cores. Should be fine from 7Mhz and up in low power situations assuming the losses are acceptable to your circuit. I just ran them up on the VNA in a small test jig i have. Wound 1uH on a variety of different cores and looked that the frequency response of each. I used type 2, 6, 43, some CoilCraft coil forms and some unknown ebay ferrites. By comparing unknown to the known, the frequency response was identical to type 6 iron with 1db of loss.

author: szoftveres
author_rank: Regular Contributor
timestamp: January 12, 2020, 08:24:01 am
content: Quote from: vk4ffab on January 10, 2020, 09:03:25 pmIn the meantime I found a quite unusual but surprisingly reliable way of testing the slugs for VHF. I had built a crystal radio for FM with an air core coil; inserting the aliexpress slug and the ferrite bead both degraded the performance significantly but the coilcraft vhf slug didn't cause too much harm.I will assume these aliexpress cores max out somewhere between 25-40MHz (took some time to figure out what type 6 core is:http://www.qrz.lt/ly1gp/amidon.html)