# Rh versus MHz/resistance for inductor beads

In my previous question question I learnt I have to use inductor beads, which is also according to the MIDI specs, see below

They state 1K@100MHz, but if I look e.g. on AliExpress for inductor beads, I only see stats like RH3.5 * 6 * 0.8, see AliExpress. How can I convert these specs?

(btw, I do not want SMD, I use DIP/normal breadboard style components).

Update:

I found probably something better: R6T 2,5T (or 3T) R6

These are according to a similar looking spec 600 or 800 ohm (of course if the components are similar, but it seems better than RH3.5's).

• Go to a source that has proper data sheets. Forget Ali baba and the magic words and use a proper vendor. Pros don't buy stuff that doesn't have a data sheet and comes from a reliable source and the reasons should become apparent to you in time. – Andy aka Jun 14 '17 at 21:29
• @Andyaka I appreciate your remark (even upvoted it), but I have to pay everything myself ... if I would use 'official' parts I would spent hundreds of euro more, and I'm not a pro ... actually, I'm just learning electronics. Of course in a pro situation, one cannot get away with components without proper specs. – Michel Keijzers Jun 14 '17 at 21:34
• @MichelKeijzers For many parts, you can get away with poor specifications (and I often do). Ferrite beads, however, are surprisingly complicated and poorly standardized, so you really need a plot of complex impedance vs frequency for that part to know whether it will help or hurt in your specific situation. – Abe Karplus Jun 14 '17 at 22:01
• @AbeKarplus yes ... it seems that way ... so far I found one item with 100 MHz on Aliexpress, but no mention of impedance... I will check better sources, thanks – Michel Keijzers Jun 14 '17 at 22:03
• @MichelKeijzers No, it's not because I'm a pro that I can afford more costlier suppliers. In the long run, those costlier suppliers work out cheaper - how much you value your time is something that you have to factor in of course but, if you are happy to spend hours on a poorly defined component (or even a fake component) then eventually buy from a better source so be it. I can only advise you what is the most cost effective route. – Andy aka Jun 15 '17 at 7:18

## 2 Answers

That AliExpress item does not have any specifications, but the cheapest through-hole ferrite bead on DigiKey (Laird 28L0138-50R-10) looks similar, and has the following specs:

The AliExpress beads are smaller, and are made from some unknown material, so they will have even less impedance. (You could put ten beads in series, but that would be silly.)

By comparison, a cheap-ish wide-band choke that comes near the suggested 1 kΩ @ 100 MHz is the Bourns FB20022-4B-RC:

This impedance is achieved by winding the wire multiple times through the ferrite:

And this is, of course, more expensive.

In practice, many MIDI devices use 600 Ω SMD ferrite beads. (Nowadays, through-hole ferrites are used only when you want a high current rating, which is not needed for MIDI.)

But ultimately, if you really want to use cheap through-hole parts, a 100 Ω bead is better than nothing, especially if you don't even know your noise frequencies …

• I checked a more local site (which is also 'professional', probably like digikey), and they have specs for the Bourne which show 100 Mhz (nl.farnell.com/bourns-jw-miller/fb20022-4b-rc/…) ... However, since I indeed do not my noise frequencies, I think I'm going to buy the cheap through hole parts, and perform some tests with noise sources nearby (cables/power speakers etc). If it still works, I'm satisfied :-). – Michel Keijzers Jun 15 '17 at 8:42
• Also I found on AliExpress 9mm instead of 6mm beads, so those are probably the 'better' ones, it doesn't mention ohms, but mentions 100MHz: aliexpress.com/item/… – Michel Keijzers Jun 15 '17 at 8:52
• I found 600 or 800 ohm RH6 ferrite beads, see update at the end :-) – Michel Keijzers Jun 15 '17 at 10:22

I suggest you experiment with RFCs aka Radio Frequency Chokes.

Get a 1Kohm 2 watt carbon composite resistor. (approximately 1" long, 0.5" diam). Wind 100 turns of enameled wire around it. Neatly wound if you wish, scramble-wound if you wish.

Inductance will be $$A^2 * N^2 / (9*A + 10*B)$$ in microHenry. A is radius inches, B is length inches.

Inductance = 1/4^2 * 100^2 / (9*1/4 + 10*1) = 600/12 = 50uH.

The F3dB of 50uH and 1Kohm is (Tau = L/R = 50 nanoseconds) or 3MHz.

• I'm not exactly understand everything, but that seems like an easy way to do it. However, putting 18 of these 1" long things in my box, will expand it greatly ... so I'm going for the cheap (possibly less ohm) solution and hope for the best :-). But interesting to see this is possible in an easy way ... btw, what would be the best way to keep the wire attached to the resistor? – Michel Keijzers Jun 15 '17 at 8:56
• I found 600 or 800 ohm RH6 ferrite beads, see update at the end :-) And they are called in the spec Axial Wide Choke Band Ferrite Beads, which is according to your answer, thanks! – Michel Keijzers Jun 15 '17 at 10:23
• I've wound a few RFCs, as a kid, tinkering with RF stuff. I used black plastic tape. I suspect the ARRL handbooks have better suggestions (the HAM guys). – analogsystemsrf Jun 15 '17 at 16:13
• Thanks for the idea ... in my case I probably can get away with pre-made (but maybe not perfect) inductor beads, but in the future if I need them with a specific/critical value, I can use this way (or at least to test). – Michel Keijzers Jun 15 '17 at 16:24