6 ms·

The truth about ferrites

79 points by lightlyused 6y ago

dognotdog 6y ago
Applying ferrites is definitely a little bit of a dark art, I'm glad to see this article. What I did miss is showing what simple turns of the wire without a core do to the signal :)
- deleted 6y ago
  [deleted]
- jhoechtl 6y ago
  Yeah, I had good results reducing unwanted QRM entering my radio simply by winding a few air coils and zip-tieying them together. As far a I can remember the diameter of that air coil maters but here we are entering dark magic.
deleted 6y ago
[deleted]
jandrese 6y ago
For people wondering what QRM stands for, it is an old code word, not an abbreviation. https://www.onallbands.com/word-of-the-day-qrm-what-is-qrm-in-ham-radio/ https://www.onallbands.com/word-of-the-day-qrm-what-is-qrm-i...
- sinoue 6y ago
  Thanks. >> "QRM stands for human-made noise, as opposed to QRN, which indicates noise from natural sources."
- fuzzybear3965 6y ago
  You beat me to it. I had it in my clipboard.
brandmeyer 6y ago
> In Australia, there are only a limited number of suppliers that carry stock. In the US, CWS ByteMark sells a wide variety of inductive components in prototype quantities.
- dpifke 6y ago
  I can also highly recommend Diz's (W8DIZ) shop: https://www.kitsandparts.com/toroids.php https://www.kitsandparts.com/toroids.php He ships internationally, for non-US folks.
kazinator 6y ago
I don't think the authors of this web page properly understand ferrites. Their methodology is flawed. According to their experiment, the best attenuation you can get out of any ferrite is a useless value in the single digit decibel range, which can be as poor as 0.5. That's almost like saying these things don't work. How ferrites work is that they are actually resistive at high frequencies (in addition to boosting the inductive reacatance of the conductor). How much attenuation that translates to depends on the circuit. We wouldn't say that a simple resistor, as such, has a certain decibel loss of voltage; it makes no sense. A resistor can exhibit no loss of voltage, if the current flowing through it is negligible. Ferrite beads work and how they work is that the target of the unwanted noise (the destination where we don't want it to go) has, at the given frequency, a relatively low impedance relative to the impedance of the bead. E.g. suppose the noise faces a 10 ohm input impedance into the device where we don't want it to go, and the ferrite manages to create a 1000 ohm source impedance. Then we have a 100:1 voltage divider, which is a 40 dB reduction. In my personal experience, the clearest directly perceptible example I've seen of their effectiveness was when I had serious noise on an LCD monitor due to a long serial line running under it between an embedded board and host. I had a clamp-on ferrite bead lying around and put it on the serial line. The display's obvious visual disturbances completely disappeared.
- formerly_proven 6y ago
  > How ferrites work is that they are actually resistive at high frequencies (in addition to boosting the inductive reacatance of the conductor). I don't quite follow. I think you just mean reactance at higher frequencies (since resistance is DC), and total impedance is then DCR + frequency-dependent reactance. Then the ratio of impedances determines attenuation, like you say. Which I think makes sense it works against interference, because I would think that the effective source impedance of radio interference is relatively high, so a half-turn (just passing the cable through once) can provide meaningful attenuation. Considering this, 3-8 dB attenuation against a low-Z (50 Ω) source with just a half-turn actually sounds good!
  - jmwilson 6y ago
    > I don't quite follow. I think you just mean reactance at higher frequencies (since resistance is DC), and total impedance is then DCR + frequency-dependent reactance. No, he's correct. Ferrites are a specifically-designed lossy inductor over a targeted frequency range. There is more than just DC resistance + a linearly-increasing reactance with frequency. Just like dielectrics exhibit loss and can be characterized by complex permittivity and a loss tangent, magnetic materials are also lossy and have a complex permeability. Here is a ferrite I used on the data lines for a GPS module: https://www.murata.com/en-eu/products/productdetail?partno=BLM15HD102SN1%23 https://www.murata.com/en-eu/products/productdetail?partno=B... Look at the impedance chart at the end and you'll see the resistive component is basically zero at DC and starts to increase in the HF range, peaking close to 1 GHz. The reactance of it is also highly nonlinear.
lightlyused 6y ago
Here is a great set of link about RFI, grouding, common mode currents both for audio and ham use. http://www.k9yc.com/publish.htm http://www.k9yc.com/publish.htm
- dbcurtis 6y ago
  Beat me to it. I was just going to post that link. Worth noting, Jim recently was given a technical service award by the ARRL for his work. http://www.arrl.org/news/arrl-technical-service-award-conferred http://www.arrl.org/news/arrl-technical-service-award-confer...
vibrolax 6y ago
Six years ago I needed to non-destructively reverse engineer a multi-tapped toroid inductor for an Ampeg SVT preamplifier clone that I was building. Starting with the inductor's nominal inductance, the physical diameter of the magnet wire and the dimensions of the wound inductor, I was able to determine the ferrite material and standard core size, the length of wire, and winding recipe without disturbing my sample of the factory part. All of this was for an audio frequency application, so the RF properties were not relevant. However, it was satisfying that my DIY inductor worked as expected, and I was able to determine the unknowns in the recipe with simple algebra. I documented my build story here: http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreampInductor.html http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreampI...
- justwalt 6y ago
  Successes like these feel so satisfying. Cool stuff.
oxfeed65261 6y ago
s/ferrite/ferret/g Hilarity ensues.
bdd 6y ago
If you found this interesting but didn't fully get some of the things explained, I can recommend checking out W2AEW's video where he explains things on paper and illustrates them on the bench in under 12 minutes. https://www.youtube.com/watch?v=81C4IfONt3o https://www.youtube.com/watch?v=81C4IfONt3o Alan Wolke (W2AEW) is a prominent YouTuber making videos on electronics, ham radio, and related topics. He is a long tenured Tektronix employee and an outstanding technical educator.
peter_d_sherman 6y ago
>"Why adding turns makes ferrites much more effective" [...] >"As happens in power transformer calculations, the impedance ratio is always the square of the turns ratio. So increasing turns through the core by x2 lifts its impedance (and attenuation effect) by x4. A x4 increase equates to a 6dB increase." Also: https://www.electronics-tutorials.ws/transformer/audio-transformer.html https://www.electronics-tutorials.ws/transformer/audio-trans... "One of the main applications for audio frequency transformers is in impedance matching."

79 points by lightlyused 6y ago

Applying ferrites is definitely a little bit of a dark art, I'm glad to see this article. What I did miss is showing what simple turns of the wire without a core do to the signal :)

deleted 6y ago
[deleted]
jhoechtl 6y ago
Yeah, I had good results reducing unwanted QRM entering my radio simply by winding a few air coils and zip-tieying them together. As far a I can remember the diameter of that air coil maters but here we are entering dark magic.

deleted 6y ago

[deleted]

jandrese 6y ago

For people wondering what QRM stands for, it is an old code word, not an abbreviation. https://www.onallbands.com/word-of-the-day-qrm-what-is-qrm-in-ham-radio/ https://www.onallbands.com/word-of-the-day-qrm-what-is-qrm-i...

sinoue 6y ago
Thanks. >> "QRM stands for human-made noise, as opposed to QRN, which indicates noise from natural sources."
fuzzybear3965 6y ago
You beat me to it. I had it in my clipboard.

brandmeyer 6y ago

> In Australia, there are only a limited number of suppliers that carry stock. In the US, CWS ByteMark sells a wide variety of inductive components in prototype quantities.

dpifke 6y ago
I can also highly recommend Diz's (W8DIZ) shop: https://www.kitsandparts.com/toroids.php https://www.kitsandparts.com/toroids.php He ships internationally, for non-US folks.

kazinator 6y ago

I don't think the authors of this web page properly understand ferrites. Their methodology is flawed. According to their experiment, the best attenuation you can get out of any ferrite is a useless value in the single digit decibel range, which can be as poor as 0.5. That's almost like saying these things don't work. How ferrites work is that they are actually resistive at high frequencies (in addition to boosting the inductive reacatance of the conductor). How much attenuation that translates to depends on the circuit. We wouldn't say that a simple resistor, as such, has a certain decibel loss of voltage; it makes no sense. A resistor can exhibit no loss of voltage, if the current flowing through it is negligible. Ferrite beads work and how they work is that the target of the unwanted noise (the destination where we don't want it to go) has, at the given frequency, a relatively low impedance relative to the impedance of the bead. E.g. suppose the noise faces a 10 ohm input impedance into the device where we don't want it to go, and the ferrite manages to create a 1000 ohm source impedance. Then we have a 100:1 voltage divider, which is a 40 dB reduction. In my personal experience, the clearest directly perceptible example I've seen of their effectiveness was when I had serious noise on an LCD monitor due to a long serial line running under it between an embedded board and host. I had a clamp-on ferrite bead lying around and put it on the serial line. The display's obvious visual disturbances completely disappeared.

formerly_proven 6y ago
> How ferrites work is that they are actually resistive at high frequencies (in addition to boosting the inductive reacatance of the conductor). I don't quite follow. I think you just mean reactance at higher frequencies (since resistance is DC), and total impedance is then DCR + frequency-dependent reactance. Then the ratio of impedances determines attenuation, like you say. Which I think makes sense it works against interference, because I would think that the effective source impedance of radio interference is relatively high, so a half-turn (just passing the cable through once) can provide meaningful attenuation. Considering this, 3-8 dB attenuation against a low-Z (50 Ω) source with just a half-turn actually sounds good!
- jmwilson 6y ago
  > I don't quite follow. I think you just mean reactance at higher frequencies (since resistance is DC), and total impedance is then DCR + frequency-dependent reactance. No, he's correct. Ferrites are a specifically-designed lossy inductor over a targeted frequency range. There is more than just DC resistance + a linearly-increasing reactance with frequency. Just like dielectrics exhibit loss and can be characterized by complex permittivity and a loss tangent, magnetic materials are also lossy and have a complex permeability. Here is a ferrite I used on the data lines for a GPS module: https://www.murata.com/en-eu/products/productdetail?partno=BLM15HD102SN1%23 https://www.murata.com/en-eu/products/productdetail?partno=B... Look at the impedance chart at the end and you'll see the resistive component is basically zero at DC and starts to increase in the HF range, peaking close to 1 GHz. The reactance of it is also highly nonlinear.

lightlyused 6y ago

Here is a great set of link about RFI, grouding, common mode currents both for audio and ham use. http://www.k9yc.com/publish.htm http://www.k9yc.com/publish.htm

dbcurtis 6y ago
Beat me to it. I was just going to post that link. Worth noting, Jim recently was given a technical service award by the ARRL for his work. http://www.arrl.org/news/arrl-technical-service-award-conferred http://www.arrl.org/news/arrl-technical-service-award-confer...

vibrolax 6y ago

Six years ago I needed to non-destructively reverse engineer a multi-tapped toroid inductor for an Ampeg SVT preamplifier clone that I was building. Starting with the inductor's nominal inductance, the physical diameter of the magnet wire and the dimensions of the wound inductor, I was able to determine the ferrite material and standard core size, the length of wire, and winding recipe without disturbing my sample of the factory part. All of this was for an audio frequency application, so the RF properties were not relevant. However, it was satisfying that my DIY inductor worked as expected, and I was able to determine the unknowns in the recipe with simple algebra. I documented my build story here: http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreampInductor.html http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreampI...

justwalt 6y ago
Successes like these feel so satisfying. Cool stuff.

oxfeed65261 6y ago

s/ferrite/ferret/g Hilarity ensues.

bdd 6y ago

If you found this interesting but didn't fully get some of the things explained, I can recommend checking out W2AEW's video where he explains things on paper and illustrates them on the bench in under 12 minutes. https://www.youtube.com/watch?v=81C4IfONt3o https://www.youtube.com/watch?v=81C4IfONt3o Alan Wolke (W2AEW) is a prominent YouTuber making videos on electronics, ham radio, and related topics. He is a long tenured Tektronix employee and an outstanding technical educator.

peter_d_sherman 6y ago

>"Why adding turns makes ferrites much more effective" [...] >"As happens in power transformer calculations, the impedance ratio is always the square of the turns ratio. So increasing turns through the core by x2 lifts its impedance (and attenuation effect) by x4. A x4 increase equates to a 6dB increase." Also: https://www.electronics-tutorials.ws/transformer/audio-transformer.html https://www.electronics-tutorials.ws/transformer/audio-trans... "One of the main applications for audio frequency transformers is in impedance matching."