Hacker News Reader

Comment by formerly_proven | Hacker News Reader

> 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!

View on HN

> 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=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.

Of note is also that at the highest Z point, most of the Z is coming from the R; X has dropped to almost zero.

Thanks, TIL!

> (since resistance is DC)

Resistance is actually a frequency-dependent quantity as well. It's the component of impedance that dissipates power (ie, current and voltage are in phase).

A very simple example of a frequency-dependent resistor is a solid copper wire. DC currents flow through the whole cross-sectional area; AC currents flow through an area that excludes an increasing fraction of the interior as the frequency increases.

Magnetic hysteresis and eddy current losses also behave as frequency-dependent resistances (although you might alternatively think about the latter as a frequency dependent coupling to a resistor).

Reply on news.ycombinator.com