The truth about ferrites
qrm.guru
qrm.guru
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.
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!
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.
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).
Wouldn’t that be to detriment of signal integrity on receiving end of the serial line? Is it so that only higher harmonics are affected, which is okay?
Why did I hear putting ferrite on USB cable will improve signal integrity? Is that nonsense?
Most USB application notes also specify a small ferrite bead, so there is normally something on the PCB for filtering as well. Wurth have a nice app note on this (search for ANP024).
USB3 though, is particularly bad for radiating with low quality cables. My logitech mouse dongle will drop if I connect a high bandwidth camera over a poorly shielded cable.
It was only 110 kbps.
Serial consists of binary pulses. A framing bit comes first which starts a clock. The signal after that just has to be in the right range for indicating a 0 or 1 at each subsequent clocking. This is probably self-correcting to some extent, if the threshold for a level change is the same as the threshold for starting the clock.
The inputs are likely Schmitt-triggered, which essentially recovers the square wave.
https://www.onallbands.com/word-of-the-day-qrm-what-is-qrm-i...
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.
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/DIYSVTBassPreampI...
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-confer...
[...]
>"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-trans...
"One of the main applications for audio frequency transformers is in impedance matching."
In the US, CWS ByteMark sells a wide variety of inductive components in prototype quantities.
He ships internationally, for non-US folks.