Electronics > RF, Microwave, Ham Radio
is this explaination of a microwave circulator correct?
coppercone2:
So this guy says its like a hall effect device. The wikipedia page says something very confusing and does not mention the hall effect. He says that the ferrite itself is just a magnetic field spreader, while wikipedia claims that the ferrite is a specific type of ferrite. Is the video explanation just over simplified?
https://en.wikipedia.org/wiki/Circulator
Also, what is a circularly magnetized material? How do they make that? And like for explainations, if you gutted a circulator and stuck it into a uniform helmholtz coil, would it work, without the ferrite? It seems that the ferrite is critical according to one explanation but not the other.
I think it would be interesting to have a
1) how its made (practically), I guess its simple machining other then the ferrite
2) how its designed (say what it you introduce a air gap between the ferrite and the copper by making the ferrite disk thinner?)
paul@yahrprobert.com:
That guy on youtube is pretty much just making stuff up. And the wikipedia article, with its "circularly magnetized" nonsense is a little to brief and misleading. But you're right, its all in the ferrite. Its a special material, known for its non-reciprocal (anisotropic) properties when magnetized. If you decompose the RF field in the ferrite into a part rotating left around the magnetic axis and a part rotating to the right, those fields will have different propagation properties depending on the frequency and the magnetic field strength.
There are some expensive texts on Microwave devices that will go into great detail, but maybe some of the references in that wikipedia page would give you a good exposition.
coppercone2:
Ok I have a little trouble imagining the decomposition but from the sound of that behavior it sounds like there will be something like fringing happening.. is it like super imposing two magnetic paths in one location or something like that? (very confused).
I guess you have to solve it to some number of points to get an idea of how the wave is reconstructed or superimposed later or however its called for a practical explaination?
It would help to know about the ferrite disk manufacture. Is it like just a special mix where the crystalline structure (possibly with special cooling procedure/initial grain structure/pressing pressure requirements?) does the magic, or do they actually have some weird way of magnetizing it after the part is made in the kiln? When I hear magnetize I imagine something like how a magnetic part is manufactured, on a special high powered machine. I thought it might have a magnetic pattern on it like a hard drive platter the way that is phrased, but it also says it does not have a field of its own so I don't know.
What comes to mind is like a tesla-valve (the one for fluids) that does not recirculate the water back into the stream, but bleeds it off in a different direction (like if you put a buncha tiny pipes hooked up to a typical tesla water valve thing that have more flow directed to the outlets depending on flow direction. I guess dropping a big concrete slab that is at some particular angle to a flow of water in a river built on a giant tilting table would do the same thing, its easy to imagine it might work for some kind of material like grain in some kind of grain chute that tilts back and forth, or some kind of 'marble machine'.
It also makes me think of lens for some reason (the one that looks like a ferrite post inside of a wave guide Y)
Might it be easier to understand how the circulator functions in a wave guide rather then a stripline for learning the mechanics? The picture I see of a ferrite pillar in the middle of a Y waveguide piece makes some what more sense to me then a strip line based one.
T3sl4co1l:
Don't think about it in terms of point fields alone.
The bias is just that, bias. Ignore it for AC purposes. Its only effect is to bias the material, which causes Faraday rotation of propagating (EM) fields through it.
Then apply the EM field. Though there is a conductor carrying EM into the system, a substantial amount of the EM field propagates through the space around that conductor -- in particular, into the ferrite. The velocity factor within the ferrite is vastly reduced compared to air, so not much material is needed to achieve a 1/4 wave section, and sufficient rotation (Faraday effect) occurs in that span to reinforce one direction while interfering (canceling out) the opposite direction. And, through design tweaks and whatever, the bandwidth can be actually quite good, despite the dependency on wave interference.
The extra width of the conductor can also probably be explained as a means of keeping the impedance matched. Zo = sqrt(Lo/Co) and Lo is greatly increased (Co is increased modestly, as ferrites have notable dielectric constant too, just not to the extreme that permeability has), so a wider conductor can compensate for that.
The ferrite material itself may be NiZn or similar types, or YIG (yttrium iron garnet) which is even less magnetic (lower mu) than power (ZnMn) or radio (NiZn) ferrites, but much lower loss still, and I think the physical properties (paramagnetism and etc.) are more important (but I don't recall offhand if that's because EPR is tuned to the bandpass frequency in these, or if it's involved at all). So, they're quite efficient despite the high frequency.
Or at least, that's one possible explanation. It's been some years since I read about circulators. Information is not hard to find. Can be a bit technical though, being on the overlap of material physics and EM.
Tim
paul@yahrprobert.com:
I found this on the web: https://www.rf-ci.com/wp-content/uploads/2015/09/KB-001_Operating-Principle.pdf, The text on the first page talks about the counter-rotating fields.
You're right the actual device doesn't contain just pure left and right plane waves, but boundary conditions and fringing all occur and make the solution very messy. I'll bet they made these devices work by trial and error long before they could solve for the exact fields.
A similar device in the optical world employs faraday rotation. If you put a magnetic field along the axis of a long glass rod, then the left circularly polarized wave goes at a slightly different speed than the right polarized wave. If you put polarizers at either end of the rod and make the rotation difference be 90 degrees you can make an isolator.
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