shielded inductor -> low frequency?

Electrostatic (conductive) Shielding does not effect the magnetic lines of flux you need high u material for that. and high u magnetic shielding material only works at low < 100Kc frequency.

so placing a metal can around a coil is only partially effective in reducing radiated emissions.

Marc

marc

link to actual mixer site

BS benevolent Stuff for engineers

I believe what Michael meant is a closed versus open magnetic path. Closed path is what inductor company marketeers call "shielded". For example, a ferrite toroid would be quite good in that respect, that's what I usually use. I found that custom fab in Taiwan/China is actually less expensive than a fancy catalog inductor. Provided the quantities are at least a few thousand a month.

Xl = 2 * pi * f * L

Inductive reactince is equal to two pi times the frequency times the inductance.

Hope This Helps! Rich

You might want to look at toroidal inductors - these tend to have less external magnetic field that solenoidal inductors.

Chris

One of these times I'm going to spend a week doing some measurements and demonstrate how effective copper shielding can really be against magnetic pickup. People are always focusing on the skin depth argument (how the surface field penetrates the bulk) and not the shorted-turn argument (how the presence of the metal reduces the surface field).

An enormous amount of misinformation gets repeated over and over on this topic.

I have 4 weeks holiday left this year--maybe I'll do it over Christmas.

Cheers,

Phil Hobbs

Inductance is proportional to the relative permeability of the core, which is relatively constant at low frequencies then starts to decrease at higher frequencies. The exact relationship depends upon the particular core material.

As they cite an inductance at 7.96MHz, it's likely that the inductance is relatively constant from DC up to that frequency, including your 2.2MHz. There wouldn't be much point in specifying the value at a frequency where the graph is no longer relatively flat.

these "shielded" bobbin cores have a ring of ferrite material placed around them, spaced out a mm or two from the OD of the bobbin flanges.

it reduces external field leakage from an awful lot to a fair bit. that might suffice.

Joerg's approach (toroidal cores eg koolmu, -52 etc) is by far the best.

Cheers Terry

They seem to be nearly pot-core in design, as I see them.

What do you think of page 52 of Linear's AN70?

there are some pot-core SMT inductors. I've seen some nice ones, with spiral strip windings, ludicrously low Rdc (eg 1mOhm), for VRMs etc.

the Sumida style shielded inductors are bobbin cores with a ring, as are the ABC ones. sometimes the "rings" are qctually square. And the gap between the bobbin inductor and the ring is invariably filled with goop.

it kinda shows how not to wind a toroid. bifilar, thats the go.

very interesting though. and of course low-perm cores (koolmu etc) have plenty of flux leakage too.

Bruce Carsten has done some nice papers showing the sorts of field leakage you can get from a toroid.

Cheers Terry

FAR OUT! Most of us will be delighted to hear the results (aka facts).

But it can also be the most expensive, especially if quantities are low and you can't find a catalog part.

Man, that prototype on page 37 sure looks ugly. The generator on page 46 is worthwhile peeking at. I wonder how they kept it so fresh. I've got one from the 50's but it looks way more tired.

Page 52: H-field EMI can be dealt with (to some extent) with crossover turns but it is an art to get these just right for best cancellation. Almost like adjusting microwave stuff. It is even tougher to then explain all that in a fabrication procedure. T'is where even us electrical guys need to whip up a mechanical CAD drawing.

BTW since it's often said that shielding doesn't work I have to disagree. Even a Faraday type shield has saved the bacon many, many times in my EMI work. And EMI is >30% of my bread and butter.

ARE YO MENAING THAT WINFDINGING THE COIL ON A TORRUS, CNTAINS THE STRAY MAGNETCI FEILD FLINES? IF SO .yes THIS WOULD BE A GREAT WAY T GO AND THEY HAVE STEP UP AND DOWN TRANSFORMERS TTHAT ARE WOUND ON A CLOSED FORM, IE TORRUS.. BIT EXPENSIVE AS T IS HARDER TO WIND!

mhp

I have some Coiltronics DRQ coils. I was going to break one to see how well the gap is closed. It looks pretty tight, but I'm not sure.

The DRQ's have filler in some parts. I've also wondered if the "goop" is impregnated with ferrite particles.

I have to build that probe he described in AN70. I must!

Well, yes, that's why those are usually outsourced. At large qties you can get that done for well under 50c for a half-incher with a few dozen turns including materials and shipping.

Right. Actually, it takes no more then a scholar math to estimate the attenuation of a static field by a conductive shield if the geometry is enough simple.

:-))) As always. SED is not a source of information - it is a place for leisure.

Good luck.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

I have a few dumb questions about that probe described in AN70. Refer to page 67; what are the wires "to signal generator" for? There does not seem to be any reference to such foolishments any where else. In other words, doesn't the coil connect *directly* to the resistor / coax? On another note, it seems that the ID of the plastic tube may be non-critical. Perhaps ceramic, glass, or even bakelite could be used. If there is an optimum capacitive coupling, then wall thickness VS material should be specified, and if dielectric loss is also an issue, then types of plastics should be mentioned ("optimim", "useable", "poor", "too lossy", etc). Not a lot of detail...

Robert, that dwg on page 67 is not the Sniffer Probe, it is a test coil to test the Sniffer Probe. Put the SP nose into the centre of that 20-turn coil, turn up the signal generator, the scope across the 12.4 ohm resistor measures the current injected into the test coil, etc.......

A rough sketch of the Sniffer Probe construction is given on page 55 and the address to buy it from on page 54.

Thanks!!