Ferrite Core Inductor Software Update
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Please check all tag descriptions before choosing one.Click a tag name below to view just the posts in that categorySPICE, CAD, EDA software etc.Physically implementing a design (e.g. PCBs, connectors).Help with circuit design: Component use & values, implementing reference designs, how-tos etc. I dabbled in electronics when I was a teen, and I'm trying to get back into it now as an adult. I've been looking to build some RF projects, so there are lots of inductors in the schematics I'm seeing. Many (maybe even most) of the hand-wound inductors are described as being wound on toroidal cores, even though the inductance values are low enough for small air-core inductors to be practical. Being both cheap and impatient, I'd rather go with home-brew air-core inductors over ordering a bunch of different toroids every time I want to start a new project.Is there a performance reason to favor ferrite-core inductors over air-core in RF circuits if the L-values are the same?Is it just a matter of the former being easier to reproduce consistently?Is it a matter of supporting the coils for durability?Is there something else entirely that I'm missing for the reasoning behind all these toroids?. The main advantage of a torroidal core is that it keep the magnetic flux contained within the core.
The means that currents in other parts of the circuit won't couple into the inductor as much and likewise currents in the inductor won't couple into other parts of the circuit.For example if an air cored inductor is used in an oscillator the frequency will drift with placement of components around it (and also, say, with how close your hands are).If the inductor is part of a high order filter then stop band attenuation will suffer with air cored inductors as they will couple with each other. Some stop band current in the first inductor can leak into the final inductor.
Epcos Ferrites
Note that this is only an issue if you want, say, 60dB isolation. If you want 100dB stop band attenuation then torroidal inductors with shielding between filter stages would be needed.TL;DR go ahead and try air cored inductors but if the circuit doesn't operate as desired (oscillator drift, poor filter attenuation, noise pickup from nearby electronics etc) then torroidal (or other shielded geometries) inductors will likely improve things.NB: The only disadvantage of ferrite cored inductors is that if the core saturates the inductor becomes strongly non-linear. This can introduce distortion (harmonics or IMD) as it causes the inductance value to change with instantaneous inductor current. This is typically only an issue in RF circuits for high power filters (ie: after a 100W power amp). For a receive only circuit you'll never hit core saturation.Edit: Forgot to mention that ferrite cored inductors are much smaller than their air cored equivalents. Not an issue if you're making a feedpoint mounted matching circuit for a HF antenna but is a big problem if you're making a small portable radio. Thanks, lecturing electrical engineering is my day job.When using ferrite note something I forgot to mention: operation frequency.
There are many types of ferrite material which have different magnetic properties and intended frequency ranges. Generally speaking a material with lower permiability is intended for higher frequency operation. As frequency increases all materials experience a drop of permiability at some point. It might be 100kHz, might be 100MHz, you just need to get the same ferrite core specified in the design being followed to get the expected inductance.Air cores, however, don't suffer permiability loss. You only have to worry about a coil's parasitic capacitance (and resultant self-resonant frequency) and be sure to operate the coil at a low enough frequency such that parasitic capacitance can be assumed zero. Online coil calculators will provide enough data to estimate or calculate high frequency behaviour of a given geometry.
All core types have advantages and disadvantages.Air cores are the best for high frequency applications, but they become excessively large as inductance and current increases.Essentially, air has an excellent frequency response but a poor ability to store magnetic energy.Ferrite has a reasonably high frequency response (not as high as air) but it also has a significantly better ability to store energy.At some point, air core inductors become too large. Likewise, at some point the frequency response of ferrite becomes unacceptable.I can only assume that ferrite might be easier to mass produce inductors on than an air core.
So it may simply be a cost of production issue.Air core and ferrite cores have a sizeable overlap in the range they are acceptable in. So the difference might be more associated with cost and profit than their electronic qualities. Is there a performance reason to favor ferrite-core inductors over air-core in RF circuits if the L-values are the same?As you note, generally not but:.
Ferrite Core Inductor Calculator
Air cores radiate and pick up more (because the flux is less confined). A toroidal core is really good at not coupling to another toroidal core right next to it. Because you need more wire, air cores can have higher series resistance. Air cores do not dissipate energy.
In some filtering applications, you want that loss, and there are special 'soft ferrites' which make the inductor resistive at high frequencies. (On the other hand, a shunt inductor in a high-pass, or a crossover application where you want both series and shunt outputs, wants to he a 'hard' ferrite.)I suspect that if there's a reason other than familiarity on the part of the author, it's the first one.