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From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
To: <rsgb_lf_group@blacksheep.org>
References: <000701c933bd$4ef22f60$4201a8c0@home> <1224687077.48ff3de587a50@imp.netikka.net>
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Date: Wed, 22 Oct 2008 17:09:03 +0100
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Subject: Re: LF: Ferrites - why low frequency limit?
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Paul,

.. I had a phone conversation  with a applications engineer over the right 
ferrite to  use  a while ago ..... only the one call .... he advised me to 
buy the amp ready made, to  save the development .... at least he was honest 
hihi

G ...

--------------------------------------------------
From: "Paul-Henrik" <phl@netikka.fi>
Sent: Wednesday, October 22, 2008 3:51 PM
To: <rsgb_lf_group@blacksheep.org>
Subject: Re: LF: Ferrites - why low frequency limit?

>
> Thank you Jim for this most interesting brief explanation!
>
> I say brief, because I have developed a vague feeling over the years for 
> how
> long it would have to be to cover every aspect we face just in the amateur
> field... And I'm still a green novice when it comes to ferrite/powder iron
> cores.
>
> Paul-Henrik / OH1LSQ
>
>
> Quoting James Moritz <james.moritz@btopenworld.com>:
>
>> Dear John, LF Group,
>>
>> There is no strict limit on the frequency range of a particular type of
>> ferrite, rather there is an optimum frequency range depending on the
>> application.
>>
>> All ferrites have losses that increase with frequency. At low frequency 
>> this
>> is mainly due to magnetic hysteresis in the core, which results in a loss
>> that rises with operating frequency at a given level of magnetic flux, 
>> and
>> at high frequencies things like eddy currents and dielectric losses 
>> increase
>> in significance too. The lower permeability ferrite materials tend to 
>> have
>> lower overall loss in the magnetic core material in the LF/MF/HF range, 
>> but
>> require more turns of wire to achieve a given inductance, or a particular
>> maximum level of flux in the core, resulting in higher losses in the
>> windings. So there is a trade-off, favouring high permeability materials 
>> at
>> low frequency where the loss due to hysteresis is relatively low and the
>> smaller number of turns needed is a benefit, and low permeability 
>> materials
>> at higher frequency where fewer turns are required.
>>
>> Whether a material is suitable at a particular frequency depends a lot on
>> what it is being used for. If one looks at the impedance of a particular
>> winding, the core losses result in a resistive component that increases 
>> with
>> frequency, and an inductance that is constant at low frequencies, but
>> decreases rapidly at high frequencies. At very high frequencies, the
>> resistive component may also reduce. So there comes a crossover point 
>> when
>> the coil impedance becomes mostly resistive, and at higer frequencies 
>> still
>> the overall impedance of the coil actually reduces (this is ignoring the
>> effect of stray capacitance, which will also cause the impedance to 
>> reduce
>> at frequencies above resonance). The crossover tends to occur at higher
>> frequencies for lower permeability materials. For a signal transformer, 
>> one
>> does not usually care too much about the resistive component, provided 
>> the
>> overall winding impedance is high, which favours high permeability cores.
>> For a high Q coil in a tuned circuit, one wants to minimise the resistive
>> component as much as possible, which tends to favour low permeability 
>> cores.
>> For noise supression, one wants to maintain a large impedance over a wide
>> frequency range, and a resistive impedance is actually quite useful in
>> damping out resonances. Here the upper limit is where the overall 
>> impedance
>> starts to decrease. For power applications such as SMPSUs and 
>> transmitters,
>> the trade-off becomes more complicated, because one also must consider 
>> flux
>> density, temperature rise, size and cost of the core, the effect of a DC
>> bias current, etc.
>>
>> So the reccomended frequency range of a ferrite material is really rather 
>> a
>> vague notion. Nothing terrible happens at low frequencies, but the 
>> windings
>> tend to get unmanageably large. At high frequencies, there comes a point
>> where the losses are too high for the circuit requirements. But in both
>> cases, the frequency limits will depend a lot on what the core is being 
>> used
>> for.
>>
>> Cheers, Jim Moritz
>> 73 de M0BMU
>>
>>
>>
>
>
>
>
>
>



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