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Author Topic: Intermodulation in iron cored inductors?  (Read 4815 times)
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Ian VK3KRI
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« on: October 03, 2005, 07:47:28 AM »

Heres a question for the iron cored inductor Gurus out there:

If I have a Pi configuration low pass filter in series with the B+ to my plate/drain/collector modulated class C/E/D final stage, there is a DC component on the winding of the inductor. The standing DC current changes the inductance of the inductor slightly. My first thought is that if I calculate or measure the inductance change and recalculate the filter using that inductance every thing should work as long as the DC current stays at that level.
 
It then occured to me, that if I apply 100% modulation at a low enough frequency I may be varying the inducatance of the inductor due to a reversing  'almost' DC bias field. If this does vary the inductance at this low frequency rate will this not cause intermodulation distortion of a higher AC frequency that happens to be in the roll off of the filter?

Does this actually happen? If it does, is there a way to calculate what the effects will be based on inducatnce, the variation of permiability with applied magnetic field etc.         

An air core is the perfect answer to the problem, but I'm trying squeeze the size down so a cored inductor seems the way to go.

I though I might find some reference to this problem in ferrite cores used in speaker crossovers. Theres no DC there, but potentially quite low frequencies at high levels. Unfortunately I get distracted by 'high end audio' sites advertising 'saturationless' laminated ironcore inductors wound on wood formers - becuse the plastic ones cause too much 'sound degredation'!

                                        Ian VK3KRI
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WA1GFZ
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« Reply #1 on: October 03, 2005, 09:48:09 AM »

Ian,
Check out Fair-Rite Co. applications in their catalog. There are formulas to determine the effects of permability with DC bias. The inductance will change with DC component. Yes you can hear the difference. I have played with iron core PDM filters and found Air core sound better. The lower the initial permability the less
the effects of dc bias.
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Ian VK3KRI
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« Reply #2 on: October 05, 2005, 07:32:11 AM »

Thanks for the pointer  to the Fair-Rite article I'll dig out some of the old books withh references to Hanna curves and read them a few times and hoepfully some will sink in.
 I was contempating using Amidon 26 Poweded Iron material toroids for a PDM LPF and there is a chart showing  intial pemiability vs B in the old copy of the catalog I have . I figure I can design around the possible inductance change with current as long as its stable although being a low power ~25watts the current is relatively low anyway. I'm trying to get small for a portable rig. Winding on ferrite rod may be another possibility but interction may be a problem if thetre too close.

When you say the air cores sounded better was it distorion or wierd frequency response or some other audio oddity?

                                Ian VK3KRI 

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WA1GFZ
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« Reply #3 on: October 05, 2005, 11:41:03 AM »

Ian,
i have Ui=60 cores in my Viking 2 that work pretty well. I tried 3C8 gapped pot cores and they were very unstable.
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N9NEO
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« Reply #4 on: October 07, 2005, 09:06:08 PM »

Hi Ian,

I have used the red and white mix from  www.cwsbytemark.com

They are low perm materials and as Frank said, stable with flux and temperature.  25w should be easy to do.  Lean towards the RF parts with small Ur because maybe the power supply types of stuff may wander too much.    I used a stack of 2 T130-15 parts for  300uh at 8 amp for my modulator.  If you are using a multi pole chebyshev or similar then you know that the inductance will probably have to be more accurate.  Frank and Todd helped me out with suggestion of a series trap on the modulator output which  really clobberd the switching noise.

One way to test inductor is to place a steady voltage across part and watch current rise.  Rise should look like straight line all the way until it curves up when it actually saturates.  Other way which may be even better is to run a dc current into the inductor and have a tertiary winding that you can measure the inductance with a small ac signal - In effect generating your own Hanna curves.

73
Bob
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Ian VK3KRI
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« Reply #5 on: October 08, 2005, 08:45:52 AM »

Bob,
   I had assumed that using a #15 material would have required too mamy turns and definately caused saturation / permiability problems, but if youre using 2 x T130-15 for 300uH @ 8A thats got to be about 70+ turns . I might try the T130-15 , if only beacuse I have about a dozen of them I picked up for cheap. I think I used a couple in the output of my 160M rig (at RF).

Ive had a play with some fiter values in SwitcherCAD and the inductance doesn't seem too crtical. I can live with a few dB ripple around cutoff as long as the switching freq stays well down.

I have actually tried to  DC bias windings on cores while reasuring the inducatnce , but I needed a hefty series choke with the power supply to stop it acting as a short on a winding.  The best I managed was  setup with 2 inductors in series for DC but parrallel for AC.   I might try that again .
                                                                     Ian VK3KRI
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