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Author Topic: RF lowpass filters and inductor orientation  (Read 6274 times)

Offline redhat

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RF lowpass filters and inductor orientation
« on: February 21, 2016, 2002 UTC »
I've been playing with filter design lately in the process of making PCB's for a few prototype transmitters and the old debate is raging.  This is what I found.

Among hams there is a notion that toroids are self-shielding, that is the magnetic flux of the winding is contained to within the core and thus very little outside influence can be expected.  With air-wound inductors, this is not true, and every pole in a filter is usually designed and build with adjacent coils at right angles to each other.

This lack of interaction in toroids is somewhat true I found, but measurable interaction does occur.

I built a 7 element MW lowpass filter of shunt type Chebyshev design with 0.1dB ripple and a cutoff frequency of 1.9 MHz.  In this design, all inductors were built  broadside to each other to save space, cores were T200-2 spaced about 0.6"

Anticipated rejection (Fc = 1720 KHz)
Fc -0.5dB
2Fc -50dB
3Fc -76dB

Measured rejection
Fc -1.9dB
2Fc -51dB
3Fc -77dB

I then changed the winding direction of the middle inductor and measured again.

Measured rejection
Fc -0.26
2Fc -51
3Fc -76

I found it interesting that the rejection of the harmonics was little affected, but reversing the winding direction had a large effect on the passband loss.  With the inductors wound the same way,  passband power loss was around 35%, meaning 100W in, 65W out!  With the inductor wound the other way, losses were reduced to just 6%.

All measurements were done with a HP signal generator and spectrum analyzer.  First, reference measurements were made with the generator set at 10dBm on each frequency, then the filter measured, and the loss calculated.

+-RH
« Last Edit: February 21, 2016, 2011 UTC by redhat »
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Offline ff

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Re: RF lowpass filters and inductor orientation
« Reply #1 on: February 21, 2016, 2126 UTC »
Quite unexpected, Redhat.  It never occurred to me that the winding direction would affect LPF operation.  I have occasionally lashed one together and had a very disappointing amount of insertion loss for no apparent reason.  Thanks for the heads up about it.  Would it also be safe to assume that the interactions would increase as power through the LPF increases?
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Offline redhat

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Re: RF lowpass filters and inductor orientation
« Reply #2 on: February 21, 2016, 2144 UTC »
No, insertion loss would remain at the same ratio regardless of power level.
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Offline SV1XV

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Re: RF lowpass filters and inductor orientation
« Reply #3 on: February 23, 2016, 0455 UTC »
cores were T200-2 spaced about 0.6"
Your observations are reasonable: three 2 in cores, 0.6 in apart, with low μ material. It is reasonable to expect some coupling between windings under these conditions as not all magnetic flux is contained in the toroids. Coupling would be less if the inductors were of smaller size, further apart and made of higher μ material.

Offline redhat

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Re: RF lowpass filters and inductor orientation
« Reply #4 on: February 26, 2016, 0114 UTC »
Smaller sized cores unfortunately could not handle the power excursions that these filters will have to reliably endure.  This is further complicated by the relatively low inductance values which when coupled with higher mu core types leads to higher losses through core saturation.  The only real way to reduce coupling is to play games with inductor phasing, spacing, or throw in the towel and go with air inductors.

+-RH
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Offline moof

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Re: RF lowpass filters and inductor orientation
« Reply #5 on: February 26, 2016, 0134 UTC »
My brain can't process this.  Do you mean if you have a 10 turn toroid and you insert the wire sticking it through the top, pulling it around, sticking through the top again until you have 10 turns (like I always do), it will behave different if you do it mirror image pulling it through the bottom (and the legs you solder to the board will be opposite sides of the toroid in the first scenario) ?  I can barely create that thought.  Or are you talking about orientation parallel toroids vs one at a right angle to the others?  My head can accept that better.  Oh my god. Help......

Offline redhat

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Re: RF lowpass filters and inductor orientation
« Reply #6 on: February 26, 2016, 0751 UTC »
A little of both.  I'll try and post a pic or two when I get some time.
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Offline redhat

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Re: RF lowpass filters and inductor orientation
« Reply #7 on: February 27, 2016, 2002 UTC »
The basic idea is to wind all the inductors the same direction, but reverse the input and output on every other pole.  So in essence, the magnetic flux is flowing in the reverse direction than it would be for the normal orientation.  This is accomplished by merely connecting the starting point of the wound toroid to the output of the pole rather than its input.
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