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71500Re: SoftRock filters and inductor variability

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  • Orin
    Jan 7, 2013
      --- In softrock40@yahoogroups.com, "Alan" wrote:
      > ----- Original Message -----
      > Subject: [softrock40] Re: SoftRock filters and inductor variability
      > >> I'm not sure there is a design flaw. It seems to me that some of the cores are just not of the nominal permeance resulting in
      > >> higher
      > >> inductance than intended.
      > >> If the components actually conform to the design then there would be no trouble.
      > >
      > >
      > > Perhaps when originally tested, that set of cores had lower A_L values than expected?
      > >
      > Orin,
      > The T37-6 has a specified AL_Value of 3.0 +/-5%
      > The 40/30/20m LPF was designed to be 0.8uH = 16 turns.
      > I tested two 37-6 cores with 16 turns and the lowest inductance I could achieve with the turns spread was 0.97uH and 0.98uH. I used
      > two completely different ways of measurement. A PIC meter and the old fashioned way with a 1% capacitor and dip meter and a RX to
      > confirm resonant frequency.
      > Working backwards this gives an AL_Value of 3.8. Only 14 turns are needed to get the corectly designed value.

      Right. I might not have been clear when I said 'originally tested', I meant that perhaps when Tony originally designed and tested the filters, his cores had low AL values.

      > >> The 15/12/10m build is hardly affected by this problem.
      > >
      > > Hmmm. See:
      > >
      > > https://picasaweb.google.com/lh/photo/peigIX9YYsJXR5W2VHPV0YLS1FToz1TiCJzHAUEj7Cg?feat=directlink
      > > vs:
      > > https://picasaweb.google.com/lh/photo/Ujjps0q183ySKsaP29mDm4LS1FToz1TiCJzHAUEj7Cg?feat=directlink
      > >
      > Sorry, I've only just got to terms with Elsie. I'm not quite sure what I am looking at, it's the 30/20/15 as built and with 2 turns
      > removed? Did you measure the inductors?

      Yes, the first scan is before tuning and the second is with two turns removed from the inductors. The mem traces in the second scan were imported from Elsie. BTW, if you haven't found it yet, try setting Stepsize to 5% from the button when displaying the plot, then hit the Monte Carlo button. This will show the expected limits in the frequency response of the filter with up to 5% component variation.

      I measured one of the inductors after removing it this weekend and with two turns removed, after tuning, it was 330nH. I may have disturbed it somewhat since that scan was taken. I got reasonable results after tuning for 10m, but the filter was inadequate for 15m.

      I don't know if I posted my results for 16 turns; I wound a 16 turn inductor this weekend just to see what happened. I got 790nH for the turns evenly spread and 825nH for 75% coverage. That inductor is no more as the Cauer design needed 240 and 320 nH inductors.

      > All I have checked with Elsie is the loss at the highest frequency, at 20m for the 40/30/20m version. Using the original design
      > values Elsie showed about -0.3dB but with my measured values about -5dB.
      > My interest is to find out why, maybe just some, of the 40/30/20s did not produce full power on 20m.
      > Paul pointed the way, I think it may be due to the cores.

      I would agree with it being due to the cores.

      > While those like you can play with different designs I was wondering what to tell people who have built a bad 40/30/20.
      > Do we tell them to rip out the LPF, get new capacitors and rebuild?
      > I think just removing a turn, or two at the most, from each coil could return the LPFs to near enough the design parameters.

      I agree - I would recommend winding evenly spaced and if power out at 20m is low, try removing one or two turns.

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