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71497Re: [softrock40] Re: SoftRock filters and inductor variability

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  • Alan
    Jan 7, 2013
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      ----- 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?


      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.

      >> 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?

      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.

      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 repeated this for the 30/20/17m version, again T37-6 cores. -2dB at 18MHz with the bad core value. Not so noticeable but one turn
      removed would be better.
      Elsie showed the 15/12/10 version to be only slightly affected by the high AL_Value.

      73 Alan G4ZFQ

      > I have replaced the filter with a Cauer design, but have yet to scan it on the VNWA. My 15m 2nd harmonic is now at -40dB and I
      > have (according the the spectrum analyzer at least), 1W output on 10m. I wouldn't recommend this change unless you have a way of
      > accurately measuring small value capacitors and can set the resonant frequency of the parallel coils/capacitors by tweaking one or
      > the other. Not too bad with the VNWA, I could put the toroid and capacitor in parallel in my test fixture.
      > Values for the filter came straight from SVC Filter Designer from Tonne Software (same source as Elsie): 5th order, 30MHz
      > bandwidth, C-input lowpass, Cauer, 0.044 dB Passband ripple, 40dB Stopband depth. This gives over 26dB 'attenuation' of the 15m
      > second harmonic. 3dB to go...
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