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Thursday, 30 January 2014

Minima - Remaking the 15MHz LPF


Now that I have some life in my Minima, I need to remake the 15MHz Low Pass Filer. You may remember the sweep I did originally:

There is a bit too much attenuation at the 14-15MHz frequency range - this will result in a very deaf radio on the 20M band!

So I used the QUCS tool to design an 11th Order filter with a cut off at 15MHz, I then changed the capacitor values to "real world" numbers and ended up with this:

I then made the filter and after winding each toroid I measured the value and carefully spaced the turns to get as close to the required value as I could.

The physical construction looks like this - all the toroids are T50-6s:

and when I sweep the filter on the Spectrum Analyser I see this:

So this looks much more like what we need - I will use this and replace the one in the radio right now.

Good, egh?


  1. I see the schematic has the higher value inductors in the 3 center positions but the picture of the filter shows the 13 turn toroids on the right 2 positions and the second from the left- did the values just work out that way or can you elaborate. I would have thought that the 13 turn cores would have all been in the center positions.

    73, Gord

  2. Very interesting observation! I guess it has to do with the winding spacings; I measured each one before I used it - the filter scans are of the filter you see photographed.