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Monday, February 17, 2014

BITX 17 Build Update: More Filter Maintenance

Having had great success in straightening out the 11 MHz filter in my BITX 2040 (scroll down for details),  today I decided to see what I could do with the filter in my 17 meter BITX (four 5 MHz crystals in Cohn MIN LOSS configuration, with 40 pf caps all around).  The key to my success in all this has been the filter programs that came with EMRFD.  After characterizing my crystals with the G3UUR method, I plugged the values into LADBUILD 8, then took a look at the expected results.  As you can see from the image above the predictions were not pretty.  Yuck.  Lots of ripple and lots of insertion loss.

I went into the rig and using my DDS sig generator and my RIGOL 'scope, measured actual performance.  It looked worse than the prediction (part of the worsening is a difference in vertical scale):

LADBUILD lets you play around with the values of the components in the filters.  I know that ripple is usually related to an impedance mismatch.  So in LADBUILD I experimented (virtually) with different impedance values at the end.   I noticed that at about 1000 ohms, the ripple and insertion loss got better:

So I went and built two broadband toroidal transformers.   4 turns primary with 12 turns secondary (1:9 Z).  I'm assuming that the BITX has around 150 ohms at either end of the filter.  That would put about 1350 ohms at the ends of the filter.

Here are the results:

Much better.

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  1. You should get really creative since you're playing with the software.

    What about a crystal from one end of the filter to ground? Done right, that could put a notch at one side of the curve, making for getter sideband rejection on that side. At the very least, in the old days it wasn't uncommon to have a crystal to ground, notch out the carrier. One phasing rig did that, I can't remember which, a good trick in the days when balanced modulators sometimes had a tendency to unbalance.

    Or some people play with crystals in parallel, certainly done in the days of crystal lattice filters, but I think I've seen some ladder filters done that way. I'm not sure anyone has given it thought, but charting it might say weather it has value or not.


  2. Tweaking the filter response...
    I'm impressed!

    Bill N5AB


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