[ARC5] Home brew passive CW filter

J Mcvey ac2eu at yahoo.com
Wed Apr 27 23:13:39 EDT 2016


Someone suggested making a filter using a 70V line transformer. I had been looking in the junk box for something with high inductance, so I tested the primary side of a line transformer using the common and 0.62W taps. Sure enough,  it was useable at 8.5 H  To resonate that at 700Hz , it needs .006 mf, I used a .0047 in parallel with a .001 mf.That got me a measured resonance of 725 Hz on the first try! Sometimes theory works!
The test load was 7500 ohms:input 1.85Voutput at resonance was 1 v 
Taking the 1 v max, I found -3 db points at 600Hz and 880Hz yielding  a BW of 280Hz  which is a Q of about 2.6.
It works pretty well, but I am going to try to find a physically smaller transformer to try to  reduce the insertion loss.  

Haven't tried it on a real radio or signal yet, but looks promising.

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