[ARC5] Home brew passive CW filter

[Dennis Monticelli]

You should look into using a toroid for both size and Q reasons.  Remember
the 88mH telephone line equalizer toroids that were the rage back in the
60's?   You want a Q approximately double what you got.....i.e. about 5.

Dennis AE6C

On Wed, Apr 27, 2016 at 8:13 PM, J Mcvey via ARC5 <arc5 at mailman.qth.net>
wrote:

> 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 1.85V
> output 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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