[ARC5] Home brew passive CW filter

[Dennis Monticelli]

FYI.  I have a homebrew CW receiver that uses two small toroids (approx
5/8") and fine wire.  The Q for each filter is 5 at 700Hz.  Q is a function
of wire resistance, the permeability of the core material and the loss
factor of the core.  Small size can be good or bad depending upon these
factors.

Dennis AE6C

On Wed, Apr 27, 2016 at 9:22 PM, Mike Feher <n4fs at eozinc.com> wrote:

> For the same inductance a smaller transformer will have a higher R
> reducing the Q. You will need more poles and if possible some zeros to make
> a decent filter. 73 – Mike
>
>
>
> Mike B. Feher, N4FS
>
> 89 Arnold Blvd.
>
> Howell, NJ, 07731
>
> 732-886-5960
>
>
>
> *From:* ARC5 [mailto:arc5-bounces at mailman.qth.net] *On Behalf Of *J Mcvey
> via ARC5
> *Sent:* Wednesday, April 27, 2016 11:14 PM
> *To:* ARC5 at mailman.qth.net
> *Subject:* [ARC5] Home brew passive CW filter
>
>
>
> 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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