[ARC5] Home brew passive CW filter

[Mike Feher]

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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