[ARC5] Home brew passive CW filter

[Bruce Long]

XL for 8.5 H at 700 Hz is +j37,400 Ohms
Assiming parallel resonant the formula for Q is XL/R
For R =7500 Ohms  

Q = 37400/7500 =5
The measured Q was about 2.6 which means the inductor losses correspond to an additional resistive load of about 7500
You can reduce the -3db bandwidth by increasing the load resistance but increasing the load resistance to infinity will only halve the resonant bandwidth.
This means your line transformer makes a really crummy inductor at 700 Hz.  I understand the pleasure and financial advantage of using junk box parts but it will be somewhere between hard and impossible to get much better performance than what you have already with the line transformer inductor.
You would get higher filter Q and reduced bandwidth if you used your line transformer inductor in a series resonant circuit with a low  (ie RL << 7500 Ohm) resistance load but your insertion loss will increase in proportion to the decrease in bandwidth.


      From: J Mcvey via ARC5 <arc5 at mailman.qth.net>
 To: "ARC5 at mailman.qth.net" <ARC5 at mailman.qth.net> 
 Sent: Wednesday, April 27, 2016 11:13 PM
 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 1.85Voutput 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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