[ARC5] Home brew passive CW filter

[Mike Feher]

Well, you would have to have some design parameters in mind first. Like passband width and ripple, transition band width, and ultimate stopband attenuation. Also, do you still want to go all passive? 73 – Mike 

 

Mike B. Feher, N4FS

89 Arnold Blvd.

Howell, NJ, 07731

732-886-5960 

 

From: J Mcvey [mailto:ac2eu at yahoo.com] 
Sent: Thursday, April 28, 2016 10:53 AM
To: Mike Feher; 'Dennis Monticelli'
Cc: 'ARC-5 Maillist'
Subject: Re: [ARC5] Home brew passive CW filter

 

anybody got specifics to build a multi-pole filter ? It would save me having to calculate it myself!  I don't think a real narrow filter would be practical on an ARC5 radio because the band spread isn't sufficient enough to tune it easily. .

 

On Thursday, April 28, 2016 8:46 AM, Mike Feher <n4fs at eozinc.com> wrote:

 

I know you want to sound smart and you did not say if that was loaded or unloaded Q. Regardless, he was talking about line transformers and finding a smaller one and that is what I responded to. Two toroids already means two poles, which was one of my suggestions. If it was for me I would just design a nice Elliptic filter for the task. 73 – Mike 

 

Mike B. Feher, N4FS

89 Arnold Blvd.

Howell, NJ, 07731

732-886-5960 

 

From: Dennis Monticelli [mailto:dennis.monticelli at gmail.com] 
Sent: Thursday, April 28, 2016 12:44 AM
To: Mike Feher
Cc: J Mcvey; ARC-5 Maillist
Subject: Re: [ARC5] Home brew passive CW filter

 

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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