How to Pick a PC mount CMC?

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- amdx
  
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posted
5 years ago

Thu, Jun 21, 2018 4:04 PM

I need some PC mount CMC chokes to prevent common mode signals on antenna lead ins and on power supply connections. The signals I want to prevent are AM band signals, 550kHz to 1700Khz. Any reason they can't be the same model choke? PS current is well under 100ma and RF is tiny. About the only ratings I see are how many milliHenries and DC ohms. How do I pick a CMC for my frequencies of interest? Just a lot of inductance?

Mikek Extra details for Jeff. Four antenna phased system. Amp at each antenna. Want to choke any CM signal on PS. Want to choke CM signal on signal feed lines going to phasing boxes. Choke anyplace else I can think of. Signal ingress degrades desired pattern. This is for receive only.

- J
- Jeff Liebermann
  
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posted
5 years ago

Thu, Jun 21, 2018 4:30 PM

Fast half answer: Type 31 ferrite for BCB. Click on the "suppression materials" tab. Note the permeability vs freq graph.

This might be useful for identifying ferrites from your junk box:

Gotta run...

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

- T
- Tim Williams
  
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posted
5 years ago

Fri, Jun 22, 2018 12:44 AM

If you already have cables made, you may find it easier to loop a dozen turns around a high permeability ferrite toroid/bead, rather than putting components in.

How much CMRR do you need? What is the differential mode impedance it's working against? How unbalanced is it?

Most CMCs have an impedance/attenuation peak in that range so it won't be hard to select among them.

There are basically two kinds: power and data line chokes. The former have lots of isolation, and intentionally poor leakage inductance, while the latter are typically bifilar wound and exhibit modest transmission line behavior (namely, low differential insertion loss, even at frequencies well above the CM peak).

Tim

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