[RFI] DSL Filter Update

Jim Brown jim at audiosystemsgroup.com
Mon Mar 9 14:10:54 PDT 2009

On Mon, 9 Mar 2009 14:40:12 -0500 (CDT), Christopher E. Brown wrote:

>Depends, 75/80 are close and 160 is in the DSL range, but shorter 
>wavelengths are more likely for find an optimal length of line to couple 
>to.  The CPE gear is built to be light and cheap, easy enough to create a 
>case of frontend overload, or more likely just a jump in the noise floor 
>(as perceived by the CPE)

OK. One can be really brute force with a common mode choke if it doesn't 
add capacitance across the line. 

>Fair-Rite 2631540002.  Published chart shows about 500 Ohms at 1Mhz and a 
>peak of around 1900 Ohms at about 30Mhz with 3 turns.

You're looking at single turn data very different from what I see in the 
online catalog. It shows 70 ohms at 2 MHz, 130 ohms at 10 MHz, 220 ohms at 
30 MHz, and resonance at 120 MHz. Below resonance it's inductive, so it 
will resonate with the line at frequencies low enough for the line to be 
capacitive (less than a quarter wave). 

>I do not have a VNA or similar avail, so going by by SWAG, but 6 turns 
>should give a peak < 4Mhz.  I would expect at least 4K Ohms if not more, 
>and I am only dealing with 100w here.

I believe Fair-Rite's published data is trustworthy. Yes, 6 turns ought to 
get you a reasonable choke for 7 MHz, but you would probably want more 
turns for 4 MHz. Not an easy core for a lot of turns. 2631102002 is 1/2-
inch i.d, and would be easier to wind turns onto. You could probably get a 
reasonable choke using one pair out of CAT5. Have you seen my measured 
data for #31 2.4-inch o.d. toroids?  That's a far easier core to put a lot 
of turns onto. 



>I have seen signals coupled strongly enough onto the line to make it 5 - 
>10 thousand feet to the CO and still be strong enough to swamp the 
>receiver in the DSLAM.

In-band or out of band?  Looks like someone needs to do some work on 
assessing the most cost-effective way of suppressing stuff like this. 

Great input, Chris. Again, many thanks. 


Jim K9YC

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