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dsl filter measurement - RF Cafe Forums

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 Post subject: dsl filter measurement
Posted: Wed Jan 20, 2010 9:02 pm 
 
Colonel

Joined: Tue Apr 11, 2006 8:13 pm
Posts: 39
How do you measure the characteristics of a DSL filter? I would like to know how to transition from the RJ45 connector to a coax to look at it on a network analyzer. Is the network analyzer the right instrument to use? Is there any other way?

Karthik


 
   
 
 Post subject: Re: dsl filter measurement
Posted: Fri Jan 22, 2010 11:45 pm 
 
General
User avatar

Joined: Wed Feb 22, 2006 3:51 pm
Posts: 113
A vector network analyzer (VNA) with sufficient bandwidth to cover the frequency range of interest is certainly the best instrument to use. Not all S-parameter test sets will work down to about 4 kHz.

Second best is a frequency synthesizer and a gain/phase meter like the HP 3575A. These are sometimes available on e-bay. You'll have to do more work this way, but the equipment costs will certainly be less.

Do note that the DSL filter is unlikely to be a 50 Ohm device, however. (Your comment about "transition from the RJ45 connector to a coax" implies that you recognize that fact).

That means using either appropriate transformers to get to and from 50 Ohms, or minimum-loss pads (which of course will add attenuation). The transformers themselves will have some characteristics which you'll need to remove from your measurements. Recent model VNA's may be able to do that automatically.

What characteristics are particularly important to you?

Good Luck!


 
   
 
 Post subject: Re: dsl filter measurement
Posted: Fri Jan 22, 2010 11:59 pm 
 
Colonel

Joined: Tue Apr 11, 2006 8:13 pm
Posts: 39
Hi fred,

Thanks for the input. I am looking to characterize s-parameters. I am aware that, as you suggested, for a dsl splitter/filter, the impedances on the phone and dsl lines are different and not 50 ohm. I guess I have to design a balun to go from coax to the RJ-45 and the differing impedances.

Thanks for the input :)


 
   
 
 Post subject: Re: dsl filter measurement
Posted: Sat Jan 23, 2010 12:18 am 
 
General
User avatar

Joined: Wed Feb 22, 2006 3:51 pm
Posts: 113
If I remember correctly, the phone companies have used 600 Ohms for open-wire lines (such as older and rural systems often use), and 110-120 ohms for cables with a large number of pairs of wires.

Transformers are available with amazing bandwidths, if you spend enough money. Unless you have experience designing transformers (and at DSL frequencies, a balun is a transformer!), I'd suggest utilizing the skills of the specialist designers.

Good Luck!




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