[RFI] DSL Filter Update

Christopher E. Brown cbrown at woods.net
Sun Mar 8 16:05:38 PDT 2009

On Sat, 7 Mar 2009, Roger Parsons wrote:

> Just to advise the group that a couple of other stations have had success with this filter. Nick, VE3FJ, also checked out its frequency response with the following results:
> 1.85MHz   -26dB
> 3.65MHz   -46dB
> 7.24MHz   -68dB
> 14.13MHz  -60dB
> 21.0MHz   -56dB
> 28.5MHz   -54dB
> Of course in theory the stop band attenuation should continue to increase with frequency, but in practice there will be strays across the filter that will cause a performance drop at higher frequencies.
> 73 Roger

Keep in mind that ADSL2+ has signaling up to 2.2Mhz, and that the default 
S/N threshold to use a channel is normally 6 - 9 db (default varies 
between DSLAM vendor/model, providers tune based on cable plant.  Metro 
area providers with newer cabling and shorter runs can leave it around 6, 
older or lower density cable plants (longer average run length, more pairs 
feeding into single location) tend to need it tweaked higher).

3 - 6 db rolloff at 2.2Mhz or below *will* limit the DSL line max rate.

A filter with a sharp cutoff starting around 3Mhz combined with heavy 
common mode choke would have alot less impact on the sync rate.

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