DSL Filters

Last modified Thu 06 Apr 2006 10:37:38 AM EDT

You already know that you need filters to save your aDSL signal for your modem (and keep the aDSL signal out of your telephones). In most cases, the Gentek BB0001 filters available for a very low price from the NCF will serve you well. But, in "special cases" other measures might be required. For example, certain telephone models might not respond to "ordinary" filtering, or if you have many connected telephone devices (don't forget faxes, answering machines, call-display devices, dial-up modems et cetera), or two telephone lines, read on.

The following compendium presents gleanings from other members' experience, and might help you.

Alan Macdonald contributed a link on filters http://www.excelsus-tech.com/Downloads/Support/tip102.pdf which mostly discourages use of "symmetrical filters", and pointed out that their "Excelsus Z-BLOCKER® 200-series filters deliver significantly better return loss performance than symmetrical filters, especially when multiple filters are used at a single customer premise." FWIW this model is what Sympatico has supplied for wall-mounted telephones.

A symmetrical filter is one that looks the same electrically from either end, so it doesn't matter "which way around" you install it. "The perceived benefit of such a filter design is that it can eliminate potential installation errors."

A non-symmetrical filter will present a different impedance/load when looked at from either end. DSL phone-line filters are Low-Pass Filters (LPF), which means that they are designed to pass only the voice-band telephone signal to the telephone, while blocking the high-frequency DSL signal.

At its simplest, an LPF has an inductor on its input end (presents a high impedance to high-frequency signals), and a capacitor on its output (telephone instrument) end. The capacitor presents a low impedance to high frequencies, "shorting out" high frequency components that get past the input inductor. Installed properly, this assymetrical filter presents a high impedance to the DSL signal, and therefore doesn't "load down" (attenuate) the DSL signal available to the modem.

Are the Gentek BB0001 filters symmetrical, providing "the same input characteristics regardless of which port connects to the line and which port connects to the telephone equipment"? I don't know for sure, but the labelling of "line" on one end and "phone" on the other suggests they are not. Sympatico-supplied filters (two-line, from Corning) are similarly labelled.

The installation instructions suggest that the filter be plugged into the wall jack (via the pigtail with the connector on it), and the telephone device plugged into the jack on the filter itself. Suppose instead of at the wall end of the telephone wire, one plugs the telephone-end of the wire into the filter, and the pigtail into the telephone device. If the filter is symmetrical, it shouldn't materially change system performance (DSL and phone). But if the filter is not symmetrical, one "symptom" would be impaired DSL performance because the modem would see less of its signal, "shorted out" by the filter's capacitor.

This post also contains a summary of my notes when trying to learn more about Vista 350 phones, problems with them, and DSL filters. I didn't learn much about the Nortel Vista 350 but it appears to be available only as "refurbished" and a visit to Nortel's site refers one to http://www.aastra.com/ for analog telephones. No wiser.

The BB0001 filters I got via NCF (and other products) can be seen at http://www.gentek.com/cgi-bin/products.cgi?cat=879&sub=883 . Sparse information. Inspection of one of my filters confirms that the plastic housing bears the words "line" at the pigtail end, and "phone" at the jack end.

Len identified the "NID-01 ADSL2+ SPLITTER / Line Conditioner", seen at http://www.comtest.ca/NID.htm. Although sparse information again, it appears that just one of these devices, installed at the telephone demarcation point, will provide the necessary segregation between the aDSL and POTS (voice) signals, without any additional filters. However, a dedicated line to the aDSL modem is required.

Following Alan's information on Excelsus products led me to an interesting variation at http://www.excelsus-tech.com/index.cfm?fuseaction=products.Category&id=2.

-------------- "Dynamic" filters minimize capacitance created by five or more filters on the phone, by activating only when a phone is in use (capacitance degrades voice quality and Caller ID performance). Mix-n-match with up to five non-dynamic Z-BLOCKER® filters -- no need to replace your original filters.

-------------- And http://www.excelsus-tech.com/index.cfm?fuseaction=support.faqs answers a lot of questions. The one for "Why does Excelsus recommend the dynamic filter if I have more than five phones?" suggests that the problems with sidetone and noise arise from having more than five phones in the house.