Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[TobyOneKenobi]

Hi all,

I've been doing some reading on ADSL filters for part of my job and have a few questions that I'm hoping that someone can answer, hope I'm posting in the right forum:

As I understand it an ADSL filter is a low pass filter, filtering out the voice signal and routing it to the phone socket but leaving the original signal unmodified and passing it straight to the modem socket, I've come across filters that have caused low ADSL speeds and even blocked the ADSL or at least stopped the router from syncing, if the signal coming into the filter is unmodified and sent straight to the router socket how can this be happening?

If a filter is not fitted, usually the router will sync and the broadband will be ok until someone uses the phone, then the sync drops, again how does this happen when the filter is not affecting the ADSL frequencies?

Thanks
Chris

[tubaman]

If a filter is stopping the modem from syncing then it is probably faulty, likely down to bad capacitors. It's also true that not all filters are made equal, with some being of a very poor quality and not even wired correctly for UK circuits.
I'm not sure why the lack of a filter would cause a sync drop during phone use unless the phone is somehow putting a high frequency component back down the line, which of course a good filter would also stop from happening.
Here's an article about the insides of microfilters - https://www.adslnation.com/support/filters.php. It's somewhat biased towards the manufacturer that published it but it gives a good idea about the differing qualities of the filters that are out there.

[TobyOneKenobi]

Typical, after days of searching as soon I post, I find this

"An ADSL filter ensures that the impedance and capacitance seen by the modem
are constant. If you don't use one, the modem will almost certainly
re-train every time the phone goes on and off hook"

Even though the filter does not modify the modem output signal and is essentially straight through, it is still part of the circuit and changes in that circuit will yield different results at the outputs.