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Author Topic: ADSL "microfilters" - any info on their circuit operation?  (Read 3079 times)

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Offline rolandpenplotter

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ADSL "microfilters" - any info on their circuit operation?
« on: January 22, 2014, 03:03:32 PM »
Type in "how does a microfilter work" into Google, and something as inane & unhelpful as "it splits the signal into voice and data" is likely to be your first hit.

Does anyone have any extended information, or sources of information, for the circuit theory of aforementioned devices?

Many thanks; for now, I am at a loss.

Matt. :)
 

Online ConKbot

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Re: ADSL "microfilters" - any info on their circuit operation?
« Reply #1 on: January 22, 2014, 03:14:58 PM »
Its a passive low pass filter. The ADSL signal has upto a few MHz of bandwidth, the LPF probably cuts off everything above 15khz and spits out the result on the voice jack.  The unadulterated signal is on the data jack to go to the ADSL modem.
 

Offline rolandpenplotter

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Re: ADSL "microfilters" - any info on their circuit operation?
« Reply #2 on: January 22, 2014, 04:45:33 PM »
Its a passive low pass filter. The ADSL signal has upto a few MHz of bandwidth, the LPF probably cuts off everything above 15khz and spits out the result on the voice jack.  The unadulterated signal is on the data jack to go to the ADSL modem.

Thank you :)

Maybe someone (me) will have to do a video on this for my YouTube channel :)
 

Online AG6QR

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Re: ADSL "microfilters" - any info on their circuit operation?
« Reply #3 on: January 22, 2014, 04:52:29 PM »
ConKbot is right, it's typically a passive low pass filter.  More details, along with a fairly typical schematic, are here: http://www.numericana.com/answer/filter.htm#dsl

The filter presents a very high impedance at the frequencies above the voice band.  Without a filter, those frequencies would go into the telephones and other gadgets on the phone lines, and some of these devices (and perhaps even the the wiring itself) might present a somewhat low impedance path at those high frequencies.  The low impedance would absorb some of the ADSL signals.  Furthermore, some of the ADSL frequencies are within the range of human hearing, and can present an annoying whine to the ears of the telephone user if they reach the phone.

The filter keeps those high frequencies away from the voice devices, which is a good thing both for the voice devices and for the ADSL equipment.

When an ADSL filter has two ports, one for data and one for voice, the "data" port is normally just the unfiltered phone line, straight through.  The ADSL modem has a high pass filter built in to it, so there's no need for an external filter between the phone office equipment and the ADSL modem.
 

Offline ve7xen

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Re: ADSL "microfilters" - any info on their circuit operation?
« Reply #4 on: January 22, 2014, 06:13:29 PM »
Here's a datasheet with schematic (no values) for a typical "POTS Splitter' type filter:

http://rf.cdiweb.com/datasheets/pulse/B893.pdf
73 de VE7XEN
 

Offline HiTech

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Re: ADSL "microfilters" - any info on their circuit operation?
« Reply #5 on: January 23, 2014, 03:35:17 AM »
The benefit of using a "whole house" POTS filter is important to that of installing those stupif filter dongles at every phone jack in a home or small business. A dedicated POTS filter (a brand like SECOR) will have a port for the DSL and another for VOICE. The DSL port can accommodate a CAT5/6 line directly to the modem thereby increasing data throughput = faster broadband response. The VOICE port connects to all phone jacks/wiring in the home and is now completely isolated from DATA. The POTS filter also provides lightning/surge protection at the point of entry (demarkation) to the facility... something you do not get when installing those filter dongles at each phone jack. The net gain in broadband speed is noticably faster--- mine increased by 200-300Kb+ throughput.
 

Offline atw60444

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Re: ADSL "microfilters" - any info on their circuit operation?
« Reply #6 on: January 23, 2014, 03:44:13 AM »
The benefit of using a "whole house" POTS filter is important to that of installing those stupif filter dongles at every phone jack in a home or small business.
I wholeheartedly agree with that statement. I worked on RFI investigation for our national Telco. and got diverted onto a lot of ADSL faults. Separate DSL and voice wiring from the demarcation point is far better.
 Jon
 


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