Filters / Splitters

Sarcastic trivia.

From the five regular solids ancient Greeks believed that the dodecahedron had mystical powers, even today some believe the universe is shaped as a dodecahedron. Greek philosophers and priests hid this regular solid from popular view stating that the power inside this solid was too much to handle by the common person.

Splitters and microfilters fall into this category. If you want to create confusion, trouble and have a few laughs ask questions about this little device is some online forums. The adsl splitter has become the modern era dodecahedron in some online discussions.

What do some of our contemporary Greeks peers have to say about splitters:

Quote from an online discussion: This device is very complicated and have a very high failure rate.
This device is a low pass filter made of only 7 passive electronic components, two or three RJ11 jacks/plugs enclosed in a plastic case. There is nothing complicated about it, when someone doesn't want or can't understand how something works then it must be complicated or broken.

Quote from an online discussion: The capacitors inside the splitter can 'leak'.
Yeah right! And when that happens the whole space time continuum could collapse.
By the way splitters and microfilters use mylar or ceramic type capacitors that don't leak like electrolytic capacitors.

Quote from an online discussion: I've seen exploded capacitors in a linksys router and it is not a pretty sight.
I've seen UFOs landing in my back yard too and t-h-a-t is a pretty sight.

Quote from an online discussion: If You get hit by lightning the splitter can be damaged.
If you get hit by lighting the splitter will be the least of your problems.

Quote from an online discussion: If you hear noises in your phone line it may be a bad splitter.
Also it could be the US government spying on you, with the help from AT&T of course.

Now, lets take off our tin foil hats and head back to the real world:

What is a splitter or microfilter and what it is used for?
One half of the splitter ( the phone part ) is basically a passive balanced low pass filter and the other half ( the adsl modem part ) is nothing but a direct connection from the input to the output of the splitter. Microfilters are only the phone part half of the splitter and are designed to be used with a phone ONLY not the ADSL modem.

Your phone line is shared by two systems, the phone or POTS ( plain old telephone service ) and ADSL. These two systems must coexist using the same communications medium or wire pair. The frequency range at which these two systems operate is different and adjacent so special care must be taken to isolate and make sure neither service interferes with the other. As discussed earlier the ADSL allocated bandwidth covers from 25.875 KHz to 1.104 MHz and POTS use from DC or 0 MHz to 4 KHz. There is a gap between 4 KHz and 25.875 KHz ( a guard band ) that separates these two systems even further.

The function of a splitter ( low pass filter part ) is to attenuate or filter out any frequencies above a certain predefined frequency and at the same time allow to pass all frequency components to the second half ( adsl modem part ) of the splitter. The range of frequencies that a filter is allowed to pass is called the pass band and the frequency at which a splitter starts to attenuate the undesired frequencies ( usually at -3 db ) is called the cut off frequency. We encourage readers to learn more about passive filters at the American Radio Relay League web site ARRL ( may require membership ) or from Wikipedia.

Microfilters or filters behave the same way as the low pass filter section of the splitter. Microfilters main use is to isolate phone, fax, alarm systems or any other type of legacy device attached to a phone line. ADSL modems should not be connected to microfilters.

Splitter Types:
3 Different types
of splitters and filters.
Wall mounted.
Low pass filter and bypass.
Dual splitter
Low pass filter and bypass.
Phone only
Low pass filter only.
  Interior of dual splitter. NID.  

Splitters come in different shapes, packages and installation locations. Some look like the examples above and others might look a little bit different but their function is the same. Some splitters are installed in line like picture 3 and 4 others are wall mounted like picture 2 and others are installed at the phone distribution access box or NID ( network interface device )

How do I know if I have a bad splitter?

Unlikely but possible. The diagram below shows that the splitter only affect POTS. ADSL frequencies are bypassed by the circuitry. A damaged C1 ( shorted ) or a damaged phone can put excessive loads on the left side of the circuit therefore affecting ADSL performance. Shorted or out of specs C2, C3, L1, L2, L3 or L4 can alter the filter's characteristics allowing udesired frequecies to reach the ADSL side.

If a splitter is suspected of failure that splitter should be removed and the line tested again with a different splitter. A good technique for testing is to bypass all the house or office internal wiring and connect the ADSL modem directly to the NID. It is recommended that if you don't need or use your phone service to remove these devices.

Schematic of a dual splitter (POTS and ADSL).
Excelsus/Pulse Z-BLOCKER model number Z-330P2J.

Operational Tests:

We did some tests on the dual splitter with the above schematic and picture using the following equipment:
• Tektronix FG504 function generator.
• Tektronix 492 spectrum analyzer.
• Tektronix 2465B analog oscilloscope.

The test conditions are:

• Function generator set for a linear sweep 100 Hz to 4.5 MHz ( top frequency way above ADSL range of 1.1 MHz ).
• Function generator output is 50 Ohms.
• Spectrum analyzer and oscilloscope inputs set for 50 Ohms.
• Function generator output set at -30db ~ 1 Vp-p.
• The output from the function generator is connected to the input of the splitter ( Wall, NIC or external line )
• Splitter output measured at the ADSL modem and phone RJ11 jacks.
Modem jack output
Spectrum analyzer.
Phone jack output
Spectrum analyzer.
Phone jack output

From picture 1 we observe that the output signal at the modem jack is not affected, all the spectral components pass and no significant attenuation is present. Please note that the ADSL bandwidth occupies a little less than the first 2.5 graticule lines ( 0 ~ 1.5 MHz ), the whole sweep of 4.5 MHz is for testing purposes only.

From picture 2 we observe that the output at the phone jack is significantly attenuated at some frequencies. The cut-off frequency for the first lobe is approximately 15 KHz and for the second lobe is approximately 42 Khz. Any frequency above 15KHz and 42 KHz will be severely attenuated.

No human voice can reach beyond 15 KHz much less 42 KHz See: Human vocal range. Your 6 year old daughter scream might feel like 60 GHz but in reality it is much less.

From picture 3. the oscilloscope is set to trigger at the start of the sweep. We observe a similar behavior as the previous photo.


This little device is not a dodecahedron, there are no mystical powers hidden inside this little device. The function of the splitter is to separate and block any unwanted frequencies to reach the phone and adsl modem. The filter/splitter was designed to be a low pass filter acting upon the phone section.

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Last Update: October 8, 2014