Wireless Waffle - A whole spectrum of radio related rubbish
Make your ADSL connection more reliable!signal strength
Saturday 11 February, 2012, 12:52 - Amateur Radio, Radio Randomness
Posted by Administrator
A while ago on Wireless Waffle, I commented on the daily loss of service that was occuring on my home broadband (ADSL) connection. It seems that this struck a chord with a number of people and remains one of the most commented-on articles on the site.

The question is, 'can anything be done to fix this problem'? The answer seems to be, that for a few pounds (a few and a bit Euros, or a few and a half Dollars) you can easily make a device which will provide additional filtering on your ADSL line and help with the problem of interference on the line. This solution is not a 100% guarantee of an improvement but does help and has resulted in an enormous (almost complete) reduction in drop-outs and, to boot, an increase in connection speed on the Wireless Waffle line!

The increase in broadband connection speed is perhaps an odd outcome but this is a result of a more stable connection on the line. This is why... The equipment at the exchange sets a target 'signal to noise (s/n) threshold' for each line depending on how reliable the line is. If the line continually drops out, the target s/n is increased to provide an additional 'safety margin' to try and stop future drop outs. If the line remains stable for a while (typically a couple of weeks), the exchange notices the difference and will lower the target s/n threshold. Each change in threshold is typically 3dB (a doubling or halving of power) and results in an approximately 500 kBps change in line speed. The Wireless Waffle line was originally synchronised with an s/n threshold of 12dB and a speed of just over 2 Mbps. As a result of applying the solution about to be described below, over a month or so, the target s/n was reduced (by the exchange) to just 6dB and the connection speed is just over 3.5 Mbps! Even with only 6dB s/n the line has remained completely stable for over two months and has not dropped out again once.

So how does this miracle solution work. Well, the first step is to gather the teeth of some bats at full moon when wolves are howling. Add to this, three drops of lizard spittle and some onyx droppings. Only kidding! It's not magic it's just simple electronics!

The solution comprises two simple pieces:
  • A common mode choke which serves to remove any non-common mode signals from the line
  • A low pass filter to get rid of any noise on the line which is outside the range of the ADSL signals
Signals on an ADSL line should, in theory, be balanced, meaning that of the two wires used for the connection, the signal on one wire should be the direct opposite of the signal on the other. But there are a number of wiring and other issues which can lead to this not being the case, and in particular can lead to interference becoming present on both wires, but in an unbalanced way, or even just on one wire. Any unbalanced signal which is on both wires [u]should[/u] be filtered out by your ADSL modem but it seems that giving this an additional bit of assistance with an external 'common mode' choke can make a helpful difference.

The low pass filter just stops any signals arriving on the line which are not needed for ADSL from getting into your modem and causing havoc. This could be strong short or medium wave broadcast signals which appear after dark, or could be strong local transmissions from a nearby radio amateur. Three different filter designs are presented which have differing degrees of effectiveness and you can choose which is the best for your situation.

To make the Wireless Waffle Anti-Daily Service Loss (or WWADSL) device, you will need:

1. Two pieces of (single core) wire, around 30 cm long
2. A ferrite toroid (eg FT50-43)
3. Two capacitors (either 1nF or 470pF - see below)
4. Two inductors (either 3.3uH, 4.7uH or 6.8uH - see below)
5. Six pieces (three pairs) of 'chock block'
6. An old phone socket to ADSL modem lead

No soldering is required and all the parts are reasonably readily available.

wire twistedLet's start with the choke. This simply consists of two pieces of (single core) wire, twisted together and then threaded through a small ferrite toroid. The exact number of turns is not critical, but the type of toroid is important. You need a ferrite toroid which are usually black in colour, rather than any other sort which are distinguishable by the fact that they are generally painted a different colour (eg red or yellow). Ferrite toroids usually have names beginning 'FT', such as FT37-43 or FT50-43. The last number (eg 43) is the type of material which is the important bit. The first number is the diameter of the toroid in hundredths of an inch. The one used here is an FT50-43 which is about the right size. An FT37-43 is a bit small. Ferrite cores ending 43, 72 or 77 are ideal for this project.

Take the two pieces of wire and twist them together, all the way along their length. Wind these around the toroid to leave yourself with about 2 inches of wire (5 cm) free at each end. The result should look something like the pictures below.

bifilar toriod 1bifilar toriod 2bifilar toriod 3

toroid chock blockConnect one end of the wires into one side of a double piece of chock block, and the other into another.

Now you need to take a decision... If your line synchronises at over 4 Mbps, the chances are you might be using ADSL2+. ADSL2+ differs in that it uses frequencies on the telephone line up to 2.2 MHz, whereas ADSL and ADSL2 only use frequencies up to 1.1 MHz. There are three choices of filter:
  • The 'mild' filter will allow all possible ADSL signals through including ADSL2+ and is suitable for all lines.
  • The 'medium' filter will still allow ADSL2+ through but might cause some loss of connection speed (at the expense of greater reliability of course).
  • The 'strong' filter will not allow ADSL2+ fully through as its frequency response begins to roll-off below 2 MHz and is therefore only suited if your line is running at speeds below 4 Mbps to start with.
None of these filters will affect ADSL or ADSL2 connections. You can see the frequency response of these filters in the diagram below: mild is the green curve, medium is blue and the pink curve is the response of the strong filter.

low pass filters

capacitors and inductorsEach filter is made of two capacitors and two inductors. The table below shows which values you require, depending on your choice of filter.


Next take two of the inductors and a capacitor. On the opposite side of one of the chock blocks that has your choke connected, insert each of the inductors into one of the remaining holes and put the capacitor between the two holes. The other ends of the two inductors should now go into the final piece of chock block, and the remaining capacitor across the same chock block.

completed filterThe results should look something like the picture on the right (if you haven't spotted it yet, you can click on the pictures to see a larger one).

Finally, take the 'phone socket to ADSL box lead' that you have and cut it about 30cm from the end which goes into the ADSL modem. Strip back the shielding on both pieces, which will reveal some wires. If you have two wires, no more action is taken. If you find you have four wires, identify which are the middle two and cut off the outer ones (these aren't used). In the wire shown below the red and green wires are the inner two and the black and yellow wires can be disguarded.

Take one piece of the cable and insert the two wires into one end of your chock block construction - it doesn't matter which end. Then do the same with the remaining piece of wire, at the other end of your construction.

adsl plugadsl cablecompleted adsl filter

Bingo! You've finished. At this point it might be worth checking that there are no short-circuits. If you have a volt meter with a setting to measure resitance, check that:
  • there is a connection between the two ends of the filter; and
  • there are no short circuits across the chock blocks.
Replace the cable connecting your ADSL modem to your phone line with the new one you just made and Bob's your uncle.

It's probably worth stating, just to be certain, that this is for ADSL connections (ie that which comes into your property on traditional copper telephone lines). It won't work with mobile broadband, cable broadband or satellite broadband!

Let us know how you get on and whether the WWADSL filter helps. Perhaps you might also like to share your construction experiences or tips with others too.
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