While rummaging around my old equipment box, I came across some in-line microfilters which I have collected as spares from discarded modem boxes or from places where they suspect having filter problems and have had their filters interchanged with a more modern unit. The filters in question are the Alcatel Speed Touch 3AU 04805 ABAA and the Li Shin International Enterprise LSF2070.
These filters come with modular jacks on both ends and a short pigtail to connect them. They are positioned between a regular POTS device and the phone line with ADSL service to act as a low-pass filter allowing the POTS signal to go through to the telephone and preventing the ADSL signal from interfering with the phone’s operation. It also works vice versa, as the phone’s loading on the line will consume most of the ADSL signal from the CPE and cause the link to fail. Such filters were very common bundled within modems or self-install packages because they did not need a technician to visit the site to install a central splitter and do allow for re-organization of the internal phone placement without rewiring.
That being said, these units are well past their use-by date and really are a museum piece in the sense that these units were installed in the ADSL days and are not ideal for usage on ADSL2+ lines. The reason is that ADSL2+ expands the frequency spectrum upwards to 2.2Mhz, and some of the older filters present more loading at higher frequencies which disturbs these signals, limiting the sync rate attained and eroding the noise margins. In some cases, some filters are known to cause drop-outs entirely on their own.
Added to this is the slow move towards “mostly IP-only” services. For example, Naked DSL does not offer home phone service at all, meaning there is no need for filters because there is no POTS service. Other than that, those who are lucky enough to be on Fibre NBN don’t have any need for ADSL filters or even modems for that matter, and instead have phone and IP delivered over fibre. Users who have signed up for ADSL2+ bundles and don’t actually use the home phone might not even have a phone connected in order to avoid complications with filters and maximise their sync rates. With the new NBN, delivery of services over HFC or VDSL2 will eliminate ADSL and ADSL-style POTS sharing by filtering entirely to maximise the service speeds, making these items a bit of a relic.
As a result, I’ve always been curious as to what is inside these units, as they will be exposed to the harsh realities of the telephone network. High impulse voltages, 90V ringing AC voltages are not uncommon, along with a need to be able to filter through the first 4Khz and block up to 1.1Mhz with sometimes unstable line impedances makes for a tough mission. Lets take a peek inside.
Alcatel Speed Touch 3AU 04805 ABAA
Without a doubt, this unit likely came from a former Telstra customer as these were common in their self-install kits. It is a fairly dense unit feeling rather “chunky” and heavy. Two screws need to be removed and the PCB falls right out.
Inside, it is absolutely crammed full of components – many MKT capacitors are visible, along with five large inductors. This makes it appear to be a multi-stage L-C filter. There is an input fuse as well, and four diodes seemingly acting as a bridge rectifier. Two transistors and a diode are seen too, which is a little curious as most of these devices were considered to be dumb and passive.
The unit is just a single sided PCB but it seems that based on the coding, this was a specially designed unit to meet Australian market requirements. The POTS network is relatively intriguing with many countries having their own stringent requirements for interconnection and different complex impedance network requirements.
Li Shin International Enterprise LSF2070
This second filter is much smaller, and is likely to have come from a customer who purchased a Billion modem at one point in time. These filters were commonly packaged in with a modem as an included accessory.
The unit has a lot fewer components and seems to be completely different in its design. It appears to have two identically marked transformers of some sort, with a capacitor and two resistors which is probably about the bare minimum.
Looking at the PCB traces, it seems that the incoming line signal is applied to a winding of the first transformer, which isolates the rest of the downstream devices from interacting with the line directly. The secondary winding has a capacitor placed over it, with it feeding into the primary of the second transformer. The resistors form a T-shaped filter, coupling the output to the secondary of the second transformer, and the secondary of the second transformer with secondary of secondary of the first transformer (also primary of the second transformer). I wonder if this is because of the need to provide a DC path for the on/off hook requirement. It seems the transformers may have specially been made to have a frequency response which performs the filtering, supplemented with a crude R-C circuit maybe. I’m no expert on analog electronics sadly.
Again, it seems this is a product designed specially for the Australian market according to the datasheet. Despite this, the fact that component footprints are unpopulated suggests one common design may have been used for different countries with different component values/combinations used to achieve compliance.