ADSL Filter

For several years I have used the popular OZ7C ADSL filter to prevent dropping my broadband when transmitting on 160m. It worked well, however reduced my download speed by 1 Meg indicating considerable loss in the passband below 1.1Mhz. Steve G3VMW improved the design by getting the roll off on the correct frequency, the downside was it needed custom inductors on iron powder cores.

New Design

I used the excellent Elsie and LT-Spice to come up with a design which is low loss and easy to build with off the shelf inductors. It’s an m-derived low pass filter, designed to match 100 ohm lines. Construction is very straightforward as all inductors are 10uH, so you only need 3 different components. The loss is typically 0.2dB across the ADSL band up to 1.1Mhz.

Performance

LT-Spice plot of performance using real inductors. Loss > -50dB on 160m and -38db on 80m.

Blue = Insertion Loss Green = 100 Ohm Return Loss

Construction

I have successfully built filters using Murata 2200R series & Epcos LBC series inductors, both have reasonable Q at 1Mhz. Capacitors should be 100v working voltage or greater, I used NOS / surplus Silvered Mica which are readily available on Ebay. If you have access to an L/C meter it’s worthwhile picking component combinations which resonates the input / output series tuned circuit around 1850 Khz.

My Buffalo ADSL router appears to measure line loss accurately, adding the filter increases the line attenuation by 0.2dB which agrees with LT-Spice. I added and removed different filters several times, the result is always 0.2dB loss with no effect on sync speed.

Common mode choke

When building a filter you should add a common mode filter, easier than wrapping the cable through ferrite at a later stage. I added a choke using 10 turns of thin twisted pair on a FT50-75 core. This results in a a choke > 1000 ohms between 0.5 and 14 Mhz. The twisted pair can be taken from a scrap CAT5 cable or formed from thin hookup wire. Two of these chokes could be cascaded or a second choke with 10 turns on FT50-43 added to improve HF performance.

Good common mode choking is important if you have a long overhead line to your property, it may improve ADSL sync speed if your ADSL router has poor common mode rejection.