DAmp-10, 10 MHz distribution amplifier, or second life of old Ethernet hub

It is now possible to build or buy a precision 10~MHz clock source by using
either rubidium frequency standard or GPS disciplined OCXO. But such devices
normally have only single 50~Ohm output which makes it impossible to clock
multiple measurement appliances. The solution to this problem is to use
so-called distribution amplifier, which provides 3 or more galvanically isolated
outputs. Such devices are available on surplus market, but also can be very
easily built at home.

The circuit described below was constructed circa'2010 and has been proven to be stable and reliable.




Distribution amplifier should meet at least these criteria:

• continuously accept an input levels up to +10~...+15 dBm;
• provide input and output impedance of 50~Ohms at 10~MHz;
• provide output to output and output to input isolation of at least 40~dB, because some devices tend to produce unwanted signals on their reference inputs;
• optional galvanic isolation on outputs, to avoid ground loops and support devices which have complete galvanic isolation, even from earth ground.

The DAmp-10 distribution amplifier meets the above criteria and have an
additional bonus of 3rd order 10~MHz bandpass filter, which reduces levels of unwanted harmonic and subharmonics components.

The active element of amplifier is the Anaglog Device's AD8056 opamp and its usage doesn't differ much from the described in datasheet.

The most interesting part is the galvanic isolation. While it possible, to buy special transformers or wound them by hands, it was decided to try and use transformers from the old 10Base-T Ethernet cards and hubs. These transformers have built-in lowpass filters with corner frequency about 15-16 MHz and almost ideally suits for our needs.

Here are the photos of assembled DAmp-10 board:

Complete documentation including schematic, BOM and routing can be found here: damp-10.zip.