Friday, 18 July 2014

Building a DC to 2.5GHz stepped attenuator

Whilst sorting out some of my late fathers electronics, I came across an attenuator he was planning to build. Unfortunately his illness didn't allow him to start the project let alone finish it. In his memory I have built the attenuator and hope that he would have been pleased with the finished item.

The unit is based around a Weinschel 3007-100 stepped attenuator. It can apply 0 to 10dB of attenuation with an accuracy of plus or minus 0.3dB. It will work from DC to 2.5GHz and handle an input of up to 1 Watt. It has a 50 ohm impedance and should give a SWR of 1.3:1. The connectors of this sturdy unit are SMA. I managed to find some suitable coax and a couple of BNC chassis sockets that would take this rather thin cable. A trip to Maplin provided a suitable box. As the attenuator is connected using coax it was possible to select a plastic box rather than having to drill and file holes in a cast metal one. The hardest part of the build was wiring the SMA connectors. The miniature coax proved problematic on the eyes and fiddly with the fingers. The first cable was a pain but the second one seemed to be going well (may be too well). A quick test with a multi-meter showed that it had a short in one of the plugs. Fortunately I hadn't cut the coax too short so was able to cut it and start again.

After spending a few minutes each day for four days, the attenuator is now finished and fitted neatly in its box. The final job is to print the details on to some paper which I'll laminate and stick to the lid of the box. I had fun making this and hope to try it out once I complete my next project, a digital VFO.

Here are some pictures of the attenuator. I had to change some of the parts my father had sourced as the coax I used wouldn't fit his connectors.

The parts
The first lead completed

The finished attenuator