Front end filtering on the FCD continues to be a hot topic for some users. Although the FCD has front end fixed and programmable filtering, particularly at VHF, in the presence of strong out of band signals the front end can become overloaded (as has been discussed before on these pages for example http://www.funcubedongle.com/?page_id=74, http://www.funcubedongle.com/?p=456 and http://www.funcubedongle.com/?p=321).
We have been working on this and with help from the team we are presently looking at using SAW filters which presents a no tune solution. Here are the latest incarnation.
The span is 130MHz to 160MHz, and as you can see the 3dB points are at about 142 and 148MHz. The attenuation to the infamous UK pager frequencies at 138 and 153MHz is 45dB.
This prototype is not pretty! The PCB has been re-laid out and is 1″x1″ to be able to fit inside the feed point of an antenna.
It is powered either from the coax (it is designed to be comapatible with the 5V bias T in the 1.1 version of the FCD) or it can be powered separately.