[Premium-Rx] Roofing filters - more

[DANIEL HIGGINS]

I've always known them as roofing filters since my days as a receiver design engineer with Racal back in the late 70s/80s.  The RA1772/RA1784 range had a first IF of 35.4 MHz with a 13 kHz wide crystal filter after the first mixer.  There was a lab version of the first mixer board with selectable 13 kHz or 1.5 kHz roofing filters, with the selection of the filter tied to the selection of the main IF filter (I know because I've still got one of the prototype boards, but since I've now gone with the RA3702/RA3712 range of equipment with a first IF of 81.4 MHz and a 12 kHz crystal roofing filter, it won't be much use to me).  I don't know what increase in performance it gave, but I hung on to it in the hope of one day making the ultimate CW receiver.

One of my first jobs there was to productionise a test rig that was sent to all three manufacturers of roofing filters so that they could check the IMPs before dispatch.  The prototype was lashed together using the front end boards from a receiver with gold plated spring contacts for the filter and a clamp to hold it in position.  When the test department had finished building the 3 production models with nice neat wiring it didn't work.  Some filters were good on one rig but poor on another.  After several weeks of effort I managed to get to the point where a bad filter was bad on all 3 rigs, but good filters had a bit of a spread.  It wasn't perfect, but in those days it was better than a hit and miss approach.

Regards,

Danny Higgins
G3XVR
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