An LC FM IF Filter

In modern FM tuners the IF filtering is usually taken care of my two or more ceramic filters. These provide an acceptable performance but can create phase distortion that may have an impact on stereo reception. LC filters are supposed to have a linear phase response and while I cannot verify this I wanted to use this type of filter.

The 10.7 MHz IF filter proved to be much easier than I had imagined. I had decided to use a top coupled LC filter with an inductive input. The output is to be directly connected to the grid of the limiter, hence termination with the unusually high 3.3k. It turned out that the coupling capacitors became unwieldingly small. Then I consulted the service manual for the Revox A720. At first I didn't understand the engineering behind its IF filter but then the quarter dropped. The LC sections were coupled at a tap! This way, the coupling capacitors can be larger and thus more manageable. I quickly plopped this insight into LTSpice and was able to generate a plausible solution. I quickly modified the LC section I had already created and used my newly built wobbulator to test it. Once I had a usable filter I measured the trimmer capacitors between each section and replaced them with the nearest fixed value. The resulting curve was nearly identical! This also confirms the adjustment instruction from Revox: just carefully peak the level at the filter center and your result will be OK.

The LC sections were made from coils I found in my junk box. Used in Philips K7 and K8 color TVs I used these coil bobbins for many projects and for this one they were perfect. Two shielded compartments with a 5 mm adjustable inductor inside. I removed the wire and replaced it with my own windings. 40 turns of 0.2 mm CuL (enameled copper wire). The input coupling has 6 turns. For some reason these parts nowhere can be found. I came across coils from Coilcraft and Neosid and found complete units from Conrad. The latter seem old stock and only available while they last. Reinhoefer Electronic also sells complete units but they need to be modified and I don't know if that is reasonably possible.










25 October 2015