LC IF Filter

My Sony ST-S555ES tuner originally came with four 250-kHz GDT (low group delay) ceramic IF filters. The first two filters are active in wide-IF mode, and all four in narrow. I wanted a sharper narrow-IF passband. I tried substituting two 180-kHz GDT filters for the narrows, but the selectivity was not good enough. I tried 180s in all four filter slots, but the center frequencies of the filters I had on hand did not match well. In frustration I decided to use two well-matched 180s in wide and build some LC filters for narrow.

This is the filter circuit from a circuit simulation program. It has two complex poles like a two-section ceramic filter. Output is at node 6. Transformers K1 and K2 model tapped inductors (0.04 uH primary inductance, 2.66 uH secondary, 0.98 coupling coefficient). I built two of these circuits using T-68-6 powdered-iron toroids, which provide a Q of about 300 at 10.7 MHz. Resistor values less than 1Ω simulate inductor losses. The 330Ω resistors simulate the filter source and load impedances. For the capacitors I used silver micas padded with ceramic trimmers. The trimmers provide complete adjustment of the filter center frequency and passband. I set the center frequency to match that of the wide-IF ceramic filters and the passband to about 150 kHz.

This shows simulated amplitude and group delay for one filter for one particular trimmer setting.

This shows four 180s prior to installing the LC filters.

Here are the LC filters installed in the tuner. I used a single perfboard to hold the LC equivalent of two ceramic filters. The filters worked well once I reworked the perfboard layout to minimize unwanted coupling between the two sections within each filter. I adjusted the trimmers for a symmetrical passband shape, and then fine-tuned them for minimum distortion. Although 180 GDTs are narrower than filters ordinarily used for a wide IF, I'm able to obtain 1-kHz stereo THD of just 0.1% in wide. Distortion is 0.2% in narrow.

More is here.

Updated May 11, 2008