LC IF Filter

My Sony ST-S555ES tuner originally came with four 250 kHz ceramic IF filters. Wide-IF mode uses the first two filters and narrow cascades all four. I wanted a more selective narrow-IF passband. I tried substituting two 180 kHz filters for the narrows, but the response was still too broad. 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 a circuit simulation of a filter section. It replaces one ceramic filter. The transformers model tapped inductors (0.04 H primary inductance, 2.66 H secondary, 0.98 coupling coefficient). I built two of these circuits using T-68-6 powdered-iron toroids, with a Q of about 300 at 10.7 MHz. Resistor values less than 1Ω simulate inductor losses. The 330Ω resistors are the filter source and load impedances. For the capacitors I used silver micas padded with ceramic trimmers. I trimmed the center frequency to match that of the wide-IF ceramic filters and the bandwidth to about 150 kHz.

This is the simulated amplitude and group delay of one filter section for a particular trimmer setting.

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

Here the LC filters are 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 180s are narrower than filters ordinarily used in a wide IF, I was able to obtain 1 kHz stereo THD of just 0.1%. THD was 0.2% in narrow. A 1kΩ trimpot replaces the 330Ω source or load resistor for each wide filter. Adjust for minimum stereo distortion. This trick is useful when you don't have many filters to select from.

June 21, 201188108 MHz