Recently [Ken Shirriff] received a secret IC from [CuriousMarc] who had found said IC in a box of various old chips. Label only with a date code of the 47th week of 1985 and a mysterious F1-10-5 next to a logo that did not lead to any hits in the typical IC databases, it was until [Ken] to find out what lurks inside this CDIP chip.
As it turns out, it’s actually a pretty interesting IC with an easy-to-identify die once the ceramic package is opened with a surgical hammer-and-chisel blow.
Putting the die under a microscope it was identified as a Harris HF10, itself a variant of the National Semiconductor MF10. This is a dual switched capacitor filter, with the modern version eg the TI MF10-N in both PDIP and SOIC, but unfortunately no more CDIP. Availability seems tight, so the MF10 may not be long for this world.
These filter ICs can tune their capacitors to act as low-, band-, or high-pass filters. How this works is explained by [Ken]with switched capacitors replacing the resistors of a typical filter circuit. The main reasons for doing this over just using a resistor include how large resistors can get compared to the relatively small capacitor, not to mention their fixed value versus the adjustable resistance of the switchable capacitor.
The rest of the circuit is mostly there to enable switching and overall operation, providing a fascinating insight into a chip that would have been incredibly useful in the 1980s, even in synthesizers where this adjustable filter feature was most convenient.