A 1980s filter chip that uses switched capacitors

A 1980s Harris HF-10 (marked F1-10-5, 1985) is a ceramic-packaged switched‑capacitor filter chip that turns out to be Harris’s version of the popular National MF10. Microscopic inspection reveals a symmetric die with two identical filter halves, 72 polysilicon capacitor squares arranged in a grid, CMOS transmission switches, three op‑amps per filter and three outputs (high‑pass, band‑pass, low‑pass). The die also includes clock‑level shifting, clock shaping, frequency‑ratio handling, startup circuitry and current sinks, plus hair‑thin bond wires and small chip art. The layout shows groups of capacitors wired to yield precise capacitance ratios despite large absolute process variation, enabling accurate internal ratios without large diffused resistors.

The chip implements state‑variable filters using switched capacitors as the effective resistors: toggling capacitors at clock phases φ1/φ2 transports charge so the equivalent resistance is 1/(fC), making center frequency easy to set and tune via clock. A four‑switch topology can invert the sampled voltage to cancel parasitics, and op‑amps are used as integrators with switched‑cap feedback networks (8 or 16 caps used for integration). CMOS switches are complementary NMOS/PMOS pairs; shielding techniques (isolation rings, grounded polysilicon) and careful clock routing reduce switching noise. Overall, the design packs tunable, accurate analog filtering into a compact 1980s IC by exploiting switched‑capacitor techniques and careful layout.