Ken Shirriff examined a ceramic Harris IC supplied by CuriousMarc after it turned up in a box of unidentified parts. The package carried the uncertain marking F1-10-5 or FI-10-5, plus a 1985 date code. Opening the package exposed a die marked HF-10, which identified the device as Harris’s direct replacement for National Semiconductor’s MF10 switched-capacitor filter.

National Semiconductor introduced the MF10 in 1981. The chip contains two independent filter sections. Each section can provide low-pass, band-pass, or high-pass output, with its behavior set through external resistors and control pins. The die’s upper and lower halves largely mirror each other. Each section uses three op-amps, producing the three filter outputs.

The most visible die feature is a grid of 72 square capacitors. They use two polysilicon layers separated by oxide, with each square estimated at about 5 picofarads. Individual squares are combined into groups of eight to create accurate 8:1 capacitance ratios. Absolute on-chip capacitance can vary by about 20 percent between chips, but matching identical structures is much more stable. The layout technique can achieve matching around ±0.01 percent.

Clock-controlled CMOS switches connect the capacitors in alternating phases. A capacitor charges during one phase and transfers its stored charge during the next. The resulting current behaves like a resistor with an approximate value of 1/(fC), so the equivalent resistance changes with clock frequency and capacitance. This saves die area because integrated resistors are large and imprecise, especially at high values. A stable clock also gives the filter a precise tuning reference.

The MF10 uses a four-switch arrangement that reverses the signal polarity during charge transfer. This reduces the effect of parasitic capacitance. Its op-amp integrators use switched capacitors at their inputs and fixed integration capacitors made from either eight or 16 capacitor squares. Those ratios produce clock-to-filter frequency ratios of 50:1 and 100:1. The underlying relationship is the clock frequency divided by 2π times the capacitor ratio. The values 2π×8 and 2π×16 equal about 50.27 and 100.5. The datasheet gives a typical frequency error of ±0.2 percent, which Shirriff attributes to layout and stray-capacitance effects.

The die contains the control circuitry for clock level shifting, clock shaping, ratio selection, startup, and fixed-current sinks. Around its perimeter, 20 bond wires lead to the chip’s 20 external pins. The die also carries a Harris logo, an outline of Florida, and initials that probably belong to members of the design team.

Each CMOS switch combines an NMOS and a PMOS transistor. Their complementary behavior gives the switch low resistance when enabled and very high resistance when open. The chip uses paired switches for the two clock phases. Non-overlapping clock logic inserts a delay after one phase turns off before the other phase turns on, preventing both paths from closing together and shorting signal nodes. Isolation rings and a grounded polysilicon shield limit switching noise in the silicon substrate.

The filter core is a state-variable design, a topology introduced in 1967. It uses a summing amplifier and two integrators to generate low-pass, band-pass, and high-pass signals. Frequency, gain, and quality factor can be controlled separately. External resistors configure the feedback and summing paths, and a mode input selects the low-pass feedback connection.

One control pin has three voltage states. A high level selects the 50:1 ratio, a midpoint selects 100:1, and a low level stops filtering and activates low-power mode. Two voltage-sensitive inverter paths distinguish the midpoint from the high and low supplies. Pulling the low-power pin low shuts down current mirrors and cuts consumption by about 70 percent. The op-amps themselves use roughly 35 transistors each, with separate differential and gain stages and a compensation capacitor.

Harris entered integrated circuits through its 1967 acquisition of Radiation, Inc., later forming Harris Semiconductor. The company left the semiconductor business in 1999 by spinning off Intersil. Renesas later acquired Intersil. Harris merged with L3 Technologies in 2019 and became L3Harris. Texas Instruments acquired National Semiconductor in 2011. TI still lists the MF10 as active, but the part is expensive and out of stock, so production may have ended. Digital signal processing has displaced many switched-capacitor filters, although state-variable designs remain useful in synthesizers because one circuit supplies several filter responses.

For the die study, Shirriff removed the metal layer with Whink rust remover containing 1.5 to 3.5 percent hydrofluoric acid and hydrochloric acid. He used the chemicals for about 20 and 16 minutes in total, alternating them in short applications, then stopped when the transistors became visible to avoid damaging the polysilicon. The package opened easily because it was ceramic. An obscure marking yielded to a microscope and a familiar circuit architecture.