Harris HF-10 teardown shows how a clock tunes an analog filter

Ken Shirriff has identified a mysteriously marked Harris chip from 1985 as an HF-10, a version of the MF10 switched-capacitor filter. The identification in his new analysis turns an obscure part into a useful explanation of how a clock can tune an analog circuit. For an electronics learner, the important idea is charge moved per cycle, rather than a digital processor calculating the filtered signal.

Shirriff's original article was published on October 10, 2026, according to the blog's own feed; Hackaday covered it on October 11. The 1985 date code belongs to the chip, not the research. The physical examination date is not specified.

The clock controls how quickly charge moves

A resistor carries current continuously in proportion to the voltage difference across it. A switched capacitor can approximate that relationship by repeatedly taking a small amount of charge from one part of a circuit and delivering it to another.

For the simplified circuit discussed by Shirriff, the effective resistance is R = 1 / (f × C), where f is the switching frequency and C the capacitance. Increasing the number of transfers per second increases the average current for the same voltage difference, so the effective resistance falls. This is a circuit relationship, not a measurement of the examined chip's performance.

That gives the clock a practical role in tuning the filter. The signal path remains analog while switches control the transfers. It is a useful reminder that clocked operation does not necessarily mean that an analog signal has been converted into digital samples for software processing.

Two blocks provide several filter shapes

Texas Instruments' MF10-N datasheet describes two independent filter blocks used with an external clock and resistors. Their outputs can provide low-pass and band-pass responses, with another output configurable for functions including high-pass or notch.

A low-pass response reduces higher-frequency content; a band-pass response selects a range. The useful configuration depends on the signal you want to preserve, not merely the presence of the chip. The clock and the surrounding resistor network therefore belong in any explanation of what an installed circuit actually does.

Identification opens documentation, not a replacement guarantee

Shirriff traced the HF-10 marking to period documentation describing it as a direct replacement for National Semiconductor's MF10. That establishes the intended relationship between the designs. For a restorer, it narrows the search for documentation; it does not establish the condition of a particular old component.

TI's current MF10-N documentation is useful related material, but a restoration decision still needs the exact part, pin assignment, supply requirements and external circuit checked. A functional family resemblance is insufficient evidence for substituting components.

The datasheet also documents clock feedthrough: switching activity can appear at the output. Clock-based tuning offers control, but the clock is part of the analog design and its limitations. For a restoration or learning project, record the existing clock and surrounding components before treating the part number as the complete answer.

Sources

• Ken Shirriff: original HF-10 analysis, October 10

• Texas Instruments: MF10-N product documentation

• Texas Instruments: MF10-N datasheet, Rev. C

• Hackaday: October 11 discovery report

The identification and hardware observations are Shirriff's research. The circuit explanation is based on that work and manufacturer documentation; TechKili has not opened or tested the chip.