The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

The New Hotness

  • Colpitts Oscillator at 32.768 kHz

    After some fumbling, a Colpitts oscillator sorta-kinda works at 32 kHz:

    Colpitts 32.768 kHz - 47p 470p waveform
    Colpitts 32.768 kHz – 47p 470p waveform

    Even though the Darlington provides enough DC gain for megohm-scale bias resistors, I think they still loaded the crystal tuning fork a bit too much:

    Quartz resonator - Colpitts oscillator - schematic
    Quartz resonator – Colpitts oscillator – schematic

    It didn’t start reliably and a JFET would probably produce better results. As it turns out, my transistor assortment currently lacks JFETs, a situation shortly to be resolved by an ePacket from halfway around the planet.

    The proto board has two oscillator circuits behind the crystal test fixture, with the 32 kHz resonator hanging from the header pins on the lashup toward the rear:

    Quartz resonators - Colpitts oscillator test fixtures
    Quartz resonators – Colpitts oscillator test fixtures

    My copy of Crystal Oscillator Circuits (Mattys, 1983) suggests useful component values:

    Colpitts JFET RC Values
    Colpitts JFET RC Values

    The value of of the recommended shunt resistor Rhi shows why I figured the Darlington and its bias resistors wouldn’t work:

    Colpitts Crystal Shunt Resistance Values
    Colpitts Crystal Shunt Resistance Values

    The other Colpitts circuit worked just fine for a 3.58 MHz crystal and reasonably well for a 60 kHz tuning fork, so some fine tuning should improve the results.