Electronics Workbench

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.