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Project Notes

#656 Crystal-Locked Schmitt Oscillator

Testing a basic crystal-locked Schmitt oscillator using a 74HC14.



I tested the basic Schmitt Oscillator a very long time ago in LEAP#022. I was prompted to try the crystal-locked variation after seeing CD40106 aka 74C14 covered in IMSAI Guy’s chip of the day:


Oscillators of this topology are variously known as Pierce Oscillators, Pierce-Gate Crystal Oscillators or CMOS Crystal Oscillators.

An alternative design using NOR Gates:


Circuit Design

The slowest crystal I have to hand is 4MHz, so I needed to add 20pF stabilization capacitors to maintain oscilation. The second inverter buffers the output and the 51kΩ feedback resistor ensure’s the oscillator is kicked into action.




Test Results

4.00019MHz according to my scope:


Credits and References

About LEAP#656 CMOS/TTLOscillators
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