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Project Notes

#688 Crystal-locked 4MHz Oscillator

Testing a 4MHz crystal-locked Pierce oscillator using CD4001 CMOS NOR gates.



This circuit is a variation on the theme from LEAP#656 Crystal-Locked Schmitt Oscillator using NOR gates and a 4MHz crystal.

Circuit Design

Key design notes:

  • Instead of simple inverters, the design uses NOR gates from a CD4001 chip. The NOR gate may either be wired:
    • both inputs tied together, so the output is always the inverse of the input
    • one input tied to ground, so the output is always the inverse of the other input
  • With a 4MHz crystal, so I needed to add small stabilization capacitors to maintain oscilation (in this case 30pF)
  • R1 is a feedback resistor uses the gate output to ensure sufficient charge on the gate input to oscillate. Values are not critical but as a rule : the higher the frequency, the lower resistance required. 4.7MΩ works fine in this case.




Test Results

Measuring 3.99991MHz, with scope connections as follows:

  • CH1 Yellow: buffered oscillator output
  • CH2 Blue: oscillator output

At 4MHz, waveforms are far from perfect square waves. Aside from breadboard inefficiencies, this is mainly because the oscillator is running in the region of the CD4001’s max propagation delay of 250ns i.e 4MHz


Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.