Sometimes we need a timed signal to use as a clock (but also for other things).

General theory

The Barkhausen stability criterion says that

  1. the loop gain exceeds unity at the resonant frequency
  2. the fase shift around the loop is (where )

bad enough seems that the Barkhausen Stability Criterion is simple, intuitive, and wrong.

RC&Phase Shift Oscillator


Quartz is a piezoelectric material. When an electric field is placed upon it, a physical displacement occurs. Interestingly enough, we can write an equivalent electrical circuit to represent the mechanical properties of the crystal.

Colpitts oscillator

colpitts oscillator schematics

  • slide with derivation formulas for the colpitts oscillator
  • Video Colpitts Crystal Oscillator Fundamentals
  • http://www.electronics-tutorials.ws/oscillator/colpitts.html
  • http://www.electronics-tutorials.ws/oscillator/crystal.html
  • https://makerf.com/posts/experimenting_with_colpitts_oscillators
  • Crystal Oscillator Design with video and web app to calculate stuffs

Negative resistance oscillator

Some active devices (like diode and transistor) have zone of negative resistance that can be used to amplify the feedback loop of an oscillating circuit.

  • Video World's Simplest Single Transistor Oscillator - BJT with Negative Resistance
  • Page with a more elaborated explanation of NR.

Voltage controlled oscillator (VCO)

  • https://www.electronicshub.org/voltage-controlled-oscillators-vco/
  • https://electronics.stackexchange.com/questions/233607/sawtooth-vco-and-current-source-for-modular-synthesizer
  • Posts (1 2 e 3) about building a VCO creating a current source
  • https://electronics.stackexchange.com/questions/207668/how-do-i-make-a-vco-with-a-lm-358
  • https://www.allaboutcircuits.com/projects/diy-synth-series-vco/
  • Crystal Oscillator Basics and Crystal Selection for rfPICTM and PICmicro┬« Devices (PDF)
  • Crystal Oscillator Circuit Design (PDF)
  • Sparkfun tutorial
  • Choosing the Right Crystal and Caps for your Design (post
  • Microcontroller Oscillator Circuit Design Considerations ((PDF)[http://www.freescale.com/files/microcontrollers/doc/app_note/AN1706.pdf])
  • Tutorial about crystals with pratical example of circuit using them.
  • Video with simple example of oscillators (with a little bit of oscilloscope)
  • Crystal tester circuit