Crystal based 440Hz reference oscillator

This module acts as a referential oscillator for the VCOs of my Formant. It is crystal based, so you don't have any problems with temperature compensation. It produces a 440Hz triangle signal ('A'-Tone).

Such oscillator is very useful for:
  • fast tuning of the VCO bank
  • long time tests of tuning stability of the VCOs
  • Volt/Octave adjustment of the VCOs

    The circuit is based on a 1Mhz crystal oscillator. The square wave output of the oscillator is divided by 2273 with four decade counter 4017 in conjunction with a 4012 based reset logic resulting a frequency of about 439,95 Hz. This square wave is additionally filtered by an RC component creating a kind of triangle signal ("shark fin") to reduce the emphasis of the odd parts of the square wave spectrum which makes acustical adjustment of VCOs more difficult.
    Finally the triangle signal is buffered for output 'formant like' with an opamp stage.
    The module can be switched off totally if not needed as I don't know how much disturbance is generated by a 1Mhz signal.
    The original circuit possesses a transistor speaker output to be used as a stand alone module, but I built it as a Formant module, so I provided the previously mentioned opamp buffer instead.

    Output of the 440Hz Module:

    Board of the 440Hz Module

    440Hz Module test:

    The circuit is not part of the Formant Pro book, but is originally created by Frank Springer.
    The Formant like adapted circuit can be found here.

    Sound example:
    Not necessary (just a 440Hz triangle).

    Reproduction hints:
  • Please send questions or remarks to:
    Carsten Toensmann