Temperature-stable, high-frequency monolithic voltage-controlled oscillators

Relaxation oscillators are analyzed and classified topologically into floating and grounded timing capacitor types. Computer simulations are described which illustrate the factors limiting the performance of voltage controlled oscillators (VCO's). It is found that the grounded capacitor configuration with a current switch and a Schmitt trigger is superior for temperature stable performance at high frequencies. In order to realize this potential, design techniques are illustrated which lead to the fabrication of a gh frequency monolithic VCO that achieves + or - 60 ppm/C temperature coefficient of frequency over 0-75 C at center frequencies from dc to 20 MHz. The circuit also exhibits good linearity of voltage to frequency and excellent triangle output waveform over the whole frequency range from low frequencies to 20 MHz. The circuit was fabricated using an eight mask high speed (F sub T) = 1.3 (GHz) IC process with a die size which was 65 x 50 sq mil. This represents a factor of one hundred improvement in frequency capability over existing circuits at this level of temperature stability.