Temperature and trim effect compensation of a VCXO using a multidimensional segmented polynomial array

Abstract

Voltage Controlled Crystal Oscillators (VCXOs) are widely used and well known frequency control products. VCXOs are typically characterized by having wide frequency pulling ranges (greater than ±50ppm). These oscillators are also uncompensated for temperature performance. This means temperature performance of ±20ppm or more is typical over the industrial range of -40 to 85 °C. Trim effect is a skewing of the frequency versus temperature performance of a crystal oscillator as the frequency is pulled (trimmed) away from the oscillator's nominal frequency. Even though unwanted, the degradation of performance from trim effect is something generally accepted as a characteristic of VCXOs. This paper focuses on a method of compensating crystal oscillator temperature and trim effect using a multi-dimensional segmented polynomial array. The inherent trim effect has been reduced from approximately ±11ppm down to ±0.5ppm. This is a 22-fold improvement over the inherent performance. The theory of this compensation method will be discussed, and data showing the results of temperature and trim effect compensation on actual oscillators will be presented.