Microelectromechanical system oscillator

MEMS oscillators are electronic oscillator circuits which are as important frequency-determining element a microsystem (English microelectromechanical systems, MEMS abbreviated ) included.

In contrast to the comparable within the scope of the quartz oscillators of the MEMS resonator, together with other electronic circuits fabricated on a semiconductor die, which allows smaller Abmassungen. The development to first MEMS oscillators, they were then called Resonistor, go back to work to Raymond J. Wilfinger at the company IBM in the late 1960s.

Construction

In a MEMS oscillator, a MEMS resonator is used in place of polysilicon of the quartz crystal in the oscillator, which is not piezoelectric, unlike quartz. For operation of a MEMS resonator is, inter alia, a direct current voltage necessary to mechanically bias the MEMS, and the impedance of a MEMS is, compared to a quartz oscillator is much higher. This requires a specially adapted to the MEMS oscillator circuit. MEMS Resonantor has, in contrast to a crystal oscillator a strong temperature dependence in the range of -25 ppm / K. For a frequency-stable over the intended temperature range oscillator, the temperature deviation in Resonantor through its own temperature sensors and electronic circuit components must be compensated.

In addition to the oscillator circuit and temperature compensation are made commercially available MEMS oscillators still made ​​of a special phase-locked loop ( PLL), usually a english fractional-N PLL, which consists of the invariant set by the mechanical size under construction Resonantorfrequenz a configurable and within certain limits arbitrarily adjustable output frequency generated. The necessary divider ratios of the PLL are stored in MEMS oscillators together with the coefficients for temperature compensation in a read only memory in the semiconductor chip. Through unique programming this memory setting the respective output frequency. The actual MEMS resonator has mechanical lengths in the range of some 100 microns, the additional electronic circuit components have approximately similar extents, whereby the total MEMS oscillator can be implemented on an area less than 1 mm2.

MEMS oscillators have relatively high phase noise or jitter, also due to the control action can the electronic temperature compensation. The stand compared to quartz resonators cost advantages in manufacturing, to miniaturize the insensitivity to mechanical vibration and the possibility of MEMS oscillators better against. The deviations in the frequency stability were in 2012 on the order of ± 10 ppm to about ± 100 ppm. Specific values ​​are different depending on the oscillator type and quality.