Pavan Gupta

A Temperature-to-Digital Converter for a MEMS-Based Programmable Oscillator With

MEMS-based oscillators offer a silicon-based alternative to quartz-based frequency references. Here, a MEMS-based programmable oscillator is presented which achieves better than ±0.5-ppm frequency stability from -40°C to 85°C and less than 1-ps (rms) integrated phase noise (12 kHz to 20 MHz). A key component of this system is a thermistor-based temperature-to-digital converter (TDC) which enables accurate and low noise compensation of temperature-induced variation of the MEMS resonant frequency. The TDC utilizes several circuit techniques including a high-resolution tunable reference resistor based on a switched-capacitor network and fractional-N frequency division, a switched resistor measurement approach which allows a pulsed bias technique for reduced noise, and a VCO-based quantizer for digitization of the temperature signal. The TDC achieves 0.1-mK (rms) resolution within a 5-Hz bandwidth while consuming only 3.97 mA for all analog and digital circuits at 3.3-V supply in 180-nm CMOS.

Frequency Stability and

MEMS-based programmable oscillators have emerged as a promising alternative to crystal-based frequency references, with previously reported work demonstrating sub-ps integrated jitter [1]. Here we show frequency stability better than ±0.5ppm from -40 to 85°C, along with Allan Deviation (i.e., long term jitter) better than 0.005 ppm for 0.1, 1, and 10 second strides. Since the MEMS resonator has a well-defined temperature dependence, the key to this performance is a stable and low-noise temperature-to-digital converter (TDC) that utilizes a thermistor on the same die as the MEMS resonator.

Integrated Jitter

Real-time clocking for space-constrained mobile and wearable applications require low-power 32.768 kHz references with small form-factor and tight frequency stability, at a competitive price built in an ultra-high volume capable manufacturing process. Legacy 32 kHz quartz-based technology has reached the limits of miniaturization, performance and cost. In this work, a temperature compensated 32 kHz MEMS-based oscillator (TCXO), in a 1.55 mm × 0.85 mm × 0.55 mm form factor, with ±5 ppm frequency stability over -40°C to 85°C, will be presented. The combination of wafer-level chip scale packaging (WL-CSP) and silicon MEMS technology has enabled the smallest and best-in-class 32 kHz clocking solution for very high volume applications. The underlying MEMS system packaging and test technologies will be presented along with the electrical and reliability results.

A temperature-to-digital converter for a MEMS-based programmable oscillator with better than ±0.5ppm frequency stability

Micro-electromechanical (MEMS) oscillators are now in production and shipping in quantity. Early development of micro mechanical devices provided the understanding that metal beams could be fabricated and they would resonate as a time reference. The issues of performance and price have prevented silicon entry into the quartz dominated market until recent developments in semiconductor processing and assembly. Today MEMS oscillators are the worlds smallest programmable precision oscillators and are displacing quartz technology in the +/- 50 ppm accuracy spec. New oscillators extend the technology with spread spectrum, voltage control, and improved jitter performance. New ultra-thin packaging, made possible by the small encapsulated MEMS resonators, provides the words thinnest precision oscillators.