MEMS oscillators target wider range of timing functions

Article By : Graham Prophet

The Elite platform comprises four product families with a wide range of frequencies, all offering 0.1ppb/g vibration immunity.

California-based SiTime has been designing and supplying oscillators and timing products based on micro-machined mechanical resonating elements rather than piezoelectric devices (quartz). Over several years, the company has gradually been improving the stability of the MEMS devices it makes, to challenge the role of quartz in an increasing portion of the market.

Now, SiTime has introduced its Elite platform of oscillators. It continues to use the terminology “TCXO” (the X originally stood for ‘crystal’) as it is in widespread use, but the Elite devices are entirely MEMS-based. They are temperature compensated by stored parameters, but claim levels of stability that can exceed those of quartz, when quartz is operated in a stable temperature, heated, environment (oven, or OCXO). With the Elite range, SiTime aims to capture a wider range of timing functions in telecoms and networking, offering performance gains in overall stability, in stability over temperature change, and in resistance to frequency change with mechanical shock or vibration (microphony).

The company sets out the progress that MEMS-based parts have made over approximately the last decade—stability has gone from around 25ppm to 0.1ppm with the Elite products, and a company spokesman attributes the greater part of this progress to optimising the compensation techniques applied to MEMS technology, and perhaps 40% to improvement in the basic fabrication technology. Jitter (short-term stability) has gone from 175psec to 0.23psec over the same time span.

The Elite platform comprises four product families with a wide range of frequencies, all offering 0.1ppb/g vibration immunity and do not have activity dips or micro-jumps. The immunity to vibration is particularly important where high stability is required in, for example, a cell site located in a noisy environment such as road or industrial noise.

Stratum 3 precision Super-TCXOs are aimed at communications and cloud infrastructure equipment, offering:

• ±100 ppb frequency stability over -40°C to 105°C, the widest operating temperature available with any TCXO
• 1 to 5ppb/°C frequency slope (ΔF/ΔT) at an extremely fast temperature ramp rate of 10°C/minute, a performance level claimed as unique among timing devices
• 3e-11 (3×10-11) Allan deviation (ADEV) at 10 second averaging time, presented as 10 times better than typical quartz TCXOs
• 0.2psec/mV power supply noise rejection (PSNR)—this removes the need for a specific stabilised voltage supply, typically by an inefficient, linear voltage regulator or LDO.
• Optional I²C/SPI frequency tuning, eliminating an external DAC

Super-TCXOs for GNSS, industrial and automotive applications offer ±0.5ppm frequency stability over -40°C to 105°C.

• Ultra-low jitter differential oscillators offer:

• 0.23psec integrated RMS phase jitter (12kHz to 20MHz)

• 0.1psec integrated RMS phase jitter under the Ethernet mask for 10G/40G/100G

• ±10 ppm frequency stability over -40°C to 95°C, enabling better system reliability

• High-temperature high-reliability differential VCXOs offer:

• Operation up to 95°C with excellent phase noise

• Wide pull range from ±25ppm to ±3600ppm

• 0.1% frequency tuning linearity under all conditions, 50 times better than quartz

__Figure 1:__ *SiTime says that this range represents a unified offering to a market that has been fragmented into multiple sectors, and highlights the products’ 30 times higher dynamic performance for small cells, microwave backhaul, Synchronous Ethernet and optical equipment.*

“Network densification is driving rapid deployment of equipment in uncontrolled environments such as basements, curbsides, rooftops, and on poles. Precision timing components in these systems must now operate in the presence of high temperature, thermal shock, vibration and unpredictable airflow. Service providers are questioning if quartz technology is up to this challenge,” said Rajesh Vashist, CEO at SiTime.

Elite timing solutions are based on a structure the company calls DualMEMS, together with “TurboCompensation.” It employs highly integrated mixed-signal circuits with on-chip regulators, a TDC (temperature to digital converter) and a low-noise PLL with better immunity to power-supply noise, 30µK temperature resolution that is 10 times better than quartz, and support for any frequency between 1 and 700MHz. SiTime chooses to make the MEMS elements on their own die with a mixed-signal companion chip; each is best fabricated in their optimum technology, rather than trying to build the single processing circuitry in the same process as the MEMS, the company said. The two dice are stacked within the package.

Supporting material on the company’s website includes a video demonstrating immunity to mechanical noise.

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