MEMS Based Oscillators Outperform Quartz-Based Oscillators

Precision clock sources such as an XTAL, XO, VCXO, or TCXO require some type of resonator in order to supply an accurate, specific frequency. For decades, cost-effective clock sources have been manufactured with a quartz-crystal slice as the resonator element. That is, a precision quartz-crystal slice is machined, polished, and plated such that it resonates at a specific frequency. The crystal is then packaged together with a CMOS IC that provides the sustaining circuit and a logic-compatible output. Though this approach has been main-stream for the last 80+ years, there are many limitations that clock system designers have been forced to live with simply due to the lack of any other alternative in the market. That is, until now!

There has been significant progress in the MEMS (Micro-Electromechanical Systems) oscillator alternative. As a result, the quartz-based oscillator has a new and significant competitor. One MEMS start-up innovator making an impact in the precision timing space is SiTime Corporation. SiTime’s all-silicon MEMS oscillators have been able meet or exceed the performance of quartz-based oscillators and overcome quartz-based limitations. A basic key feature of any oscillator is frequency stability. When comparing standard oscillators, also called XO, the SiTime SiT8208 and SiT8209 guarantees less than 10 ppm (parts-per-million) over the -40°C to +85°C operating temperature range. This represents a 2x (100%) performance improvement compared to the crystal-based 20 ppm alternative. Similarly, the SiTime SiT5001 TCXO family features 0.5 ppm stability over the -40°C to +85°C operating temperature range. This represents a 5x (400%) improvement compared to a 0.5 ppm crystal-based TCXO. And last, SiTime’s MEMS Oscillators, VCXO, and TCXO long term aging and jitter performance are comparable to, or better than, quartz-based oscillators. As you can see, the performance barrier has been shattered.

SiTime’s MEMS oscillators simplify system designs and open the door to new applications. First, standard crystal-based oscillators do not operate at frequencies beyond approximately 70 MHz. Beyond that frequency, crystal oscillators use different techniques that sacrifice accuracy (such as Surface Acoustic Wave (SAW) oscillators) and reliability (Overtone Mode). SiTime’s single-ended LVTTL/CMOS compatible oscillators operate at any frequency up to 220 MHz without any frequency holes and the differential oscillator family expands the frequency to 800 MHz. Second, crystal-based oscillators cannot support any frequency that the customer may want. Instead, they are available in standard, pre-set frequencies already defined by the cut of the crystal. Any non-standard frequency requires the crystal oscillator manufacturer to develop a new device and make it manufacturable in higher volumes, and the lead time for the non-standard frequency is typically 16-weeks. As a result, the high selling price is usually prohibitive for mid-to-high volume applications. This limitation goes away with the MEMS-based approach. SiTime’s oscillators are all programmable to any frequency within their operating range as previously described, samples are shipped within 48 hours, and the price is similar to any standard frequency.

And last, MEMS-based oscillators are significantly more rugged and reliable. Shock and vibration are two standard figures of merit. SiTime’s MEMS-based oscillators feature 50,000 G and 70 G tolerance to shock and vibration, respectively. This represents a 7-10x improvement compared to crystal-based oscillators with their shock and vibration tolerance of only 5,000 G and 10 G, respectively.

In summary, the SiTime’s MEMS-based oscillators meet or exceed the performance of crystal-based oscillators plus they include the advantages of any frequency between 1-to-800 MHz without any delivery delay, improved reliability, and no price premium for non-standard frequencies!

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Quartz Oscillator

Silicon Oscillator

What is a TCXO Timing Component? Why is it Important in Electronics?

The frequency stability (variation of frequency) of quartz oscillators varies significantly with temperature changes. At the same time, however, electronics manufacturers need extremely stable clock devices with very low variation of frequency stability over temperature. A traditional crystal oscillator (XO) cannot meet these requirements, which is why TCXO devices were created. In a TCXO device, the oscillator and resonator devices are designed and manufactured to have excellent frequency stability, usually an order of magnitude better than crystal oscillators. For example, a TCXO will have stability from 0.5 to 5 PPM over the same industrial temperature range.

Many applications need such stringent stability requirements. For example, a base station needs transfer calls from a mobile cell phone. If the cell phone is not completely synchronized with the base station in time, then dropped calls will occur. Therefore, almost every cell phone has a TCXO on it - to ensure that it remains synchronized to a base station.

MEMS TCXOs Replace Quartz Crystal TCXOs
MEMS Oscillators (XO) also has frequency variation with changes in temperature. On a MEMS oscillator, the analog circuitry compensates for this variation. Recently, MEMS TCXOs have entered the market and offer a level of stability that was not previously seen. Again, the analog circuitry on the MEMS TCXO takes care of the compensation, enabling it to achieve 0.5 - 5 PPM stability over industrial temperature range. Plus, the MEMS TCXOs offer all the other benefits of MEMS devices and are 100% pin compatible with crystal oscillators.

Features and Benefits of MEMS TCXOs
* As good as 0.5 PPM stability over the industrial temperature range, rarely available from quartz crystal TCXOs.
* The ability to specify any frequency from 1 to 220 MHz, with 6 decimal places of accuracy, allowing the user to customize the device for their application and enhance system performance. Again, because of the stringent manufacturing requirements of quartz devices, such frequency customization capabilities are just not available in quartz crystal TCXOs.
* MEMS TCXOs offer a pull range (for fine-tuning in the system) which is 10 times better than quartz crystal TCXOs.
* MEMS TCXOs are 10 times more reliable than quartz TCXOs
* MEMS TCXOs are available with much shorter lead times than quartz TCXOs.

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Clock Generator

Clock Oscillator

What is a VCXO? Why is a VCXO used in Electronics?

A VCXO – Voltage Controlled Oscillator – is a reference timing component that is commonly used in telecom, consumer and industrial electronics. A VCXO differs from an XO (Oscillator) in that it offers an capability to “fine-tune” the output frequency within a certain range, after the device is already populated in a system. This capability is not available on XOs.

Typically, a VCXO is used wherever a clock and data recovery function is performed. Usually, that involves receivers such as wireless base stations and other telecom equipment as well as consumer electronic devices such as Set Top Boxes (STB). The VCXO is part of a control loop that involves a PLL – Phase Locked Loop – that synchronizes the recovered clock on the receiver with the transmitted clock – to ensure that the two devices are locked and data integrity is maintained.

Pull range and Linearity are two key specs for VCXOs. Typically, crystal oscillator based VCXOs will offer a pull range of up to ±200 PPM of the output frequency. The maximum value of this “fine-tuning” capability is limited in a crystal-based VCXO because of the type of crystal and circuit that is used. This control is implemented on the analog oscillator circuit using varactors or switched capacitor arrays, which switch in and out as needed. In addition, a pullable crystal is also required – which is more expensive and not as easily available as a standard crystal resonator.

Pulling is achieved by an analog control voltage that is provided as input into the VCXO. The variation of the output frequency with this control input voltage is measured as linearity – the more linear a VCXO, the better control and characteristics it exhibits.

A silicon based VCXO offers much better pull range and linearity than a crystal-based VCXO. It does so because the “pulling” is done using Phase Locked Loops instead of varactors and pullable crystals – thus achieving a far larger pull range at a far better linearity. VCXOs that are based on Silicon MEMS will offer pull range of up to ±1600 PPM, as well as linearity of less than 1%, at comparable phase noise and jitter as crystal-based VCXOs. Thus, they are becoming the VCXOs of choice in electronics applications.

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Silicon Oscillator
Differential Oscillator

Features and Applications of MEMS Clock Generators

Clock Generators are timing components which integrate the clock functionality of many different discrete devices into one semiconductor component. They offer the traditional semiconductor benefits of integration, lower cost, smaller size, and full customization with minimal additional expenses. Historically, the use of clock generators started with the expansion of the PC motherboard market in the early 1990s. At that time, the functionality of a PC motherboard was increasing dramatically – audio, networking, high-end-graphics, video and various interfaces were being added to a PC. Each of these additional components (in addition to the core processor and memory) required a clock, which necessitated the use of as many as 7 clocking devices on a single PC motherboard. Semiconductor companies started developing clock generators which integrated the clocking function of these 7 devices into one or two clock generators, which resulted in the benefit of lower cost and smaller footprint.

MEMS-based Clock Generators are completely Integrated
Every clock generator requires multiple PLLs (Phase Locked Loops) which are used to generate any specified frequency from a standard fixed frequency reference, which is usually an external clock source such as a quartz crystal or crystal oscillator. With the advent of MEMS resonators (which are available in the form of semiconductor die and can be completely integrated inside a plastic package), the need for having an external crystal or clock source goes away. Thus, a MEMS clock generator provides a completely integrated solution with no external reference clocks. A MEMS clock generator also eliminates the matching of the crystal with the clock generator circuit, which is a time-consuming problem to solve and may sometimes affect the performance of the system.

Features and Benefits of MEMS Clock Generators
* Completely integrated solutions, no external components required. MEMS resonator die (reference) is integrated inside package with analog circuit.
* Functionality of 3 – 6 clock generators in one 7.0×5.0mm package, results in up to 66% board space savings.
* Independent operating voltage on each of the PLLs, eliminates the need for external level translators, thus reducing component count and cost
* Mixed differential and LVCMOS outputs in the same device, addressing the need for different clocking devices in complex systems.
* Available spread spectrum capability to reduce system EMI and pass compliance testing.

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Clock Oscillator 

VCXO