Programmable Clock Oscillators Provide the Right Reference Clock, Every Time

A reference clock oscillator in electronics has long been the domain of fixed-frequency devices. These devices have historically consisted of a quartz crystal, which has been cut and manufactured to operate at a precise, fixed frequency, married to an analog oscillator circuit, which operates at the same fixed frequency and a fixed voltage. The process of manufacturing these fixed-frequency oscillators has been optimized over time, and results in accurate clocking devices that are very inflexible in offering features.

 

With recent advances in semiconductor-based MEMS technology and analog circuits, these fixed frequency clock oscillators are rapidly being replaced by a programmable clock oscillator. Programmable clock oscillators use a fixed frequency MEMS resonator, married with a programmable analog circuit which can offer a host of features that are not available from fixed-frequency oscillators, such as

 

1. Any frequency within the operating range, achievable by using a highly accurate Phase Locked Loop (PLL) which can multiply the MEMS resonator frequency up to any desired frequency of operation. The output frequency is accurate up to 6 decimal places of accuracy (1 Hz). This is particularly useful in applications where non-standard frequencies are desired to improve performance and reduce error rates.

 

2. Ability to operate at any voltage between 2.5V and 3.3V, as well as 1.8V, which are the most common input-output voltages used in electronics.

 

3. Programmable drive strength control using SoftEdge technology, which allows the user to accurately match the output impedance of the clock oscillator with the trace impedance of the board, and thus reduces reflections. Higher drive strength can also be used for driving multiple loads, while lower drive strength can be used for reducing electromagnetic interference (EMI).

 

4. Ability to configure the output control pin into Output Enable or Standby

 

In the case of more-featured oscillators such as Voltage Controlled Oscillators (VCXOs) and Voltage-Controlled, Temperature Compensated Oscillators (VC TCXOs), the programmability of the device also allows easy configuration of the pull range.

 

With such a host of features available from programmable clock oscillators, it is no surprise that the devices are rapidly gaining in popularity and have already replaced a significant number of fixed frequency reference oscillators.

 

 

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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 SiT5001TCXO 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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MEMS-Based Silicon Oscillator - Features and Benefits

Micro-Electrical Mechanical Systems (MEMS) have been greatly expanding in popularity over the past decade. While the automotive industry found MEMS vital for vehicle safety applications (tire pressure monitoring, air bag sensors, etc), these MEMS components have more recently been put to use in systems including sensors, gyroscopes, and microphones. These systems are used in high end electronics devices like Smartphone's, Digital cameras, tablets, gaming devices, etc.

In the past five years, MEMS-based Silicon oscillators have been replacing quartz oscillators in electronic applications. MEMS-based Silicon oscillators have numerous advantages over quartz including more features, higher performance, faster availability, higher robustness and reliability, along with a lower cost.

MEMS-based Silicon oscillators were first used in consumer electronics in such devices as laptops, DVRs, digital cameras, stereos, etc. But since SiTime has recently released a much higher performance MEMS–based Silicon Oscillator, these devices are being used in high performance applications like:

Telecom infrastructure – highly stable MEMS-based Silicon Oscillators, such as MEMS TCXOs (Temperature Compensated Oscillator) have been designed in by companies developing core routers based on SONET and Synchronous Ethernet and optical networking systems.

Differential MEMS-based Silicon oscillators have also replaced high frequency SAW (Surface Acoustic Wave) crystal oscillators, leading to their usage in Storage Area Networks and RAID systems based on SATA, SAS and FibreChannel protocols.

MEMS-based Silicon Voltage Controlled Oscillators provides a level of pullability and precision-tuning that is crucial in synchronization of clock signals. This leads to wireless applications, such as cell-phone base-stations and repeaters to start adopting the usage of differential and single-ended MEMS-based Silicon oscillators, which are well-known for their robustness, reliability, and low phase noise.

The addition of programmable signaling levels makes customizing your oscillator for any product or application easy. Available signaling levels include LVPECL, LVDS, CML, HCSL, or LVCMOS on various Silicon oscillator devices. Frequency is another customizable feature to enhance your system performance. Operating voltage is also a programmable feature, with options of 1.8V, 2.5V, 2.8V, and 3.3V, allowing interface with a wide variety of SOCs and ASSPs.

The long lead time for crystal oscillators is another disadvantage in which Silicon Oscillators have a solution. Lead times of 3-5 weeks enables better inventory control/ management, flexibility in meeting upsides, and simplified supply chain.

Because of a superb frequency stability, as low as 10PPM, the system timing margin and reliability of your electronics will increase greatly. Better stability also offers improved immunity against low frequency environmental noise from power lines, fluorescent lights and transformers.

With a robustness and reliability as high as 50,000 G of shock resistance, 70 G of vibration resistance, and 500 million hours of mean time between failure, MEMS-based Silicon oscillators are 10 times more reliable than quartz.

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Features and Applications of MEMS Clock Generators

History of 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 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 7.0x5.0mm 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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Voltage Controlled Oscillator - Usage of Voltage Controlled Oscillators

A voltage controlled oscillator is a timing component that allows the fine tuning of a clock frequency within an electronic system, thus allowing a receiving system to be synchronized with a transmitting system that is not at the same location. The fine-tuning (pulling) of the output frequency can be controlled with an analog voltage input that is offered on one of the pins. Voltage controlled oscillators (VCXO) are generally made of quartz crystals, though recently, all silicon oscillators that utilize MEMS technology have also begun to offer VCXO devices.

Today's programmable and MEMS-based Voltage Controlled MEMS Oscillator that are flexible and provide innovative solutions for various applications such as telecom clock synchronization, wireless networking, FPGA-based systems, instrumentation, audio and video. Two main categories of Voltage Controlled Oscillators are the Differential VCXO and High Performance LVCMOS VCXO. The salient features and benefits are listed below-

Differential VCXO

* 100% compatible with quartz crystal VCXOs

* Provide a differential clock output, at LVPECL, LVDS and other popular signaling levels.

* Excellent jitter and frequency stability allows designers to resolve timing margin concerns and increase system reliability. Increased timing margin translates into fewer system errors and higher long-term reliability

* Programmable differential VCXOs are easily customization and can be optimized for the system application for the best system performance.

* Extremely short lead times (3-5 weeks for production volumes) minimize inventory overhead and reduce cost

* Better availability reduces shortage risk.

High Performance VCXO

* LVCMOS signaling levels, with frequencies up to 220 MHz

* 100% drop in replacement for quartz crystal VCXOs.

* Excellent jitter and frequency stability allows designers to resolve timing margin concerns and increase system reliability. Increased timing margin translates into fewer system errors and higher long-term reliability

* Programmable high performance VCXOs are easily customization and can be optimized for the system application for the best system performance. Best linearity results in simpler loop monitoring in software controlled system PLL

* More reliable PLL bandwidth over operating range

* Quicker lock time and calibration

* Decrease in of modulation harmonics

* Tighter PLL bandwidth and simpler system design

Voltage controlled oscillators have a number of uses, for instance in clock functions to control computer processes. They are also used to give out electronic signals in stationary as well as wireless devices like radio receivers, radio transmitters, routers, modems, game consoles and television. In addition to that, it can be also located on function generators and electronic jamming equipment.

 

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