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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Crystal Oscillator

Top Five Reasons to Replace your Quartz Oscillator

A quartz oscillator (also known as a crystal oscillator) has been the reference clock of choice in the electronics industry for many decades. Recently, this legacy device has been under attack by a host of clock devices that use a newer, more advanced technology - Silicon MEMS. Here are the Top Five reasons why you should replace yourquartz oscillatorwith a Silicon MEMS device.

1. Higher Performance:
Silicon MEMS oscillators offer higher performance across a wide spectrum of parameters. A quartz oscillator has been optimized for specific parameters - such as phase noise at a particular frequency and operating voltage and does extremely well. Silicon MEMS oscillators not only perform extremely well on these specific parameters - but they also offer high performance other parameters, such as:

a.Stable and reliable startup over temperature (which is an inherent problem in quartz oscillators due to the activity dips of crystals)

b.Full frequency range available at 1.8V, which is not commonly available from quartz devices.

c.Oscillator stability as good as 10 PPM, which is not commonly available from quartz devices (MEMS TCXOs offer stability as good as 0.1 PPM)

2. More Features:
Silicon MEMS oscillators offer many more features than a quartz oscillator. Some of these features are listed below.

a.Any frequency, up to 6 decimal places of accuracy. This capability is useful for generating higher performance or lower error rates from systems.

b.Drive strength control for better impedance matching, ability to drive multiple loads, or reduced EMI.

c.Thinner packages for thinner electronics.

d.Operation at custom voltages between 2.5V and 3.3V.

3. Better availability:
Production lead times of Silicon MEMS oscillators are 3-5 weeks, while that of a quartz oscillator is 6 - 16 weeks. This reduction of lead time ensures that Silicon MEMS oscillators can reduce inventory and cost of ownership.

4. Better robustness and reliability:
Silicon MEMS oscillators are based on Silicon, and use no quartz. They offer 10 times more robustness (ability to withstand shock and vibration) than quartz oscillators, as well as ten times better reliability.

5. Better cost trajectory:
Silicon MEMS oscillators have a better cost trajectory than quartz oscillators because they are based on Silicon and leverage the semiconductor industry infrastructure.

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Crystal Oscillator

The Clock Oscillator: The Shift from Quartz to MEMS

The technological era has given birth to numerous electronic gadgets. One of the most popular electronic devices ever invented is the clock. Eventually technology enabled the use of electo-mechanical  resonators to generate a reliable, accurate frequency for all clocks.  This became known as the modern reference clock.  With its stable, repeatable waveform, the reference clock is the heartbeat of an electronic product.  In essence, a clock is the most important component for operation of an electronic device as it controls timing. Depending on the application requirements, various timing components such as resonators, oscillators and clock generators can be used as reference sources. However, most electronic systems such as hard drives, cameras, video cards and computer systems use several clock oscillators to provide accurate timing necessary for the proper operation of the system.
 

Based on quartz oscillator technology, these clock oscillators were made by using precision manufacturing techniques.  Consisting of three components; a resonator, an analog excitation circuit plus a standard logic output driver, and a package, the oscillator’s main performance parameters include - frequency range and resolution, stability and tolerance, jitter, power consumption, operating supply voltage, and temperature range. However, the advent of technological innovation has paved way for the wide spread adoption of the Micro-Electro-Mechanical Systems (MEMS) resonator instead of quartz in the electronics industry.  This has given rise to the design and development of a programmable clock oscillator, which can customize the oscillator frequency and has been adopted in devices such as Smart Phones, Tablets, Digital Cameras and gaming systems.
 

The past few years have seen the quartz crystal oscillators being replaced by the MEMS Oscillators in various electronic applications. With a number of benefits such as better features, higher performance, faster availability, higher robustness, lower cost and increased reliability, the MEMS Oscillators score higher than its counterpart in the adoption processes. These features have given these oscillators a place in Telecom infrastructure routers and optical networking systems, which uses the TCXOs (Temperature Compensated Oscillators) for their high performance applications. Further, VCXOs or Voltage Controlled Oscillators provide a level of pullability and fine-tuning that is required for synchronization of clock signals, in wireless applications, cell phones and base-stations. These are also used in Storage Area Networks and RAID systems based on SATA, SAS and Fibre Channel protocols.
 

The MEMS based clock oscillators can be programmed for any frequency levels with performance optimization and flexibility.  MEMS oscillators achieved unparalleled stability and low jitter performance.  XOs achieve +/-10 PPM stability over the -40C to +85C temperature range, and their TCXOs achieve 0.500 PPM stability over the same operating temperature range.  Both devices also feature ultra-low jitter as low as 0.5psRMS. Further, these can be programmed for any supply voltage as low as 1.8V. Most importantly programmable MEMS oscillators simplify the supply chain with shorter lead times and fewer dedicated parts at discrete frequencies. The superior frequency stability as low as 10 PPM (XO) improves  system timing margin and reliability and leads to longer life of the electronic systems.

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Quartz Oscillator 
TCXO Oscillator 

What is a Differential Oscillator? Where can it be used in Electronics?

There are two major kinds of electrical signaling –Single-ended (LVTTL, TTL, LVCMOS, CMOS, etc.) and differential (LVDS, LVPECL, HCSL, etc.). Differential signaling makes the use of 2 signals that are exactly opposite in phase to each other, thus eliminating common mode noise and resulting in a higher performance system. Differential signaling is used by many high performance protocols such as SATA, SAS, FibreChannel, 10G Ethernet, etc.

Differential oscillators are usually used to provide higher frequencies in very high performance systems where single-ended clocks do not perform well, such as the examples listed above. Typically, differential oscillators are used at frequencies above 100 MHz, because the rise times of differential clocks are usually much faster, and can support these high frequencies. However, it is not unusual to see differential clocks at even 25 MHz. Differential oscillators can output frequencies as high as 1 GHz.

One of the main reasons why differential oscillators are used in electronic systems is that they offer more robustness against power supply noise (and therefore, a higher PSRR) and reduce common mode noise coupling in the system. This is especially crucial for very high speed circuits, typically above 6 Gigabits per second data rates.

Historically, LVPECL protocols have been very popular in differential signaling. However, recently, LVDSs signaling has started to gain in popularity, driven by the lower power consumption of this protocol. Since the output frequencies of differential oscillators are very high, they have typically operated at 3.3V and higher voltages. However, newer differential oscillator devices from Silicon MEMS timing companies have offered differential oscillators at voltages as low as 1.8V.

Typical quartz-based differential oscillators are offered in industry standard, 6-pin footprints, either 7x5mm or 5×3.2mm. Silicon MEMS-based differential oscillators also fit in these footprints, ensuring that they can replace differential quartz oscillators with no changes in design or layout. Some newer devices are also available in extremely small, 3.2×2.5mm packages.

Silicon MEMS-based differential oscillators are usually programmable, i.e. their frequency, voltage, stability, drive strength, and other features can be customized exactly to the required specification, which is different than what quartz differential oscillators can achieve. The ability to customize is an extremely important feature that can be useful in a variety of scenarios –such as –reducing EMI, bit error rates, higher performance, higher throughput, etc.

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Silicon 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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TCXO Oscillator

VCXO 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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TCXO Oscillator

Clock Oscillator

Silicon MEMS VCXO Surpasses Quartz

Quartz has been the predominant technology of choice for timing references. Quartz oscillators consist of piezoelectric crystals with analog circuits in ceramic packages. They provide accurate and stable clocks that meet the requirements of most electronic applications. However, they have many inherent limitations, such as:

* Long lead times and lack of easy availability, due to complex manufacturing
* Lack of customization due to inflexibility
* Quality and performance variations over different lots and over different manufacturers
* Lack of integration

As semiconductors have grown in popularity, electronics manufacturers have turned to Silicon MEMS Timing solutions for their clocking needs. Silicon MEMS Timing solutions provide the following benefits:

* Better availability and short lead times through semiconductor manufacturing infrastructure
* Easy customization due to programmability
* Consistently high quality and reliability
* Significant integration
* High performance, particularly for high speed serial interfaces and programmable logic (FPGAs).

In the case of VCXOs (Voltage Controlled Oscillators), the benefits of MEMS solutions become even more evident. Below is a comparison of Quartz based VCXOs vs. Silicon MEMS VCXOs, which shows that MEMS is clearly superior to Quartz.

Quartz VCXO
* Core Technology - Pullable Crystals
* RMS Integrated Phase Jitter (12kHz - 20MHz) - < 1ps
* Frequency Offering - 10-15 standard frequencies (< 60 MHz)
* Pull Range - ±50PPM, ±100PPM, ±200PPM
* Pull Range Linearity - 10%
* Tuning Slope (Kv) Consistency - Poor Consistency
* Drive Strength for Impedance Matching, EMI Reduction - Not Available
* Product Coverage - Limited options for 1.8V, pull range or 3225 package
* Lead Time - 8-12 weeks for standard device & 16 weeks for non-standard device

MEMS VCXO
* Core Technology - All-Silicon MEMS
* Integrated Phase Jitter (12kHz - 20MHz) - < 1ps
* Frequency Offering - Any frequency up to 800MHz
* Pull Range - up to ±1600 PPM
* Pull Range Linearity - <1%
* Tuning Slope (Kv) Consistency - Excellent Consistency
* Drive Strength for Impedance Matching, EMI Reduction - Configurable
* Product Coverage - Any combination of voltage, pull range and package
* Lead Time - Samples in 48 hours & Production in 3-5 weeks

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Crystal Oscillator

Silicon MEMS VCXO Surpasses Quartz

Quartz has been the predominant technology of choice for timing references. Quartz oscillators consist of piezoelectric crystals with analog circuits in ceramic packages. They provide accurate and stable clocks that meet the requirements of most electronic applications. However, they have many inherent limitations, such as:

* Long lead times and lack of easy availability, due to complex manufacturing

* Lack of customization due to inflexibility

* Quality and performance variations over different lots and over different manufacturers

* Lack of integration

As semiconductors have grown in popularity, electronics manufacturers have turned to Silicon MEMS Timing solutions for their clocking needs. Silicon MEMS Timing solutions provide the following benefits:

* Better availability and short lead times through semiconductor manufacturing infrastructure

* Easy customization due to programmability

* Consistently high quality and reliability

* Significant integration

* High performance, particularly for high speed serial interfaces and programmable logic (FPGAs).

In the case of VCXOs (Voltage Controlled Oscillators), the benefits of MEMS solutions become even more evident. Below is a comparison of Quartz based VCXOs vs. Silicon MEMS VCXOs, which shows that MEMS is clearly superior to Quartz.

Quartz VCXO

* Core Technology - Pullable Crystals

* RMS Integrated Phase Jitter (12kHz - 20MHz) - < 1ps

* Frequency Offering - 10-15 standard frequencies (< 60 MHz)

* Pull Range - ±50PPM, ±100PPM, ±200PPM

* Pull Range Linearity - 10%

* Tuning Slope (Kv) Consistency - Poor Consistency

* Drive Strength for Impedance Matching, EMI Reduction - Not Available

* Product Coverage - Limited options for 1.8V, pull range or 3225 package

* Lead Time - 8-12 weeks for standard device & 16 weeks for non-standard device

MEMS VCXO

* Core Technology - All-Silicon MEMS

* Integrated Phase Jitter (12kHz - 20MHz) - < 1ps

* Frequency Offering - Any frequency up to 800MHz

* Pull Range - up to ±1600 PPM

* Pull Range Linearity - <1%

* Tuning Slope (Kv) Consistency - Excellent Consistency

* Drive Strength for Impedance Matching, EMI Reduction - Configurable

* Product Coverage - Any combination of voltage, pull range and package

* Lead Time - Samples in 48 hours & Production in 3-5 weeks

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Crystal 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

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.

Learn more about :- 
Silicon Oscillator
Differential Oscillator