Every electronic system needs a timing device. And crystal (XTAL) resonators are often the go-to solution. However, oscillators, which pair a resonator with an oscillator IC into one complete integrated timing device, offer several benefits compared to XTALs. These benefits are further extended with MEMS timing technology. System designers no longer need to work around the limitations of XTALs and accept the headaches and risks of designing with crystals.
If you are looking for more details, kindly visit our website.
Image
1. Plug-and-play oscillators simplify system design
On the surface, oscillator design using quartz crystals might seem straight forward, especially considering the maturity of this technology. But there are a myriad of design parameters to consider when matching a crystal to an oscillator circuit. Among these parameters are crystal motional impedance, resonant mode, drive level, and oscillator negative resistance which is a measure of oscillator gain. Additionally, load capacitance must be considered for parallel resonant mode crystals and it should account for the PCB parasitic capacitance and potentially the on-chip integrated capacitance included in the oscillator circuit.
All of these parameters must be carefully considered to ensure reliable start up and operation of the circuit. Because an oscillator circuit requires close matching of the resonator to the oscillator circuit, crystal vendors cannot guarantee startup of the crystal. By contrast, oscillators are a completely integrated solution. The oscillator manufacturer matches the quartz resonator to the oscillator circuit, thus relieving the board designer of this burden. Because matching errors are eliminated, oscillator start-up is guaranteed by SiTime. In short, oscillators are a plug-and-play solution that greatly simplifies system design.
Design concerns eliminated with MEMS oscillators
Crystal motional impedance and oscillator negative resistance
The oscillator circuit must have enough gain and phase shift to meet the Barkhausen criterion for oscillation. Of particular importance is the motional impedance (ESR) of the crystal and the negative resistance (equivalent to gain) of the oscillator. If the oscillator has insufficient gain to overcome the motional impedance of the quartz resonator, the circuit may not start up. These issues are eliminated with the use of oscillators.
Crystal resonance mode, frequency tuning capacitance, and on-chip oscillator capacitance
Quartz crystals can resonate in either series or parallel resonant mode, but they are typically calibrated for only one of these two modes. If calibrated for parallel resonance, they require a specific load capacitance which is usually specified. However, the proper capacitance is not used, the frequency error may exceed the datasheet specifications. The oscillator IC may or may not have integrated chip capacitance which must be taken into account along with any parasitic capacitance from the printed circuit board connections, bond wires and lead frame of the oscillator IC to ensure the best frequency accuracy.
In contrast, MEMS oscillators integrate the resonator and oscillator/PLL IC into one package, eliminating the need for an external capacitor to tune the resonant frequency.
Crystal drive level
Care must be taken to ensure the oscillator circuit does not overdrive the crystal resonator. Overdriving the resonator can lead to accelerated aging of the crystal resonator and at extreme levels, it can damage the crystal. In contrast, MEMS resonators do not experience aging.
Image
2. MEMS oscillators offer much better quality and reliability
Quality and reliability are critical &#; not only are company reputations at stake, but re-work can be costly and time consuming. Moreover, systems that are deployed outdoors and exposed to environmental stresses must be especially robust. Quartz resonators, while a mature technology, involve a rather complicated manufacturing process in which each individual resonator is tuned to the desired frequency, usually by ablating the metal electrode with an ion beam. This step occurs before the crystal is encapsulated and causes the resonator to be susceptible to contamination. This process, along with other quartz manufacturing complexities, result in the mean time between failures (MTBF) of quartz to be as low as 14 to 38 million hours. The defective parts per million (DPPM) is up to 50 for the best quartz manufacturers and as high as 150 for tier 2 quartz suppliers.
In contrast to the specialized manufacturing processes of quartz crystals, MEMS oscillator manufacturers use standard semiconductor batch mode techniques. This includes wafer level production of resonators and oscillator ICs, and die bonding to standard lead frames with plastic encapsulation. SiTime MEMS resonator die are made from a single mechanical structure of pure silicon. During the manufacturing of SiTime MEMS, an Epi-Seal process is used to clean the resonator, after which polysilicon is deposited to seal the structure. The ultra-clean hermetic vacuum seal ensures the resonator structure is protected and free of contamination, eliminating aging mechanisms.
As a result, the DPPM and MTBF of SiTime oscillators are about 30 times better than quartz, providing a very reliable technology platform that endures severe environmental stresses and delivers a high quality product for the end user.
Image
Image
3. MEMS low-frequency oscillators consume 65% less board space
Oscillators are a completely integrated solution and do not require external components such as power supply decoupling caps. SiTime&#;s 1.5 mm x 0.8 mm () footprint is smaller than the smallest quartz crystal footprint at 1.6 mm x 1.2 mm. And when taking into account load capacitors that are needed for the 32 kHz quartz crystal, the total board area or the XTAL solution is over three times larger.
Image
4. Oscillators can drive multiple loads, reducing costs, BOM and board space
An oscillator is an active circuit with an output driver usually capable of driving 2 to 3 loads depending on the drive strength. This allows the oscillator to replace several crystals and their associated capacitors, further reducing BOM, system cost, and board area.
Image
5. MEMS oscillators are much less sensitive to EMI
Electromagnetic energy, which is common in most systems, can be picked up by exposed PCB traces that connect the crystal resonator to the IC containing the oscillator circuit. This noise can be coupled into the oscillator circuit and passed to the output, potentially adding jitter and noise to the system. However, integrated oscillators have no exposed PCB connections between the resonator and oscillator IC, and the bond wires or balls that connect the MEMS resonator to the IC are extremely short. This makes MEMS oscillators much less sensitive to EMI. As shown in the following table and plot, SiTime oscillators are up to 11.3 dBm less sensitive (134x on a linear scale) than crystal resonators.
Image
This test was performed per IEC -2 standard that injects electromagnetic energy into a transverse electromagnetic (TEM) cell where the device under test (DUT) is mounted.
6. MEMS oscillators are much less sensitive to vibration
Image
Some systems that require a very stable frequency, such as wireless base stations and small cells, can experience system failure and service interruptions due to vibration.
MEMS oscillators are vibration resistant because the mass of a MEMS resonator is approximately 1,000 to 3,000 times lower than the mass of a quartz resonator. This means a given acceleration imposed on a MEMS structure, such as from shock or vibration, will result in much lower force than its quartz equivalent and therefore induce a much lower frequency shift. The figure on page 5 shows that SiTime MEMS oscillators are more than 10 times lower (better) in vibration sensitivity compared to quartz oscillators. Note this figure is based on measurements of quartz oscillators rather than passive crystal resonators, but comparable results are expected on quartz crystal resonators.
7. MEMS oscillators are readily available in any frequency
The quartz supply infrastructure has several constraints which can result in long lead-times, on the order of 12 to 16 weeks or even longer. One constraint is the limited number of ceramic package suppliers. Another constraint is the limited availability of frequency options. With quartz products, every frequency needs a different crystal cut unless a programmable phase locked loop (PLL) is used. Therefore, lead-times for non-standard frequencies can be very long.
Image
In contrast to crystal resonators, MEMS resonators are based on a standard resonator configuration. The output frequency of MEMS oscillators is generated by programming the PLL to different multiplication values. This enables a very wide frequency range with six digits of accuracy. In addition, silicon MEMS oscillators are manufactured using standard semiconductor processes and packaging. Because MEMS oscillator vendors leverage the very large semiconductor industry infrastructure, capacity is virtually unlimited.
MEMS oscillator samples can be programmed and available within one day, even for non-standard frequencies. By using SiTime&#;s low-cost Time Machine II programmer and field-programmable oscillators, designers can instantly program oscillators in their lab to create a device with any frequency, any supply voltage and any stability within the device&#;s operating range. Production lead-times are only 6 to 8 weeks.
8. One qualification for an entire product family
Qualifying components for end-use (system) conditions can consume significant time and resources. However, qualification efforts can be reduced with MEMS oscillators. SiTime products are based on a programmable platform which allows each device within a base product family to generate a wide range of frequencies, supply voltages and stabilities. If for example, resources have been invested in qualifying a SiTime device at a particular output frequency and a new board design requires a different frequency, the existing qualification data may be extended to a part with a new frequency.
In contrast, each XTAL frequency requires a different quartz blank. And if a design requires frequencies above 60 MHz, a different technology other than fundamental mode quartz is often used. Third overtone quartz crystals are often used for higher frequencies. This mode can introduce additional challenges to ensure reliable start up (i.e. higher motional impedance and different oscillator circuit than fundamental mode) which requires qualification.
Image
Summary
Despite inherent limitations, crystals have been the standard in electronic timing for several decades. SiTime&#;s MEMS oscillators overcome these limitations and offer many benefits compared to traditional quartz crystal resonators. Designers no longer need to accept the headaches and limitations associated with XTALs.
The top 8 reasons to replace XTALs with MEMS oscillators are:
- Oscillators are &#;plug and play&#; &#; much easier to design, guaranteed startup
- 30X better quality and reliability &#; lowers cost, increases robustness
- Smaller package and no/fewer caps &#; reduces PCB area
- Drives multiple loads, replacing 2 to 3 quartz crystals &#; reduces costs, BOM and PCB area
- Up to 134X lower sensitivity to electromagnetic energy &#; more robust
- 10X lower sensitivity to vibration &#; more robust
- Available in any frequency &#; very short lead-times
- One MEMS product covers a large frequency range &#; reduced qualification effort
Do you know when to use a crystal or an oscillator? The wrong answer can cost you.
Have you ever thought about the total cost of using a crystal versus a MEMS oscillator? This question may not be at the forefront of your selection process when the price of crystals seems so cheap&#;at least on the surface. But although the unit cost of crystals is generally lower, once the total cost is calculated, the picture looks much different.
3 Common Crystal Design Issues
At SiTime, we hear from many customers when they have crystal design issues such as cold startup failures, oscillator circuit problems from mismatched crystals, or failure to pass EMI tests. These problems cause engineering cost overruns during development and can create costly quality issues. Plus delaying the product release date equates to lost revenue. Here we share three situations where customers came to SiTime to decrease their overall cost of ownership when facing crystal design concerns.
The difference between a crystal vs an oscillator
But first let&#;s briefly cover the basics &#; what is the difference between a crystal (XTAL) and an oscillator (XO)? In simplified terms, a crystal (sometimes called a resonator) is a moving/resonating passive component [1] that connects to an external oscillator circuit in the chip that it is timing, like an SoC, microcontroller or processor&#;as shown below on the left.
Image
An oscillator, shown on the right, is an integrated timing solution that contains both a resonator and an oscillator IC, in one active device. In the case of SiTime oscillators, the resonator is based on silicon MEMS (micro-electro-mechanical systems) technology instead of a traditional quartz crystal. This architecture enables robust &#;plug-and-play&#; oscillators that are flexible and very easy to design into a system.
Total cost of ownership
Oscillators are easier to design into a system since they include functionality and features that solve common and often difficult timing design problems. We&#;ve laid out some total cost examples which are based on pricing from Digi-Key and SiTimeDirect for XTALs and XOs with the same output frequency, stability, and package size [2]. Then we add the cost of the engineering workhours (based on $100 per hour) that is required to remedy the problem.
Each case has a different breakpoint based on production volume and engineering time. In general, the cost of designing with a crystal is lower when quantities are very high and design costs are amortized over large volumes. Conversely, the cost of using an oscillator is lower when quantities aren&#;t in the tens of thousands. But there&#;s more to the story.
What&#;s NOT factored into the following examples is the opportunity cost (lost revenue) due to project design delays, which can be tremendous in some markets. In some cases, there are additional costs for outside services and testing&#;and these can also be significant. Plus there are other penalties that add up. These include items such as the expenses for additional materials and components needed for board re-spins, the cost of load capacitors that are required with crystals (and not with oscillators), and the additional board space consumed by the capacitors&#;all of which further tilt the equation toward using an oscillator.
For simplicity sake, in the following examples we&#;ve included ONLY the cost of the timing component and the engineering time to correct the crystal problem.
1.
For more information, please visit Huixun.
Cost of crystal vs oscillator &#; cold startup failure
Unlike crystals, MEMS oscillators do not have startup problems. In this customer case, 15 hours of engineering work was required to correct the crystal startup problem. Here, with a relatively quick fix, the cost benefit of using a MEMS oscillator is realized when production volume is around 6,500 units or less. In other words, unless you are manufacturing in very large volumes, you could be paying more for a crystal in the long run.
Image
2. Cost of crystal vs oscillator &#; mismatched crystal causes oscillator failure
Because oscillators are an integrated solution (combining the resonator and oscillator IC in one package), matching errors are eliminated. Designers don&#;t need to worry about crystal motional impedance, resonant mode, drive level, oscillator negative resistance, or other pairing considerations. In this customer case, 40 hours of engineering work was required to correct the matching issue, making it less expensive to use an oscillator at volumes of around 17,000 or less.
Image
3. Cost of crystal vs oscillator &#; EMI compliance failure
The clock is often the largest contributor to EMI (electromagnetic interference) in a system and it can cause a prototype to fail compliance testing. SiTime MEMS oscillators offer multiple techniques for quickly and easily reducing EMI. One such technique is spread spectrum clocking. Another feature is FlexEdge&#;, a programmable feature for adjusting the rise/fall time (slew rate) of the clock signal to lower EMI.
As a passive component, crystals don&#;t have these EMI-reduction features. If designers need to use shielding or add a spread spectrum clock generator IC with their crystal, this adds expense and board space. Plus renting an anechoic chamber for additional testing could incur another $3,000 or more. To redesign the board and retest, it can take 50 hours of engineering work, making it more beneficial to use a MEMS oscillator at volumes of around 23,500 or less. And this doesn&#;t include the additional materials and test facility costs mentioned above.
Image
Bottom line &#; savings across the board
In addition to direct costs, there are other factors that affect the cost of designing with crystals. For example, oscillators can drive multiple loads. That means one oscillator can replace multiple crystals, which can provide a timing signal for only one device.
Additionally, SiTime MEMS oscillators are based on a programmable architecture that makes them readily available in any frequency, stability, and voltage within a very wide range. This provides great flexibility for designers to optimize their design. In fact SiTime oscillators can be programmed by key distributors or even by customers in their own lab using the Time Machine II.
Image
Programmability can also reduce the cost of qualification efforts if specification changes are needed. This cost- and time-saving benefit is possible because a MEMS oscillator (before programming) can generate millions of part numbers. Once the base part is qualified it can be configured to support a huge variation of specifications.
Perhaps one of the biggest indirect savings comes in the form of higher quality and reliability. SiTime MEMS oscillators have less than 1 DPPM and over 2 billion hours MTBF (mean time between failure) compared to typical quartz devices, which is up to 50 times better. Plus, SiTime MEMS oscillators have much better survival rates against shock and vibration compared to quartz crystals.
The higher failure rates of quartz crystals can increase costs in many ways, such as the added resource costs for root-cause analysis or extra service and replacement costs. And, the damage that quality issues do to a brand&#;s reputation can have a huge and long-lasting negative effect on a company&#;s bottom line.
Using an oscillator in place of a crystal can lower costs in many ways. Why not skip all the headaches and extra expenses, and use an oscillator? When procurement is focused on lowering component costs, remember that looking at the big picture will ultimately save in the long run. To learn more about the benefits of oscillators beyond cost, read our white paper: The top 8 reasons to use an oscillator instead of a crystal resonator.
&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;
References:
[1] ESC Components: Active & Passive Components - What Is The Difference Between The Two?
[2] Based on Digi-Key pricing as of February 23, for a ABM8W-25.MHZ-4-D1X-T3 crystal with 25 MHz frequency output, ±20-ppm frequency stability and ±10-ppm initial frequency tolerance, 3.2 x 2.5 x 0.75 mm package, -40 to 85°C operating temperature.
[3] Based on $100 per workhour.
[4] Based on SiTimeDirect pricing as of February 23, for a SiTBI-21-YYS-25. oscillator with 25 MHz frequency output, ±20-ppm frequency stability, 3.2 x 2.5 x 0.75 mm package, -40 to 85°C operating temperature.
[5] Difference in cost between using an oscillator compared to a crystal passive component with similar specifications and with engineering time included.
&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;&#;
Related Blogs:
Custom Programming SiTime MEMS Oscillators at Digi-Key
Top supply chain benefits of programmable timing
Top size and power saving benefits of programmable timing
Capacitor shortage? Use oscillators &#; they don&#;t need caps
How to Solve Costly Design Problems with EMI Reduction
SiTime Oscillators Replace Discontinued Murata Resonators, While Upgrading System
Related Course:
MEMS Oscillators Fundamentals
Related White Paper:
The top 8 reasons to use an oscillator instead of a crystal resonator
Related Products:
MHz Oscillators
SiT Low Power Oscillator
How can we help you?
Contact Us Request Samples Buy Now
If you want to learn more, please visit our website crystal oscillator manufacturer.
Comments
Please Join Us to post.
0