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Top 8 Reasons
To Use An Oscillator

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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.

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SiTime oscillators comprise a MEMS resonator and an oscillator IC in one active device as shown on the right. This architecture enables robust, high-performance, flexible timing products that are easy to design into a system.

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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.

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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, MEMSresonators do not experience aging.

 
 
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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. This 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.

3. MEMS low-frequency oscillators consume 65% less board space

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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 (1508)  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.

 

4. Oscillators can drive multiple loads, reducing costs, BOM and board space

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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.

 

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.

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This test was performed per IEC 62132-2 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

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Vibration resistance is important  since electronic systems are  often exposed to environmental  stresses, especially systems  deployed outdoors. Wind, heavy  vehicles and trains are a few  examples of the many sources of  external vibration. In addition,  systems often employ cooling  fans that cause vibration. These  vibration stresses can induce  frequency shift and noise on the  crystal resonator.


 

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.

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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.

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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:

1.Oscillators are “plug and play” – much easier to design, guaranteed startup
2.30X better quality and reliability – lowers cost, increases robustness
3.Smaller package and no/fewer caps – reduces PCB area
4.Drives multiple loads, replacing 2 to 3 quartz crystals – reduces costs, BOM and PCB area
5.Up to 134X lower sensitivity to electromagnetic energy – more robust
6.10X lower sensitivity to vibration – more robust
7.Available in any frequency – very short lead-times
8.One MEMS product covers a large frequency range – reduced qualification effort

Download SiTime Top 8 Reasons To Use An Oscillator Instead of a Crystal Resonator

ES Components can 100% up-screen or sample test SiTime’s products
in accordance with MIL-PRF-55310 requirements and MIL-STD-883 methods.

These screenings can be selected per our Standard flows, individual selection or per Customer Drawing.


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