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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High Frequency Crystal Oscillators

The heartbeat of an electronic system is the timing component. For the past 50 years, the timing reference in all electronics has been based on a quartz crystal. Components such as crystal resonators and crystal oscillators (also known as quartz oscillators) have been extensively used. Tens of billions of these devices are shipped every year.

In spite of their popularity, there are some significant limitations to crystal resonators and crystal oscillators. The most basic limitation is that typically, a crystal resonator cannot resonate at higher than 75 MHz in fundamental mode. To overcome this limitation for high frequencies, there are three alternatives:

* Use overtone mode – such as 3rd overtone, 5th overtone, etc. In this case, the oscillator operates at a frequency that is the third or fifth harmonic of the fundamental frequency. The disadvantage of this mode is that the costs are higher, and startup is not guaranteed over the entire temperature range.

* Use Phase Locked Loops to multiply the frequency – this is a very popular mode for MEMS-timing companies, but is not very popular with the crystal-based companies as they do not have analog circuits expertise. The benefits of this mode are that the intelligence is now in the electronics, leading to more features and higher performance at lower cost.

* Use Surface Acoustic Wave (SAW) technology – in which the crystal device operates differently than a standard AT-cut crystal.

Below is a comparison of the key parameters between SAW oscillators and MEMS Oscillators

SAW oscillators

* Frequency Range- Each device has to be cut to operate at a unique frequency

* Frequency Stability - Generally, 50 PPM, occasionally 25 PPM

* Package & Size – Large, 5032 and 7050 packages only

* Power Consumption- Typically more than 40mA

*1.8V Operation - Limited availability

* Aging - ± 3 PPM per year

* Shock & Vibration sensitivity – Sensitive, can break easily. Typical shock resistance is 2,000 to 5,000G

* Available Add-on features – Varies with part and frequency. Very few standard options available.

MEMS Oscillators

* Frequency Range- Programmable up to 800 MHz

* Frequency Stability - Can support as good as 0.5 PPM over industrial temperature range

* Package & Size - Small 2520 and 3225 in addition to 5032 and 7050 packages

* Power Consumption- Typically more than 20mA

* 1.8V Operation – Easily available

* Aging -Typically less than ± 1 PPM per year

* Shock & Vibration sensitivity – Extremely robust – 50,000G shock

* Available Add-on features - VCXO, SSXO (spread-spectrum) available in all frequencies and voltages, programmable drive

As can be seen from the above comparison, MEMS oscillators can offer superior characteristics compared to SAW crystal oscillators for high frequency applications.

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