What is the first thing you think of when selecting electronic components? Chances are it&#;s the processor or something else central to the system. The timing component may be the last thing on your mind, even though the clock provides the heartbeat on which all signals in the system are dependent.
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Selecting these essential timing components may appear to be a straightforward process, but there are a number of factors to consider that affect system performance. So what are the most important specifications and considerations? Here&#;s a short list of the top oscillator parameters and why they&#;re important. Of course there are more details to consider, so we&#;ve created an in-depth glossary that covers a broader range of oscillator characteristics.
Frequency
The most basic parameter for any oscillator is the frequency. It is the repetition rate (cycle) of the signal output from the oscillator and is measured in Hertz (Hz) per second. SiTime oscillators are available in frequencies as low as 1 Hz for low-power devices and as high as 725 MHz. The frequency of SiTime oscillators is programmable within this range to 6 decimals of accuracy. The use of custom frequencies can optimize system performance. Frequency can be factory programmed by SiTime, programmed by key distributors, or programmed for lower volumes in the customer&#;s lab using an oscillator programmer.
Frequency Stability
Frequency stability is a fundamental performance specification for oscillators. It is typically expressed in parts per million (ppm) or parts per billion (ppb) which is referenced to the nominal output frequency. It represents the deviation of output frequency due to external conditions; therefore, a smaller stability number means better performance. The definition of external conditions can differ for different oscillator categories, but it usually includes temperature variation and initial offset at 25°C. It may also include frequency aging over time, solder down frequency shift, and may include electrical conditions such as supply voltage variation and output load variation.
Output Signal Format
Chipset vendors may specify the required output signal mode for timing chips, or the system designer may have some leeway. Output types fall into two categories: single-ended or differential. Single-ended oscillators are lower cost and easier to implement, but they have limitations. They are somewhat sensitive to board noise and are therefore typically better suited for frequencies below 166 MHz. LVCMOS is the most common single-ended output type which swings rail-to-rail. SiTime also offers NanoDrive&#; output, which is similar to LVCMOS, but has programmable output swing down to 200 mV to match the input requirements of the downstream chip, and to minimize power consumption.
Differential signaling is a more expensive option, but it enables better performance and is preferred for higher frequency applications. Since any noise common to both differential traces will be zeroed out, this mode is less sensitive to external noise and it generates lower levels of jitter and EMI. The most commonly used differential signal types are LVPECL, LVDS, and HCSL.
Supply Voltage
Supply voltage, specified in volts (V), is the input power required to operate the oscillator. Supply voltage powers the oscillator through the VDD pin and is sometimes referred to as VDD. Standard voltages for single-ended oscillators include 1.8, 2.5, and 3.3V. Voltages for modern differential oscillators typically range between 2.5 and 3.3V. SiTime offers oscillators that operate as low as 1.2V for regulated supply applications such as coin-cell or super-cap battery backup. The supply voltage of most SiTime oscillator families is programmable, which reduces the need for external components such as level translators or voltage regulators.
Supply Current
Supply current is the maximum operating current of an oscillator. It is measured in microamps (µA) or milliamps (mA) at the maximum and sometimes nominal supply voltage. Typical supply current is measured without load.
Operating Temperature
The operating temperature range is the temperature span in which all oscillator parameters are specified within in the datasheet. Common temperature ranges are listed below.
Packages
Oscillators are usually housed in metal, ceramic, or plastic packages. And they&#;re available in a variety of industry-standard package dimensions. The pad (pin) arrangements may vary among vendors, but the overall x-y dimensions are standardized. Here&#;s a list of common oscillator package sizes for single-ended oscillators, which usually have 4 pins. Differential oscillators, which have 6 pins, are typically available in the larger packages: , , and .
Some specialized oscillators, such as OCXOs are housed in significantly larger packages, often measuring 25.4 x 25.4 mm but can range from 9.7 x 7.5 mm to 135 x 72 mm.
In addition to these standard package sizes, SiTime offers a couple of unique packages to solve difficult design challenges. One is a tiny (1.5 mm x 0.8 mm) chip-scale package (CSP), which is the smallest oscillator package available. Another option is a leaded SOT23-5 package for applications that require higher board-level reliability and easier visual inspection during board assembly.
Jitter
Jitter is an important parameter, especially for digital communications applications. It is the short-term deviation from an ideal clock signal and is measured in picoseconds (ps) or nanoseconds (ns). Because jitter can be one of the main contributors to system timing errors, it&#;s critical to account for oscillator jitter when evaluating the total timing budget. This is not necessarily a simple matter. Oscillator manufacturers do not all specify jitter in the same way. Jitter requirements vary by application and there are various types of jitter and different integration ranges for integrated phase jitter which is measured in the frequency domain. To help sort this out, our glossary includes definitions for cycle to cycle (C2C) jitter, integrated phase jitter (IPJ), long-term jitter, period jitter, and phase noise.
Click here to download the Clock Jitter Definitions and Measurement Methods app note for even more information.
Other Parameters
The eight parameters listed above are the most common factors used when selecting an oscillator. But depending on the application, there can be many more characteristics and features that are important to consider. These include EMI reduction features, pull range options for fine-tuning frequency, start-up time, and quality/reliability (Q, DPPM, MTBF, FIT rate).
And for high-performance applications, there are a number of additional stability-related specifications to consider beyond basic frequency stability. These include aging, frequency vs temperature slope (ΔF/ΔT), thermal hysteresis, Allan deviation, Hadamard variance, holdover, and retrace.
To learn about these parameters and more, see the glossary created by SiTime &#; one of the most extensive oscillator definition guides available.
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.
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.
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.
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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.
Cost of crystal vs oscillator &#; cold startup failure
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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.
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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.
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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.
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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.
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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.
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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.
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