By Carolyn Mathas, contributing writer
Once digital systems took hold, oscillators moved front and center into microprocessor-based systems to provide clock signals and frequency stability. The use of quartz crystals in oscillators provides that high-frequency stability. Miniaturization and the need for high-performance electronic devices that require tighter frequency tolerance over a wider operating temperature range have driven technology and product improvements. Two of the biggest market drivers have been automotive electronics and wireless communications.
Oscillators, for example, are currently widely used in automotive applications such as brake controls, anti-blocking systems, airbags, and tire-pressure–monitoring systems. They are also increasingly used in advanced driver-assistance systems (ADAS), light detection and ranging (LiDAR), GPS systems, engine control, in-vehicle Ethernet, and autonomous driving applications that need accurate timing solutions and tight frequency stability.
Timing and clocking are critical for high data rate transmission with 4G and 5G networks, creating even greater demand for oscillators. They are also one of the most critical components in systems such as SONET SDH that require a high-stability system clock to prevent time slips or data loss.
Oscillators are used in a variety of applications, including consumer electronics products such as smart wearables, cellphones, video games, and cable TV systems. Other growing applications span space tracking, sensors, measuring instruments, timers, phase-locked loop systems, medical devices, oscilloscopes, and signal generators.
Crystals and oscillators
There are many manufacturers in this marketplace, and it’s important to note that other types of oscillators exist, such as MEMS oscillators that are not crystal-based and are often used in high-vibration environments. Silicon MEMS oscillators are more compact, require less power, and have high efficiency.
Kyocera Corp., for example, produces a variety of crystal products, including crystal units and crystal oscillators. The company addresses internet of things (IoT) segments with its products’ unique structures, low current consumption, and decreased lead time. It offers clock oscillators (XOs), voltage-controlled crystal oscillators (VCXOs), and temperature-compensated crystal oscillators (TCXOs) and kilohertz-range crystal devices.
Surface-mount–type crystal units including the CX3225SB , a miniature low-profile crystal, is designed for applications such as digital electronics, mobile communications, consumer products, and car audio and accessories.
In the crystal oscillator area, the KT2520K is a miniature 2.5 × 2 × 0.8-mm TCXO, providing frequency characteristics of ±2.0 × 10–6 /–30°C to 85°C and ±0.5×10–6 /–30°C to 85°C (for global positioning system, or GPS). Target applications include GPS units, mobile communications, W-LAN, and low-power radio communications.
Silicon Labs also has a broad portfolio of products. Two of its newest crystal oscillators include the Si56x Ultra Series VCXO and XO and Si54x Ultra Series XO . The Si56x addresses next-gen high-performance timing apps that need ultra-low-jitter oscillators. The VCXO/XO can be customized to any frequency to 3 GHz, which is 2× the operating frequency range of previous Silicon Labs VCXO offerings with half of the jitter. The family features devices with typical phase jitter as low as 90 fs.
In comparison, the Si54x Ultra Series XO family targets applications requiring tighter stability and guaranteed long-term reliability. These include optical transport networking (OTN), broadband equipment, data centers, and industrial systems. Used as a low-jitter reference clock, the Si54x XO maximizes signal-to-noise ratio (SNR) headroom, minimizes bit errors, and enhances signal integrity. It features typical phase jitter as low as 80 fs.
IQD Frequency Products recently launched the IQOV-220 OCXO, featuring high stability and low phase noise. Frequency stability performance is delivered down to ±0.5 parts per billion (ppb) over the full industrial temperature range of –40°C to 85°C, with a short-term stability of 0.5 ppt (tau = 1 s). The IQOV-220 is suited for high-performance synthesizers, network clocks, radar, and satellite communications.
The IQOV-220 is a high-stability OCXO in a hermetically sealed package with excellent phase noise and Allan deviation. (Image: IQD Frequency Products)
On the MEMS front, Microchip Technology claims that its DSA family of automotive-grade MEMS oscillators provide 20× better reliability, 500× greater tolerance to shock, and 5× better vibration resistance when compared to traditional quartz-crystal devices. The multiple-output MEMS oscillators can replace multiple crystals or oscillators with one device. The DSA1001, DSA11x1, DSA11x5, and DSA2311 family members resist mechanical shock and jarring in harsh environments over a frequency range of 2.3 MHz to 170 MHz. AEC-Q100–qualified, the devices provide ±20-ppm stability over temperatures ranging from –40°C to 125°C.
The DSA family of automotive-grade MEMS oscillators delivers higher reliability and greater tolerance to shock and vibration compared to traditional quartz-crystal devices. (Image: Microchip Technology)
For 5G applications, SiTime Corp. offers a family of MEMS-based timing and synchronization solutions that offer high reliability, tight stability, and the ability to withstand harsh environmental conditions. SiTime’s Elite family of Super-TCXOs offers tight stability (±0.1 ppm to ±2.5 ppm) over a frequency range of 1 MHz to 220 MHz. They can replace quartz-based OCXOs and TCXOs in 5G and IEEE 1588 synchronization applications. The devices also offer 30× better dynamic stability, I2 C digital frequency tuning, and on-chip power supply noise filtering. These devices offer an operating temperature range from –40°C to 105°C, which is important for 5G applications that are expected to run in hotter environments.
Crystal oscillators in deep space
An example of just how far crystal oscillators have come is their use in NASA’s Jet Propulsion Lab’s Deep Space Atomic Clock (DSAC), launched on June 24, 2019. The miniaturized, low-mass atomic clock now in space is substantially more accurate and stable than any previously possible. Stability determines how consistently a clock measures time without drift.
The frequency of quartz-crystal oscillation by itself is not sufficient for deep-space navigation. According to NASA , quartz-crystal oscillators lose a full millisecond within six weeks. To put it in perspective, an atomic clock requires one-billionth-of-a-second precision. The DSAC now in orbit doesn’t just use the oscillations of mercury atoms; it also uses charged mercury ions, resulting in 50× more stability than clocks on GPS satellites.
Should the quartz oscillator move off-frequency in space, a correction determined by the atoms is applied to the quartz oscillator to steer it back to the correct frequency. This type of correction is calculated and applied to the quartz oscillator every few seconds in the DSAC compared to twice-daily updates for atomic clocks used onboard GPS satellites.
Applications for crystal units, crystal oscillators, and MEMS oscillators continue to rapidly grow. Fueled especially by automotive and telecommunications applications like 5G radios, designers can expect newer devices that offer higher reliability in radio synchronization to meet the demand for fewer service disruptions and better user experiences.
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