Use MultiVolt Automotive Crystal Oscillators for Stable Timing Under Variable Supply Conditions

The quartz crystal oscillator, which leverages the piezoelectric effect, has come a long way since its development in the 1930s. Use of this frequency-determining element soon supplanted radio tuning based on an inductor-capacitor (LC) “tank” circuit. Back in those days, the crystals were ground by hand, mounted in a holder, and then compared to a standard using Lissajous figures on a crude oscilloscope (frequency counters as we know them did not exist).

Today’s crystals and their oscillators are far smaller, more accurate, higher frequency, and made via a highly automated process. Their role has expanded far beyond selecting and tuning a desired signal in the wireless spectrum to include precision timing and synchronization for circuits, systems, and networks.

Crystals are now available for many commonly used frequencies ranging from under 1 to over 100 megahertz (MHz). Sorry, old-timers and nostalgia buffs, the classic EIA/NTSC analog TV standard was officially discontinued in 2009. As a result, the ubiquitous 3.579545 MHz crystal (often referred to as 3.58 MHz) that was used to encode and decode color information in those TV signals is now only a museum piece.

Modern crystal oscillators are often supplied as a complete package, comprising an oscillator circuit and the crystal element. There are also many situations where the enclosed crystal alone, along with a standalone oscillator circuit, is preferred.

Mass-market crystal-based oscillators have come a long way since their singular use in radios and TVs in the 1950s and 1960s. Cars of that era had no “electronics” except for the radio, which adopted crystal-based synthesized tuning as soon as it was available, reliable, and affordable.

In sharp contrast, today’s cars have upwards of 100 processors ranging from dedicated microcontroller units (MCUs) to high-performance applications processors and communication controllers. These processors support functions that include sensing, safety, entertainment, connectivity, and hands-free driving, many of which are often summarized as ADAS (Advanced Driver Assistance Systems) (Figure 1).

Figure 1: The use of advanced electronics in core power-train functions, as well as ADAS features in today’s cars, is beyond what was imagined just a few decades ago, and there is more to come. (Image source: ECS)

Each of these functions requires crystal oscillators to provide clock signals for processors, controllers, communications devices, and related circuitry. Some of these will need simple, lower-performance crystals, but many others will require low-jitter, high-stability clocks. For functions such as vehicle-to-vehicle (V2V) and related vehicular interconnects, even higher levels of timing performance will be required.

Regardless of the crystal’s function, oscillators for cars must meet the rigorous AEC-Q200 standard for electronic automotive components. This standard encompasses much more than basic reliable performance despite temperature, vibration, and shock. It also mandates tests covering electrical and mechanical systems, thermal stress, moisture and solvent resistance, connector/lead integrity, and production solderability.

To meet these requirements, ECS Inc. International developed the ECS-2520MVQ AEC-Q200 SMD MultiVolt Crystal Oscillator series, almost all of which come in an ultra-tiny, four-pad ceramic package measuring just 2.5 × 2.0 × 0.8 millimeters (mm).

However, small size is just one of this series' attributes. The oscillators are offered in a frequency range of 1 MHz to 160 MHz (with a 1 kilohertz (kHz) special unit), standard stability of ±50 parts per million (ppm) with additional stability options of ±20 ppm, ±25 ppm, and ±100 ppm, and temperature ranges of -20 to 70°C, -40 to 85°C, -40 to 105°C, and -40 to +125°C.

As a result, users can choose a device that meets the required performance specifications.

What about supply voltage?

The ECS-2520MVQ SMD MultiVolt crystal oscillators are compatible with static (nominal) power-supply rail voltages of 1.8, 2.0, 3.0, and 3.3 volts, but that’s only part of their supply-voltage story. Their “MultiVolt” design provides a feature that sets them apart and offers designers additional flexibility.

How so? These devices use a cutting-edge, small-form-factor, high-performance ASIC. The oscillator-circuit stage uses a low-current linear voltage regulator, significantly reducing current consumption compared to traditional oscillator designs.

In addition, this internal regulation allows the MultiVolt oscillators to operate over a wide range, including from a deteriorating battery supply as well as a traditional fixed supply. This feature largely eliminates the dependence of performance on supply voltage, a traditionally detrimental characteristic of oscillators.

As a result of the internal regulator, the devices can operate across a 1.7 to 3.6 volt supply, which is wider than their static supply voltages. At the same time, the no-load operating current is low, ranging from 8 milliamperes (mA) at the lower frequencies up to 20 mA at the highest.

Two examples, the ECS-327ATQMV-AS-TR (Figure 2, left) and the ECS-2520MVQ-1250-BN-TR, illustrate the wide span of oscillator options. The ECS-327ATQMV-AS-TR is a miniature 32.768 kHz surface-mount device (SMD), AEC-Q200-qualified HCMOS oscillator that operates from 1.62 to 3.63 volts. The four-lead 3.2 × 2.5 × 0.9 mm ceramic package (larger than the others in the family) has just four pad connections: power (V_dd), ground, output, and tri-state output-mode control (Figure 2, right). It is well-suited for automotive as well as low power, portable, industrial, and Internet of Things (IoT) applications.

Figure 2 : The ECS-327ATQMV-AS-TR (left) is housed in a 3.2 × 2.5 × 0.9 mm ceramic package with just four pad connections: power, ground, output, and tri-state output-mode control (right). (Image source: ECS)

The ECS-327ATQMV-AS-TR’s CMOS output can drive loads up to 15 picofarads (pF) at a 50% duty cycle. It requires just 0.2 mA in active state, and the user can invoke a low-power tri-state mode via its pad connection #1, reducing current to just 20 microamperes (µA).

Why the need for that “odd” 32.768 kHz frequency? It provides time-of-day clock functions because dividing that frequency by 2¹⁵, a trivial hardware operation, yields pulses at a 1 second (s) rate.

For its part, the ECS-2520MVQ-1250-BN-TR is a miniature SMD HCMOS 125 MHz device, placing it slightly over three orders of magnitude higher in frequency than the 32.768 kHz oscillator. It also comes with MultiVolt capability across a 1.6 to 3.6 volt supply. Housed in a 2.5 × 2.0 × 0.8 mm ceramic package, the same as all other members of the series, except for the ECS-327ATQMV-AS-TR, this device features 1 picosecond (ps) jitter from 12 kHz to 20 MHz. The input current is 20 mA, and the output load rating is 15 pF. Frequency stability is ±50 ppm from -40 to 85°C.

Conclusion

Designers of automotive electronics need consistent, precise, and reliable frequency sources. The AEC-Q200-qualified crystal oscillators in the ECS-2520MVQ series from ECS meet these requirements with the added advantage of tiny packages. They incorporate the MultiVolt feature, so performance does not degrade when the supply voltage deviates from the nominal value, thereby reducing uncertainty from the design error budget.

Related Content

1: Understand Crystal Oscillator Parameters to Optimize Component Selection
https://www.digikey.com/en/articles/understand-crystal-oscillator-parameters-to-optimize-component-selection