Timing Technology

With respect to different applications, there are different types of oscillators:

Standard clock oscillators typically offer a single frequency that is set at the factory. Also referred to as simple packaged crystal oscillators (SPXO), these are the most basic oscillators that combine a quartz crystal with an oscillating circuit. Applications: <100MHz - replacements, consumer electronics. >100MHz – SONET/SDH, 10GB Ethernet, Fibre channel.

Programmable oscillators are integrated chip oscillators offering commonly used reference clocks. The desired clock frequency is typically configured via pin selection as directed in the datasheet.

TCXO (also TCO) stands for “temperature compensated crystal oscillator;” is a highly stable combination of a crystal oscillator with a temperature-compensated circuit. TCXO is excellent for applications requiring precision timing. Often used for RF applications such as tactical radios, wireless devices, small cell access applications and nearly all smartphones.

TCVCXO stands for “temperature compensated voltage controlled crystal oscillator.” This is a crystal oscillator solution that is both temperature compensated and frequency-controlled via voltage. (Also abbreviated as VCTCXO, or VC/TCXO) Due to its precision and variable frequency control aspects, applications include cell and cordless phones, mobile and radio communication equipment.

VCSO stands for “SAW Voltage Controlled Oscillators.” VCSO are used for applications requiring low jitter and low noise at fundamental frequencies up to 1GHz. On the down-side, VCSOs have very low vibration sensitivity such that their frequency changes with physical vibration; but this problem is deterministic, or predictable, such that it may be compensated for. The rugged VCSO is used in various commercial, telecomm, and military applications.

VCXO (also VCO) is a voltage-controlled crystal oscillator; an oscillator whose frequency is tuned via a variable voltage input. This offers the option of a stable crystal oscillator with the ability to fine-tune the frequency within a finite range. These are often used in industrial equipment, communication relays, digital TV, digital audio, set top boxes and many other applications.

OCXO is an “oven controlled crystal oscillator,” and is the most precise and frequency-stable solution available. The quartz crystal, with a zero temperature gradient at high temperatures, operates in a miniature temperature controlled environment and is available in surface mount packages measured by the millimeter. OCXOs find applications in base stations for mobile phones, broadcasting equipment, navigation system and clock frequency standard, radar, and test & measuring instruments.

In addition to piezoelectric devices such as crystal or ceramic resonators, there is a newer addition to this fundamental basis for timing technology. A MEMS resonator is a small structure (0.1mm or less) that is designed to vibrate at high frequencies under electrostatic excitation. For precision timing applications, MEMS oscillators often include temperature compensation by using an on-chip temperature sensor. MEMS oscillators have superior resistance to shock, are thin, and are well-suited to co-fabrication in CMOS integration.

Quartz, ceramic and MEMS resonators are one option for the resonating device in a timing circuit. Another low-cost timing option is a simple RC oscillator, which uses a resistor and a capacitor with an amplifier to produce an oscillating signal using positive feedback. Most common output signal is either a sine wave, a CMOS-compatible output, a TTL-compatible output, or an ECL-compatible output.

However, for precision applications, the traditional solution has been a circuit based on a vibrating resonator. Crystals and ceramic resonators have well-known oscillating properties. MEMS resonators are micro-mechanical and are inherently integrated, resulting in a much smaller footprint, but worse noise performance than quartz. Quartz has a temperature dependency that is almost zero, whereas a MEMS oscillator (if etched in silicon) significant temperature dependency and must be compensated with additional circuitry. Nevertheless, MEMS appears to be well-suited as a real time clock (RTC) since they have very low power and size requirements, are reliable at low frequencies, and noise/jitter is not as much of a concern with an RTC. (See Application Specific Clocks for more information on RTCs.) Any oscillator and circuitry on an integrated single chip, etched in the same silicon or similar substrate, has all of the components on a single die, giving more precise operation and improved performance over temperature.

The clock scheme for PCI bus systems is very precise. Deviations may result in bit errors, reduced throughput, or the system may fail to train, which means the clocks did not synchronize between boards on the PCI bus.

JESD204B Clocks

JESD204B defines a serial interface standard between a high speed data converter and the host processor. In these systems the data converter clock is also used to generate the clock for the JESD204B output of an ADC, or the JESD204B input of a DAC. With a data rate of up to 3.125Gbits/sec, it is important that the JESD204B clock be low jitter and also have deterministic latency.

Real-Time Clocks

A real-time clock, commonly abbreviated RTC, is an IC that provides an accurate timebase when the main power is disconnected from the system. An RTC has its own dedicated 32.768kHz oscillator and is powered by a small battery that is enabled in the absence of system power. An RTC can maintain an accurate calendar including month, day, year, and exact time down to the second. An RTC can even have a programmable alarm or count-down timer. While RTCs can be a peripheral inside the main microcontroller, a separate RTC chip can also be used.

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Clock Buffers

Clock buffers allow a clock source to be distributed to many destinations at the proper time with little loss, adding stability to the clock distribution system. Clock buffer chips need to have a high input impedance so they will not load the clock source. Buffers must also minimize skew and jitter. Clock buffers can also be used when a delay needs to be added in order to synchronize the clock across a network.

Redrivers

Redrivers, also called repeaters, are used to improve the signal quality of high speed clocks. These devices clean up the clock signal, which includes minimizing jitter and boosting the strength of the signal.

Frequency Multipliers

Frequency multiplier/dividers are used to increase or decrease a clock signal by a multiple of its frequency. This is useful for circuits that need to generate another clock that is a multiple of the main system clock.

Clock Development Tools

Clock and timing circuit development tools can be invaluable in the design of complex clock circuits. These tools allow modifying the operating characteristics of one or more programmable clock chips. With an interface to a host PC, the developer can change on-the-fly clock system parameters while allowing easy measurement of signal characteristics like jitter and frequency.