Whether it is a microprocessor-based control circuitry, RF transceiver for wireless communications, or a linecard for optical networks, clocks are needed as timing circuits to perform functions like synchronization, distribution, control and protection. They are critical components when it comes to overall system performance.

Over time, these simple discrete components have become quite sophisticated. To keep pace with the needs of modern and future timing and frequency requirements, clock oscillators have also been the beneficiaries of silicon chip technology. Ever since Micro Oscillator Inc. first introduced its CMOS-based oscillator in the mid-90s, silicon clock chips have continued to bring cost, integration and performance benefits of silicon processes to the designers with the advantage of immunity from shock and vibration.

Over the years, we have seen clock ICs with onboard techniques like spread spectrum to minimize electromagnetic interference (EMI) problems, and programmable outputs with digital technology. Others have provided multiple outputs from a single die, while some have incorporated circuit techniques to offer features like low jitter, better system reliability, wider operating temperature range and tighter frequency tolerance in smaller packages. Developers like IDT, Linear Technology Corp., Maxim Integrated Products, On Semiconductor and Pericom continue to improve reliability, jitter, noise, accuracy, size and cost performance of silicon clocks.

Speaking of digital, several suppliers have developed proprietary digital phase-locked loop (PLL) techniques to enhance the synch-ronization and noise performance of their respective silicon clock solutions. IDT and Silicon Laboratories are a few of them. And, the progress continues. For instance, last year, Silicon Laboratories unveiled crystal oscillators (XOs) and voltage-controlled crystal oscillators (VCXOs) with its patented DSPLL technology with advanced 0.13 µm CMOS process with quad frequency outputs for applications like networking equipment, base stations, test and measurement equipment, storage area networks and video systems.

According to Silicon Labs, the optimized DSPLL technology offers a new class of frequency synthesizer that permits the designer to move frequency control and tuning from a complex resonator to a single CMOS IC, thereby delivering significant improvement in initial frequency accuracy when compared to designed based on traditional high-frequency SAW or crystal resonators. Plus, it offers low intrinsic noise, more clock rate flexibility, and true peak-to-peak measurements.

Now, Analog Devices takes it to another level of sophistication. This month, the developer released an unprecedented clock IC that combines low phase noise clock generation with 14-channel clock distribution at jitter levels below 1 picosecond (ps). It provides a new level of integration to replace several discrete components for significantly cutting board real estate and bill-of-materials costs for clocking.

In fact, the new 0.35 µm BiCMOS-based 14-channel clock generator with on-chip 2.8 GHz VCO, integrates an integer-N synthesizer, two reference inputs, a VCO, programmable dividers, adjustable delay lines and 14 clock drivers, including LVPECL, LVDS and CMOS. Bringing the VCO on-chip diminishes the risks associated with the failure of discrete oscillators, thereby contributing to the overall improvement in system reliability, stated Scott Behrhorst, marketing and applications manager for ADI's Clock and Signal Synthesis products.

“For designers working on wireless base stations, optical networks, and telecom applications, the on-chip VCO eliminates a large discrete oscillator, while the 14-output channels will often provide all of the clocks needed on the transceiver or line card, simplifying space considerations and lowering system costs, added Behrhorst.

In fact, there are five versions in this series with each supporting a specific frequency range. On the high end, a version includes a VCO that tunes from 2.60 GHz to 2.95 GHz. The other parts cover lower frequencies, down to the range of 1.50 GHz to 1.90 GHz. While derivatives for low channel counts are in the works, the supplier is also investigating higher channel parts.

Despite the progress in silicon clocks, ceramic resonators and quartz crystal oscillator clocks will continue to dominate applications that need high performance at the lowest cost.