If Jitter Is Your Problem, Try This Clock

As high-speed communications protocols move to 10 Gbits/s, 40 Gbits/s, and higher data rates, timing budgets increasingly get squeezed. Most existing clock and synthesizer solutions cannot achieve the low jitter necessary to minimize dropped packets, enable interoperability between computer systems, and ease performance bottlenecks.

Jitter is the key parameter designers want to minimize to unlock higher system throughput. If you’re designing datacom products and jitter is giving you fits, Multigig’s QuietClock family can give you more headroom with an rms jitter spec of less than 60 fs.

The QuietClock clock synthesizers can open constricted eye diagrams, enhance bit error rates (BERs), and maximize precious timing margin. The family provides three to five times lower jitter and significantly lower phase noise than the typical solution—and it does so with 30% to 50% lower power consumption. Other benefits include higher reliability, frequency programmability, flexibility, and lower cost.

These benefits are derived from Multigig’s patented RotaryWave technology, which makes an oscillator with a travelling wave technique. The heart of the oscillator is a differential transmission line whose length, inductance, and capacitance set the frequency. It comes with taps and amplifiers to make it a distributed amplifier. The transmission line creates a closed loop with the ends connected together with a half twist like in a Mobius strip. The result is a very stable oscillator with low jitter and low phase noise.

QuietClock synthesizers provide an ultra-low jitter and phase noise reference clock signal to communication, computing, and networking interface devices. Typical applications include Gigabit Ethernet (1GE), 10 Gigabit Ethernet (10GE), Sonet/SDH, Fibre Channel, Rapid I/O, SATA, and InfiniBand. Most standard frequencies are available. The family uses supplies of 3.3 and 2.5 V, incorporates one to 10 outputs from a mix of single-ended (LVCMOS) and differential (LVPECL and LVDS) outputs, and supports the full industrial temperature range of 40°C to 85°C.

More than a dozen different models are available (Fig. 2). Sampling now, prices start at $2.75 for 1000-unit quantities.

MEMS Oscillators Becoming Popular

Oscillators based on MEMS silicon resonators are gaining in popularity as a crystal replacement. Their precision and stability specifications are certainly competitive. And despite the factory programming, delivery times tend to be shorter than those of crystal oscillators.

The Ecliptek EMO series of MEMS oscillators offers frequencies ranging from 1 to 125 MHz with a tolerance of ±100 ppm and a stability of ±50 ppm max. That’s not bad at all, and it’s certainly good enough to replace XOs in many applications.

The EMK13 oscillator comes in a standard 5- by 7-mm package. It uses a 3.3-V supply and has an LVHCMOS output. The jitter spec (peak-to-peak) varies with output frequency, but it’s typically 200 ps at frequencies up to 12 MHz and 100 ps in the 12- to 125-MHz range. It also has 30,000-G shock resistance. Other package sizes include 3.2 by 5 mm and 2 by 2.5 mm, which is much smaller than available XO packages. Models for supply voltages of 1.8 and 2.5 V are available as well.

The SiT8002XT from SiTime Corp. is one of the thinnest programmable oscillators available, measuring 3.5 by 3 by 0.25 mm. It can be programmed from 1 to 125 MHz with a tolerance of ±100 or ±500 ppm. Peak-to-peak jitter is typically 60 ps at 100 MHz.

SiTime’s SiT9002 is programmable from 10 to 220 MHz. It comes with tolerance ranges of ±25, ±50, or ±100 ppm. Jitter is commonly less than 30 ps. Packages measure 5 by 7 mm and 3.2 by 5 mm. Voltage supply options include 1.8, 2.5, and 3.3 V. Its frequency can be modulated over a narrow range at a 31.5-kHz rate, greatly reducing the EMI in some applications (Fig. 3).

The SiT9001 is available in a 2.5- by 2-mm package and is programmable from 1 to 200 MHz. SiTime promises samples in 24 to 48 hours with full production quantities in two to three weeks.

Goodbye Crystal Oscillators?

Not so fast. While the LC, MEMS, and other non-crystal oscillators offer real advantages such as programmability, rapid delivery, and shock and vibration resistance, their tolerance and stability figures leave something to be desired for some more critical applications.

When you need stability figures in the ±1- to ±10-ppm range, it’s hard to beat an XO. You can get a temperature-compensated crystal oscillator (TCXO) that easily meets those specs. If you need stability figures in the parts per billion (ppb) range, an oven-compensated crystal oscillator (OCXO) will do the job. But for the less critical frequency-control needs, other options are worth considering. And the new MEMS and LC oscillators are available in smaller sizes that are perfect for the next generation of smart phones.

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