As they say, timing is everything. That’s certainly true for most electronic products. Every electronic product includes a clock oscillator for timing its operations, whether it’s a simple embedded controller, a 3G handset, or the most complex telecommunications gear with multiple timing chains.
Try to name a product that doesn’t contain a clock oscillator. True, there are vacuum-tube guitar amplifiers. But modern solid-state guitar amps have built-in controls with a microcontroller, so they have a clock too. Anyway, you get the picture.
Up until recently, virtually all clocks were of the quartz crystal-oscillator (XO) type. They’re precise, stable, and widely available. Yet they have some downsides, such as their sensitivity to vibration and shock as well as their high price. And if you want a special frequency other than some standard off-the-shelf (OTS) values, the shipping time ranges from many weeks to months as the crystal house grinds the quartz to your special needs.
In the past, you had to put up with these problems. But now you have some real options when it comes to selecting your clock. Over the past several months, many silicon oscillators based on micro-electromechanical systems (MEMS), LC, and other technologies have appeared on the market with stability and precision figures competitive with quartz oscillators at prices and delivery times you can love.
LC Oscillators Make a Comeback
How can an ordinary IC oscillator whose frequency is set by a resonant inductor-capacitor (LC) combination meet XO stability and precision specs? By automatically compensating for supply voltage, temperature, load changes, and other variations, it can meet these modern XO requirements. At least , that’s how the Silicon Laboratories Si500 series does it.
The Si500 uses a 3-GHz on-chip LC oscillator with feedback stabilization (Fig. 1). Combined with Silicon Labs’ famous digital phase-locked loop (PLL) and some programmable dividers, the result is a fully silicon oscillator that can be programmed for any frequency in the 0.9- to 200-MHz range. Silicon Labs performs the programming, but the usual ship time is at least six times faster than what traditional XO manufacturers can deliver.
As for specs, there are Si500 models that can meet stability specifications of ±100 to ±150 ppm. The jitter spec for the Si500 is 1.5-ps rms phase jitter and 2-ps total period jitter. Typical supply current is 8 mA. The Si500 also can support differential clock outputs such as low-voltage positive emitter coupled logic (LVPECL), low-voltage differential signaling (LVDS), and hybrid computation and simulation (HCSL), in addition to single-ended complementary metal-oxide semiconductor (CMOS) and stub series terminated logic (SSTL) formats. In its dual-output CMOS mode, a single device can produce two output clocks at the same frequency, eliminating the need for external clock buffers.
The Si500 package is a standard 3.2- by 4-mm dual flat no-lead (DFN) with four or six pads. There is no hermetically sealed ceramic or metal package like those used in XOs, so it’s immune to the potential contamination that can plague XOs. The packaging is also immune to shock and vibration that can affect XOs or even MEMS mechanical resonators. Pricing is in the $0.95 to $2.24 range in 10,000-unit quantities depending on the model.
A while back, Mobius Microsystems introduced its CMOS Harmonic Oscillator (CHO) technology, which uses a gigahertz LC oscillator with extensive temperature and voltage feedback stabilization with a programmable divider that can deliver an output frequency ranging from 100 kHz to several hundred megahertz. It incorporates spread-spectrum modulation from 0% to 6% of the operating frequency, helping designers reduce electromagnetic interference (EMI) in RF designs by up to 15 dB.
The Mobius MM8511 is available in any frequency in the 10- to 100-MHz range. Its temperature stability is well within the ±100-ppm range. It operates from a 3.3-V supply and comes in an eight-pin DFN package of 3 by 3 by 0.75 mm or 3 by 6.4 mm. Pricing is $1.35 in 1000-unit lots.
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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