Clock Oscillators - The Heart Of The Handset
As they say, timing is everything. Thatís certainly true for most electronic products. Every electronic product includes a clock oscillator for timing its operations, whether itís a simple embedded controller, a 3G handset, or the most complex telecommunications gear with multiple timing chains.
Try to name a product that doesnít 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. Theyíre 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 , thatís 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 Labsí 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 Ī100 to Ī150 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 itís 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 Ī100-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.
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