Temperature is an important factor in determining the stability of crystal oscillators. By compensating for the effects of temperature on the oscillation frequency of the crystal, a TCXO (temperature compensated crystal oscillator) can exhibit up to 50 times better stability over its operating temperature range than a comparable uncompensated oscillator.
For example, where an uncompensated device might offer ±20 ppm stability over the temperature range from -40° to 85°C, a TCXO of otherwise equivalent specifications might be stable to within ±0,4 ppm over the same temperature range.
Adding temperature compensation to an oscillator requires considerable extra circuitry, which leads to a bulkier device with higher power consumption. However, the trend in communication systems design, particularly for mobile equipment, is toward ever-smaller component sizes and power requirements, without sacrificing performance, of course.
To reconcile these apparently contradictory requirements, C-MAC MicroTechnology has developed Pluto, a new analog temperature compensation chip for high-stability TCXOs. So named during its development phase because it was to be 'small and cool', Pluto allows high-stability TCXOs to be made more compact and more versatile than their predecessors. Not only that, the parts can operate from a lower supply voltage and at a higher frequency.
The Pluto device is a full custom ASIC implemented in 0,8 µm CMOS on a 3,2 x 2,3 mm silicon die. Despite its small size, it crams in a high degree of functionality, including digitally-controlled analog temperature compensation, frequency adjustment by voltage control (frequency pulling), linearisation of frequency pulling, overtone selection, synchronous voltage multiplication for low-voltage operation, and three different output buffers all with tristate.
Fourth-order compensation
Pluto's key function - fourth-order analog temperature compensation of output frequency - is achieved by generating zero to fourth-order polynomial Chebyshev functions of temperature. These functions are weighted via multiplying D/A converters, superimposed through a summing amplifier and applied to the VCXO (voltage controlled crystal oscillator) signal generator. Due to C-MAC's proprietary linearisation technology, target TCXOs using fourth-order compensation can achieve stability previously only available using larger, more expensive digitally-compensated or higher-order analog compensated devices.
Pluto's linearisation technology also means that frequency pulling is possible without significantly degrading compensation accuracy at the extremes of the adjustment range. This offers two key benefits: performance is maintained even after adjustment for crystal ageing effects; and 'holdover' performance is improved, making the product suitable for use in SETS (station equipment timing synchronisation) applications, which are required to meet Stratum 3.
Pluto incorporates circuitry that allows either the crystal's fundamental frequency or its third overtone to be selected, giving an oscillation frequency range in excess of 10 to 80 MHz. An on-chip two, four or eight times frequency divider is also incorporated, so that output frequencies less than 10 MHz can be achieved. Three separate output buffers can provide sinewave, low-power square wave (HCMOS) or high-drive square wave (ACMOS) output, all of which may be inhibited to give a high impedance output. Power consumption can be kept at less than 7 mW at low power supply and load conditions, making Pluto-based oscillators particularly suitable for high-performance portable systems.
As 'synchronous' voltage multiplication - a DC/DC converter fed from the crystal oscillator - is included within Pluto, the frequency pulling range can be maintained even for low-voltage mobile applications (down to 2,4 V) without creating spurious sidebands.
Family planning
C-MAC incorporates the Pluto chip within several families of TCXOs. The CFPT-9000 range of miniature TCXOs offers one of the world's highest stability per unit volume - better than ±0,3 ppm over the temperature from 0 to 50°C from a surface mountable device measuring just 7 x 5 x 2 mm.
The CFPT-9000 design concept, with the Pluto device sandwiched between the packaged crystal oscillator and an LTCC (low-temperature co-fired ceramic) substrate, was a response to requests from telecom manufacturers for a small, standard-footprint SMD compatible with existing low-specification oscillators, but exhibiting Stratum 3 stability.
Stratum 3 is the Bellcore/ITU-T (SETS) requirements for retransmission equipment used in non terminal nodes of SDH/SONET telecommunications networks. Its key stipulations as far as oscillator performance is concerned are: free run accuracy to within ±4,6 ppm; holdover stability to within ±0,37 ppm for the initial 24 hours without a reference clock; and sufficient frequency pulling to allow reference clock synchronisation to within ±4,6 ppm accuracy (ie, around ±9,2 ppm to account for deviations in both the oscillator and the reference clock).
The CFPT-9000 delivers its optimum stability rating - within ±0,28 ppm - over an operating temperature range of 0 to 50°C for standard telecommunications output frequencies at 12,8, 13, 16, 384, 16,8 and 19,44 MHz. However, frequencies up to 40 MHz are available at lower stabilities, and wider operating temperature ranges can also be specified, extending the range of applications beyond the original Stratum 3 brief. For example, a device offering ±1 ppm stability over an extended -40 to 85°C temperature range is likely to prove popular in 3G mobile applications.
The CFPT-9050, a more versatile TCXO family, uses Pluto's fundamental/third overtone selection circuitry to provide output frequencies from 1 MHz right up to 80 MHz. Packaged as an FR4-based 14 x 9 mm SMD with an industry standard six-pad SOJ-20 compatible footprint, this device is also Stratum 3 capable.
Versatile frequency pulling characteristics fit the CFPT-9050 for a wide range of applications. Frequency pulling can be disabled completely for high stability - for instance when used in SETS applications incorporating synthesizer-based digital PLLs. It can be set to around ±10 ppm for simple ageing adjustment or for use in analog PLLs. Alternatively, it can be specified to more than ±32 ppm for Stratum 4 PLL applications, providing the 'low-stability clock' in SDH/SONET terminal nodes.
Low phase noise
Low phase noise characteristics, (-120 dBc/Hz at 100 Hz) achieved by optimisation and attention to fine detail in the design of both the functional blocks and individual components of the Pluto ASIC - bring additional potential applications in GPS systems and mobile communication base stations.
Plans are already being made for 'Pluto 2', with chip geometries reduced to 0,6 µm and additional functionality aimed at key applications like broadband mobile communications. In a few years' time, when you are streaming a new movie from the Internet onto your PDA for instant playback via a lightweight video headset, it may well be Pluto 2 that is keeping the signal on track.
For more information contact Kevin Murphy, Arrow Altech Distribution (local C-MAC MicroTechnology representative), 011 923 9600, [email protected]
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