Technical Paper: Pluto -- A Novel 4th-Order Temperature Compensation ASIC Brings Stratum 3 Frequency Stability Within a Miniature SMD
By Nigel Hardy
C-MAC MicroTechnology
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, whereas 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 size and power requirement, without sacrificing performance of course. To reconcile these apparently contradictory requirements, C-MAC MicroTechnology has developed Pluto,a new analogue 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 at once more compact, more versatile, lower voltage and higher frequency than their predecessors.
The Pluto device is a full custom ASIC implemented in 0.8 µm CMOS on a 3.2 mm x 2.3 mm silicon die. Despite its small size, it crams in a high degree of functionality, including digitally controlled analogue temperature compensation, frequency adjustment by voltage control ("frequency pulling"), linearization of frequency pulling, overtone selection,synchronous voltage multiplication for low-voltage operation, and three different output buffers all with tristate.
Pluto's key function, "fourth-order" analogue 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 linearization technology, target TCXOs using fourth-order compensation can achieve stability previously only available using larger, more expensive digitally compensated or higher-order analogue compensated devices.
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