DOUBLE OVEN CRYSTAL OSCILLATOR (DCXO) WITH TEMPERATURE
COEFFICIENT <1×10-10 -55°C TO 70°C |
A new oscillator designed to meet the requirements of today's CDMA base station will be presented. The design consists of three basic parts, the oscillator, frequency synthesizer and thermal controller. Computer simulations revealed new insights into the temperature characteristics of crystal oscillators and patent pending designs for the frequency synthesizer and thermal control architecture will be presented.
The DCXO architecture is based on an oscillator designed primarily for the lowest and most predictable temperature coefficient possible. Classically, initial frequency element placed in series with the crystal. This requires the crystal frequency to be within its tuning range and adds temperature sensitive components such as inductors and varactor diodes to the oscillator circuit. A frequency synthesizer was designed to perform the frequency calibration and aging adjustments. The oscillator and crystal subsequently were designed for the optimal temperature stability, phase noise and aging performance. A 5MHz 5th overtone crystal was designed for low aging, high stability and suppression of the 3rd overtone C and B modes thus further simplifying the oscillator design.
Direct synthesizers (DDS) have some major limitations such as limited frequency resolution and a highly spurious output spectrum. With a unique architecture a PLL was combined with a 32 bit DDS resulting in a frequency synthesizer with resolution less than 3×10-12 and an output spectrum free of spurious signals.
The thermal control consists of a double oven which has thermal gain greater than 100,000. This is accomplished with a zero temperature gradient outer oven. The zero gradient oven uses a system of satellite heating semiconductors controlled by a main controller. The heat dissipation of the satellite heaters and the controller is optimized over the outer oven surface in such a way as to minimize the temperature change of the crystal to less than 0.001°C over the ambient temperature range of -55°C to 70°C. |
