MultiMode: The concept behind SmartXtal
Crystal Oscillators are used as frequency and timing reference in electronic equipment. Their frequency stability is of outmust importance. The stability over a short time interval is limited by the frequency drift due to changing temperature conditions. A 1st approach to stabilize the temperature drift is the approach in Oven Controlled Crystal Oscillators (OCXOs). In these conventional OCXOs specialized SC-cut crystals are used that exhibit zero temperature drift at about 80°C. As the name indicates these crystals are then placed in an oven that operates at that specific temperature of 80°C. Then the Oven should be controlled to keep it at exactly this temperature. A disadvantage of this solution is that this results in a relative large and costly device. A different approach is to implement temperature compensation: temperature measurement, calibration and correction. These are the so called Temperature Compensated Crystal Oscillators (TCXOs). The performance is however limited since the temperature of the sensor does not perfectly track the temperature of the crystal.
In SmartXtal we use MultiMode technology, we determine the temperature by means of a triple mode oscillator and correct this by means of an algorithm running on the internal microprocessor. The difference between the 3 frequencies is highly temperature dependent. This relationship is stored during calibration and during operation used by a frequency synthesizer to correct one of the outputs of the triple mode oscillator. The advantage of this is that it uses regular AT-Cut crystals and the whole circuitry can be implemented in silicon, resulting in a small and very cost effective, high performance crystal oscillator solution.
The MultiMode Technology
The MultiMode technology consists of:
- A triple mode oscillator
- Frequency measurement system
- Microprocess with memory
- Digital controlled frequency synthesizer
connected to a low-cost AT-Cut crystal resonator.
The triple mode oscillator produces 3 independent output frequencies, each linked to a different resonance of the crystal resonator. These 3 frequencies each have a different temperature behavior, and thereby the delta frequencies also have a unique temperature dependency. This temperature dependency is calibrated during production, and during normal operation this informed – stored in memory – is used by the on-board algortihms to accurately temperature compensate the output frequency of the SmartXtal by means of the onboard digitally controlled frequency synthesizer.