Other Crystal Oscillator Types
In addition to the obvious use of a microprocessor to “calculate” a frequency correction voltage to apply to an electrical tuning input, microprocessors may employ syntheses to achieve frequency accuracy. SC-cut crystals offer many stability advantages over AT-cuts but they have a steep frequency drop at colder temperatures, dropping in frequency far more than they can be tuned, making them unsuitable for ordinary TCXOs. However, a microprocessor programmed with the crystal’s offset frequency over temperature may be used to synthesize the correct frequency. The synthesizer may be a crude “pulse swallower” or a modern numerically-controlled oscillator (also called DDS – direct digital synthesis) depending upon the phase noise and resolution requirements. In addition to correcting for the crystal’s temperature drift, the DDS can generate an arbitrary output frequency on demand. The correction function need not be in the same package as the oscillator if the system includes a computer-controlled synthesizer. The oscillator may simply supply crystal temperature data and correction coefficients or a correction look-up table.
A microprocessor outfitted with a non-volatile time accumulation register can be programmed to apply aging correction to a precision ovenized SC-cut crystal oscillator achieving stabilities approaching Rubidium standards but with superior short-term stability and phase noise. These systems usually require the application of an external standard for an extended period of time so that the microprocessor can “learn” the oscillator’s aging rate. When a standard is not available for extended periods, the correction rate may be manually adjusted as part of a regular calibration procedure.
When Rubidium accuracy is needed but the power consumption must be low, a “sleeping” Rubidium system is appropriate. These systems combine a low power oven oscillator with a Rubidium standard that is normally off. Once or twice per day power is applied to the Rubidium standard just long enough for it to stabilize so that the crystal oscillator may be corrected. The total energy required per day is far below the energy required to keep the Rubidium standard operating.
Oscillators may also be disciplined with transmitted timing signals including Loran-C, GPS, WWVB, Omega, cellular, CDMA and similar systems providing precision time and frequency information either as a primary or secondary function. Shortwave band transmissions including those from WWV are also useful but sky-wave variations limit the accuracy.
Oscillators with more than one crystal address special stability or frequency agility requirements. A VHF crystal may be phase-locked to a low frequency crystal to achieve low VHF phase noise with the close-in noise and low aging of the low frequency crystal. Electronically switched crystals can provide multiple frequency outputs with phase noise performance not available from synthesizers.