Comparing the differences between AT and SC crystals and their impact on oscillator performance.
Inside of every quartz oscillator is something called a crystal blank. The crystal blank is the resonating element of the oscillator that when subjected to a voltage potential will begin to vibrate and oscillate at it’s "fundamental frequency.” As you can probably imagine, the way that crystal blank is manufactured can have a significant impact on the oscillators performance. One of the most impactful manufacturing steps to crystal, and therefore oscillator performance, is the orientation of the quartz when the crystal blank is cut.
The crystal blank is cut from a larger piece of bulk quartz and since quartz is a crystal the internal structure of the material is a predicable lattice structure. Therefore, the orientation or angle of the lattice structure can create different ‘cuts’ of crystal blanks. Two of the most common quartz crystal cuts used in OCXOs (oven controlled oscillators) are AT cuts and SC cuts. And just like any other engineering or design decision, they both come with their own set of advantages and disadvantages that must be traded during the design phase of the frequency control device.
The AT is a temperature compensated cut, meaning the cut is oriented such that the temperature coefficients of the lattice will have minimal impact on crystal performance.
The SC (stress compensated) is also temperature compensated. Originally developed in 1974, the SC cut is a double rotated cut (similar to a compound miter cut for the woodworkers out there).
When selecting an oscillator for a specific application, it is important to understand the type of crystal at the heart of the oscillator. The key parameters that differ between AT cuts and SC cuts are frequency vs. temperature stability, crystal aging, g-sensitivity, initial frequency accuracy, availability, and cost.
Frequency vs. Temperature
Frequency vs. temperature stability is a spec that describes how the frequency output of the oscillator changes over temperature. FvT performance is measured in parts per billion or PPB. The SC cut will deliver improved performance in the oscillator. Main reason is the slope of the curve in the vicinity of the crystal’s turning point. Typically, the slope of the SC cut is 1/3rd that of an AT cut with same turning point. Another area where the SC cut delivers improved FvT performance is operation at elevated temperature ranges (-20C to +200C) seen in downhole applications. With an inflection temperature of approximately +90C compared to +27C for AT, the SC will achieve tighter stability over these ranges. The improvement could be as much as 5X over this extended temperature range.
The crystal aging is a spec that describes how the frequency of the crystal changes over the life of the crystal. Crystal aging occurs as a result of impurities within the oscillator. Aging is also measured in PPB. The SC cut is less sensitive to some of the aging effects, such as crystal mount stresses, crystal blank plating stresses, changes in performance of the electronics.
G-sensitivity describes the impact of shock and vibration to the frequency output of the oscillator. G-sensitivity is measured in PPB/G. Because the quartz crystal is an electro-mechincal device that vibrates under a voltage potential, introducing a vibration can cause the electrical output characteristics to change. In fact, g-sensitivity is a major contributor to the phase noise of an oscillator.
The following table summarized the comparison between AT and SC crystals when used inside OCXOs: