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AT-cut vs. SC-cut Resonators |
| Of the many crystal cut types available, most applications use either AT-cut or SC-cut quartz resonators, and in many instances both cuts are candidates. It is worthwhile to compare them on the basis of eight characteristics: |
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Static f-T Characteristic: Both AT- and SC-cut resonators have a static frequency-temperature characteristic that is well described by a third-order polynomial in temperature. For the AT-cut, the inflection temperature [at which the curvature changes sign] is within a few degrees of room temperature. In other words, the f-T curve is approximately antisymmetric about a temperature close to 25°C. This makes the AT-cut suitable for non-temperature-controlled applications, such as simple oscillators and TCXOs. By contrast, the inflection temperature for the SC-cut resonator is 90° to 95°C. Most SC-cuts are used in OCXOs, where the higher inflection temperature results in an important advantage -- the f-T curve is quite flat in the vicinity of the oven set point, which is adjusted to the resonator upper or lower turnover temperature. Consequently, temperature control is less critical than if an AT-cut were used. The improved dynamic f-T characteristic of the SC is also a contributing factor to allowing less stringent temperature control.
Dynamic f-T Characteristic: When a step change is made to the ambient temperature, the frequency of an SC-cut resonator changes smoothly, without overshoot or ringing, in a manner corresponding to a critically- damped system. The AT-cut dynamic f-T characteristic, on the other hand, has very pronounced overshoot and ringing. An important reason for choosing an SC-cut for an OCXO is its greatly improved dynamics, which is, in fact, the hallmark of a properly designed SC-cut. For critical applications, the term "properly designed" should not be taken lightly. It means that for each design, the orientation angles must be selected to obtain this thermal transient compensation. Small changes in orientation angle can lead to significant degradation of the thermal transient response. Skipping this design step may save design time, and money, but the user pays in performance.
Aging: If we compare equal frequency and overtone, the SC-cut is slightly better than the AT-cut, on account of its slightly greater thickness. This difference, however, is usually insignificant. Because the greater current-handling capability of the SC renders its frequency less sensitive to current changes, in oscillators the SC-cut may exhibit better aging.
Acceleration Sensitivity: At present, AT-cut and SC-cut resonators are equally good choices for low acceleration sensitivity. It may be worth noting that when the SC-cut resonator was introduced, there was a belief, by some, that it would be insensitive to acceleration. This has not proved to be the case.
Current Handling: This refers to the maximum current at which a resonator can be operated without a significant (reversible) change in frequency. Generally, this is markedly higher for SC-cut than for AT-cut resonators. The consequences for oscillator applications are an improved phase noise floor by operating a higher current, and reduced sensitivity to drive level change, which may affect aging of the oscillator frequency.
Impedance Level: If we compare AT-cut and SC-cut resonators having the same frequency and overtone, the motional inductance (L1) and motional resistance (R1) of the SC-cut resonator will be significantly higher than for the AT-cut resonator, while the motional capacitance (C1) will be lower in the same proportion. The static capacitance (C0), however, will be nearly the same for the two. (The exact ratios depend upon a number of design details.) For oscillator applications, high impedance is desirable if aging is important, because it reduces the effect of the sustaining circuit on the oscillator frequency. The trade-off is that high resonator impedance reduces the tuning range of the oscillator frequency, limiting corrections for manufacturing tolerance and for aging. In VCXOs, aging is, at most, a secondary consideration; the required tuning range will determine the highest practical overtone. For either cut, the impedance level increases roughly as the square of the overtone for a given frequency, but also depends upon details of the resonator design.
Size: For most applications, the sizes of the AT-cut and SC-cut resonator packages are the same.
Cost: The user should consult PTI Sales
with regard to his specific application. Generally speaking, an SC-cut
resonator is somewhat more expensive than an AT-cut resonator because of
the tighter orientation angle requirements of the SC-cut, but the SC provides
important advantages in OCXOs, as discussed below.