Issue 91, February 1998
Choosing the Right Crystal For Your Oscillator

Start •Why Quartz Crystals • Timing Budget & Accuracy • Frequency Tolerance • Frequency Stability • Aging • Load Capacitance • Series and Parallel Resonance • Frequency Tolerance and Load Capacitance • AT vs. BT Cut • Mode of Operation • Package Considerations • Crystal Placement • Crystal Clear • References

MODE OF OPERATION

The crystal mode of operation largely depends on the operating frequency. Up to 50 MHz, the mode of operation is fundamental. Above 50 MHz, the mode is probably overtone.

Note that overtone frequencies are not harmonics of the fundamental frequency, although they are close. Harmonics are exact integer multiples, while the overtones are not.

However, overtone frequencies are always odd multiples of the fundamental frequency. Both AT- and BT-cut crystals are available for overtone use.

Overtone operation is a nontrivial effort. Oscillators that run over 50 MHz must run in the overtone mode.

You don’t see high-frequency fundamental crystals because the crystal becomes too thin. Crystal thickness is inversely proportional to resonant frequency, so high frequencies translate to thin crystals. And, thin crystals are expensive because they’re difficult to manufacture and handle.

With overtone crystals, the thickness is greater than that of the fundamental crystal [3]. The overtone mode multiplies the thickness. As Figure 4 shows, a third overtone crystal is three times thicker than the comparable fundamental crystal.

Figure 4—The thickness of the third-overtone crystal slab is three times that of the fundamental-mode crystal slab.

There are a few disadvantages to using overtone crystals. The first is that the oscillator must be designed to specifically operate at the overtone frequency and not the fundamental frequency. Therefore, the oscillator must contain a filter to avoid the fundamental frequency.

Another disadvantage is that overtone crystals tend to be thicker than the fundamentals. This translates into larger ESRs, hence lower Q. Care should be take to ensure reliable oscillator startup and operation.

A third disadvantage is that overtone crystals can contain spurs (i.e., short for spurious mode, an unwanted type). The crystal manufacturer has to make sure that spurious modes are sufficiently suppressed. If they aren’t, the oscillator can run at the wrong frequency.