The most obvious method of using a quartz crystal resonator is to operate it at its findamental frequency. However as frequencies increase, the crystal blanks become much thinner and mosr fragile. To overcome this and extend the frequency of oepration, crystals can be operated in an overtone mode.
For an AT cut crystal, these overtone modes are nearly at the overtone number times the frequency of the fundamental. In fact the actual frequency of the overtone more nearly equates to the series mode fundamental frequency times the overtone number.
It should also be remembered that crystals may also have many other modes of operation that can be exercised. These modes are unwanted and may be excited to lesser or greater degrees by different circuits. Care should be taken especially when using crystals in unturned digital circuits as unwanted modes may unexpectedly dominate. Any tuning will naturally tend to suppress these unwanted modes.
The major applications for overtone crystals are for frequencies above 10 MHz and more. Here the crystals typically vibrate in a thickness shear mode and the crystals can be excited in either fundamental or odd overtones. It is found that the motional capacitance C1n of an overtone crystal decreases. It follows an approximate law where the capacitance for the nth overtone is the capacitance for the fundamental divided by the square of the number of the overtone.