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It is often necessary to operate quartz crystal resonators at very high frequencies for some RF designs - it is sometimes necessary to be able to use the high Q and performance of a quartz crystals at high frequencies.
Using an overtone mode of operation for the quartz crystal resonator enables the electronic component to operate at the very high frequencies whilst still remaining robust and easy to manufacture.
Reason for using overtone operation for quartz crystals
Quartz crystals use the mechanical resonances of the quartz element to provide the high levels of Q and general performance. This is linked to the electrical circuit as a result of the piezoelectric effect.
As the frequencies of operation increase, the fact that the mechanical resonates determine the frequency of operation means that the quartz blanks become smaller.
Although manufacturing techniques have improved immeasurably in recent years it is still found that as frequencies increase, the cost of processing blanks that are very thin increases and the reject / failure rates rise.
Quartz crystal overtone operation
Whilst the most obvious method of using a quartz crystal resonator is to operate it at its fundamental frequency, operating a crystal in its overtone mode provides some distinct advantages.
Overtone frequencies are at around three, five, seven, etc times the fundamental frequency. This means that the quartz crystal blanks can have a fundamental frequency much less than the final intended frequency of operation, making the blanks much easier to manufacture and much less fragile.
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 untuned 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 30 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.
When ordering crystals for operation in an overtone mode, this must be specified tot he manufacturer so that the blank can be ground to operate correctly at the frequency required. Often manufacturers have a specific order form for crystals, and one of the questions to be answered is whether the crystal is to be operated in overtone mode.
Circuits for overtone crystal oscillators
When using a crystal in its overtone mode it is key that the electronic circuit design ensures that the quartz crystal operates in its overtone mode and it does not operate in its fundamental and a required harmonic selected later.
The reason for this is that the harmonic of the fundamental, and the frequency of the crystal operating in its overtone mode will be slightly different. This can make a significant difference in some instances.
To overcome this, the RF design for the circuit must incorporate a tuned circuit that ensures the feedback int he oscillator occurs at the overtone frequency, and rejects the fundamental frequency. This will ensure that the oscillator operates at the overtone frequency and not the fundamental.
Overtone quartz crystals are very useful in many RF design or other general electronic circuit design applications where the stability and performance of a crystal oscillator is required at a frequency that is higher than that which can be comfortably achieved using a standard fundamental mode crystal. In these cases the cost of the overtone crystal is much less than that of a quartz crystal that would be manufactured to operate at a fundamental mode at the high frequency needed.
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