Advantages and challenges of third-overtone IC crystals

  
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ICs with on-board oscillators requiring low-frequency fundamental crystals are commonplace, but nowG‚IC frequency multipliers require the higher frequencies of third overtone crystals. Compared to its fundamental implementation however, the third-overtone (3OT) crystal oscillator is more complex, with different crystal characteristics plus additio
Advantages and challenges of third-overtone IC crystals - schematic

nal reactive elements and higher gain. Most crystals use the fundamental thickness-shear vibration mode of AT-cut quartz. The practical upper limit of fundamental operation is around 50 MHz because the fundamental resonant frequency is inversely proportional to thickness. Frequencies higher than 50 MHz become too thin to handle during the manufacturing process. While a few crystal manufacturers can make fundamental crystals to 600 MHz or more using the inverted mesa process, 3OT manufacturers are more prevalent. The 3OT is a crystal mode that resonates at three times its fundamental frequency. There are in fact an infinite number of odd harmonics that exist on the same quartz plate. The first, third and fifth harmonics are shown in Figure 1a with some anharmonic modes or spurs shown in between. Spurs are by-products of other vibrational modes and a good crystal manufacturer makes sure that these are minimized. G‚ The AT-cut crystal has the equivalent circuit (see Fig. 1b) of a series RLC for the fundamental and each harmonic mode plus a capacitor, C0, in parallel due to the electrodes. The 3OT series resistance, Rs, is more than three times that of the fundamental mode. Its series capacitance, Cs, is nine times less. This lower Cs in the 3OT will make tuning or calibration harder in the oscillator. If the crystal is too far off nominal frequency, there is less chance to electrically correct this in the oscillator. The 3OT...



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