Parasitic problem of quartz crystal component

Problem and Phenomenon: After installing quartz crystal resonator and electro-circuit, frequency is not in accordance with the required value, which will make electro circuit not work.                           

Cause analysis: In most cases the condition is related with parasitic oscillation, sometimes concerning the unreasonable design of electro circuit. As mentioned above, quartz crystals usually have many parasitic responses, and above all, the overtone resonator is liable to generate parasitic responses. To thickness shear quartz crystal resonator, the frequency is above 20MHz. The difference between fundamental resonator resistance of overtone crystal component and resonator resistance of 3rd overtone unit sometimes is little, which may oscillate on fundamental frequency in unfiltered circuit，therefore, it is advisable to avoid this situation by the addition of frequency selection circuit in the design of electro circuit.                                                   

Another reason is the 3rd harmonic of fundamental frequency of overtone crystal resonator is close to the frequency of 3rd overtone unit, so it is difficult for frequency selection circuit to filter, which may cause frequency hopping.                 

If this situation happens, it is suggested that the interval requirements should be proposed when ordering products, usually between 150-250 PPM. It can also reduce circuit gains duly to decrease 3rd harmonic element.