How to choose the correct temperature crystal
Precision, low power consumption and miniaturization, is still the study of temperature compensation crystal. In the miniaturization and chip type, faced with many difficulties, including the main two points: First, miniaturization will make the quartz crystal oscillator frequency variable amplitude smaller, more difficult temperature compensation; Second, after the chip package It is difficult to change the frequency change of the temperature compensation crystal to ± 0.5 if the welding temperature is much higher than the maximum allowable temperature of the temperature compensation crystal, and the frequency of the crystal oscillator will be changed. × 10-6 or less. However, the technical level of warming up the crystal did not enter the limit, the innovative content and potential is still large.
We briefly introduce the parameters of temperature compensation crystal requirements:
1. Frequency accuracy, the maximum allowable deviation of the frequency of the quartz crystal oscillator relative to its specified nominal value shall not exceed the standard supply voltage, the standard load capacitance impedance, the reference temperature (25 ° C), and other conditions remain unchanged ± 50.
2. The temperature stability and other specifications are kept constant. The maximum variation of the output frequency of the quartz crystal oscillator in the specified temperature range is the allowable frequency offset of the sum of the output frequency extremes in the temperature range, ie (fmax-fmin ) / (Fmax + fmin).
Frequency adjustment range: Change the range of the output frequency by adjusting a variable element of the crystal.
FM (voltage control) features: including frequency offset frequency, FM sensitivity, FM linearity.
① frequency deviation: voltage-controlled oscillator control voltage from the nominal maximum value to the minimum output frequency difference.
② FM sensitivity: voltage-controlled crystal oscillator changes the unit plus the control voltage caused by the output frequency of the amount of change.
③ FM linearity: a measure of the transmission characteristics of a modulation system compared to an ideal straight line (least squares method).
Load characteristics: other conditions remain unchanged, the load within the specified range of crystal oscillator output frequency relative to the nominal load output frequency of the maximum allowable frequency offset.
Voltage characteristics: other conditions remain unchanged, the power supply voltage within the specified range of crystal oscillator output frequency relative to the nominal power supply voltage output frequency of the maximum allowable frequency offset.
Clutter: The power ratio of the discrete spectral component to the main frequency in the output signal with no harmonics (except for the sub-harmonics) is expressed in dBc.
Harmonics: the ratio of the harmonic component power Pi to the carrier power P0, expressed in dBc.
Frequency Aging: The system drift process of the output frequency over time due to the aging of the element (mainly the quartz resonator) under the specified environmental conditions is usually measured by the frequency difference within a certain time interval.For a high stable crystal, Since the output frequency is approximately linearly unidirectional drift in the longer working time, it is often measured with the aging rate (relative frequency change per unit time).