Crystal Oscillator

Crystal Clock Oscillators

Introduction:

Simple packaged oscillators of moderate stability are referred as Crystal Clock Oscillators. These are available in 14 pin and 8 Pin Dual in Line Packages.

Initial Accuracy of Clock Oscillators

The initial accuracy of the oscillator is the difference in the frequency between the actual output frequency of the oscillator and the specified frequency at 27° C at the time of manufacture and a specific time following shipment for non frequency adjustable oscillators such as the clock oscillators. If there is a provision for frequency adjustment by the user by means of Voltage control or mechanical trimming, then the need to specify the initial frequency accuracy is no longer needed as the user can adjust the oscillator to nominal frequency.

Stability

The frequency vs. temperature stability in a Clock oscillator is the characteristic of the crystal blank used in the oscillator. The frequency Vs. Temperature characteristic of an AT cut crystal unit describes a cubic curve that has an inflection or inversion point at room temperature. Inversion points are the points of zero temperature coefficients and there are two inversion points: the lower and upper inversion point. Changing the crystal blank cutting angle q can change the slope of the curve and the points of inversion.

Temperature Stability is defined as the peak to peak change over the entire operating temperature range referenced to the nominal frequency and not referenced to the frequency at room temperature or any other specified temperature.

MIL-0-55310 described the overall frequency temperature accuracy as :

The initial maximum permissible deviation of the oscillator from the assigned nominal value due to operation over the specified temperature range at nominal supply and load conditions, other conditions remaining constant. For non-frequency adjustable (Manufacturer calibrated) oscillators, the initial frequency-temperature accuracy applies at the time of manufacture and a specified period following shipment. For frequency adjustable (manufacturer–user calibrated) oscillators, the initial frequency-temperature accuracy applies at the time immediately following calibration by manufacturer or user. Initial frequency–Temperature accuracy considers the combination of frequency-temperature stability and calibration related errors and is the preferred general method for specifying the frequency–temperature performance of crystal oscillators.

In moderately stable clock oscillators with an initial frequency-Temperature accuracy of say ± 100 ppm, it is the general practice to define the stability as inclusive of calibration tolerance, temperature stability, ageing, change of frequency with load changes and changes to shock & vibration.

Output

The most common clock oscillators have an HCMOS or TTL (TTL drives 10 gates but sometimes only 2 - In case your application needs to drive more than 2 TTL gates please specify while ordering). The general Square wave output of the clock oscillator is shown in figure 1:

Symmetry

Symmetry or duty cycle is measured at 1.4 V DC in the case of TTL output oscillators and in the case of HCMOS oscillators it is measured at ½ VCC.



Vlo= Logic 0 Level
Vhi = Logic 1 Level