QUARTZ CRYSTAL UNITS

1. Cutting angles and vibration mode
Cutting angles differ depending upon the applications.(oscillation frequencies and electrical characteristics). Taking the most popular AT-cut crystal wafer for example, it is in a plane which makes an angle of 35?5´to the Z axis and the wafer thickness is approximately 0.16mm in the case of 10MHz fundamental-mode thickness-shear vibration.

2. Frequency-Temperature Characteristics
The frequency-temperature characteristics for a thickness-shear mode AT-cut with the angle of cut as a parameter. Since the AT-cut frequency-temperature
Characteristic is equivalent to an equation of the third degree, it displays excellent frequency stability over a wide
Temperature range. The frequency-temperature characteristics of the mode of vibration other than AT-cut. All vibration modes show an equation of the second degree. Therefore, when the operating temperature range is wide ,the frequency change is much larger than that of the AT-cut. Therefore, the AT-cut is most widely used.

3. Crystal unit and load capacitance (CL)
The load capacitance CL is a factor for determining the “conditions?of a crystal unit when used in the crystal unit is used in a range where it functions as an inductive reactance, In such usage, the oscillation circuit operates as a capacitive reactance. In other words, when the oscillation circuit is seen from both terminals of the crystal unit, this oscillation circuit can be expressed as a series circuit of negative resistance and capacitance. At that time this capacitance is called the load capacitance.

4. Crystal holders
Solder seal: This method is cost effective and accepted for military and commercial applications.(Mil-c-3098)
Nickel-Silver cans and bases are soldered together with a tin-lead alloy.
Resistance Weld: Resistance welded enclosures are sealed together by melting the metal for a very short time with a large predetermined electrical pulse, thus welding the crystal cover to the base. No fluxes are used in this process. The resistance-weld method has the advantages of approaching the long term stability of the cold-weld method. Typical aging per year for resistance weld is <5ppm.
Cold weld: In the cold weld method a nickel plated copper can and base are sealed together under a concentrated high pressure bonding technique. The enclosure is evacuated using a cryogenic vacuum and backfilled with a mixture of helium and nitrogen. The cold weld method is exclusively used when long term stability is required (low aging characteristics). Typical aging per year for cold weld is <3ppm. Cold weld sealed crystals are slightly higher cost.