Synthetic Quartz Quality

The "Quality Factor" of an oscillating device is defined as:

Q = Energy stored over a cycle/Energy lost over a cycle

The stored energy is in the form of kinetic energy of the moving structure plus the potential energy stored in the deformed structure. The energy dissipation is usually caused by energy losses in the acoustic device. Energy loss is caused by a variety of effects from internal dislocations and inclusions to external factors such as acoustic loss into the environment.

Certain imperfections which produce absorption bands in the infrared absorption spectrum of the crystal correlate with the degradation of the quality factor "Q" of the resonators made from quartz. One of the factors that determine the maximum achievable resonator Q is the hydroxyl ion (OH^-) content of the quartz. Infrared absorption measurements are routinely used to measure the intensities of the temperature-broadened OH defect bands.

The absorption can be measured by directing infra-red light through a known thickness of single crystal quartz material. Absorption at two different frequencies are measured. Typically the absorption at 3800 cm^-1 is measured to provide a reference level and absorption at 3800 cm^-1 is compared to absorption at 3500 cm^-1 .

EIA Standard 477-1 defines the infrared absorption coefficient as follows:

Alpha = (A₃₅₀₀- A₃₈₀₀)/Y

Where

A = The log₁₀ of the fraction of the incident beam absorbed

A₃₅₀₀= Absorption at 3500 cm^-1 wave number

A₃₈₀₀= Absorption at 3800 cm^-1 wave number

Y = Cut thickness in cm

The table below defines the five standard Q levels for commercial quartz stones.

Grade	Alpha	Minimum Q Level Correlation
A	.033	3,000,000
B	.045	2,200,000
C	.060	1,800,000
D	0.12	1,000,000
E	0.25	500,000

Grade "A" quartz material will have the most regular single crystal structure. It exhibits a high Q because it has fewer dislocations in the crystal structure. Normally, this grade is used in applications where maximum stability is required. An example would be precision crystal oscillators which are expected to achieve stabilities of 1 x 10^-10 per day.

Grade "B" material should be used for most high stability applications such as TCXOs and DTCXOs. It is also the minimum useable grade if the quartz is to be deeply etched. Deep etching will cause selective etching. Quartz with low Q grades will exhibit selective etching which will cause channel to be etched through the quartz. These channels can result in greatly degraded resonator parameters and poor environmental shock performance.

The "C" grade material can be used for less demanding applications.

"D" grade material should only be used for very undemanding applications. The Q is less that one half the Q of the "B" grade and units made from this material will have a response that has double the bandwidth or half the frequency stability of the better grade.

A more serious problem with low Q material concerns the temperature vs. frequency (T/C) characteristic of the finished resonator. Normally the shape of the T/C curve is determined by the angle of cut (ZZ' angle) for an AT resonator. When the Q falls below 1,000,000 the relationship between T/C and ZZ' angle becomes very fuzzy. The two parameters will correlate very poorly and the T/C of the resonators built from such material will deviate substantially from the average possibly causing significant yield loss.

Updated: 11/15/2010