Article | February 23, 2006

TCXO Approaches OCXO Stability - Without The Power

Source: Greenray Industries, Inc.
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By Greenray Industries, Inc.

For years, manufacturers have sought to provide an oscillator that offers tight stability in a compact package. An Oven Controlled Crystal Oscillator (OCXO) provides excellent stability, but for portable applications, and particularly for demanding military applications, the OCXO package may be too large and it may also require too much power from a battery.

Greenray Industries' new T75 Series TCXOs (temperature compensated crystal oscillator) satisfy the need for tight stability performance, a compact package and low power consumption.

The heart of any oscillator is the crystal. The new T75 Series TCXOs utilize an AT Strip crystal, from sister company Statek Corporation, that offers distinct advantages over a normal circular blank crystal.

The AT Strip crystal is extremely rugged and can handle very high shock and vibration levels. Special versions of this crystal have survived up to 100,000g's of shock, making the crystal ideal for use in severe environments such as Smart Munitions. Typical g-sensitivity of this crystal is 2.5 x 10-9/g in the worst axis, which is the Z axis, and less than 2 x 10-10/g in the X and Y axes.

G-Sensitivity can be further improved to approximately 3 x 10-10/g in the Z axis, thereby minimizing stability and phase noise affects from shock and vibration. This is very important for portable and airborne applications that rely on consistent performance under shock, acceleration, and vibration. The downside to using a lower g-sensitivity crystal is that aging performance is degraded slightly.

One of the limiting factors in making a TCXO perform closer to OCXO stability is crystal perturbations. Perturbations are isolated events which happen at specific temperatures and normally over just a few degrees, causing the frequency of the crystal to jump by up to several parts per million (ppm).

All crystals inherently have perturbations – what is important is how many and how large they are. Perturbations are very repeatable over temperature within a manufacturing run. There are many theories about the origin of crystal perturbations, including blank stress, cleanliness, and particulate matter leaving the crystal.

Greenray has worked with Statek to significantly reduce AT Strip perturbations to typically less than 1 x 10-7 to allow compensation to temperature stabilities as tight as ±2 x 10-7. It is important to test tight stability TCXOs every two degrees over temperature to ensure that perturbations do not exceed specifications.

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