A TCXO is best viewed as a derivative of a standard Crystal Oscillator for use in applications where an improvement in the devices Temperature Stability is required. The premise holds that the TCXO designer can flatten the Temperature versus Frequency curve in order to spec a device meeting these more precise characteristics. Aging characteristics can also be improved upon versus a standard Crystal Oscillator.

A Temperature Compensated Crystal Oscillator (TCXO) utilizes the following design features differing substantially from a basic Crystal Oscillator design:

1. Specially processed tight tolerance crystal manufactured for both accuracy and to offer minimized long term aging.

2. Temperature/frequency compensation network made up of thermistors, resistors, and capacitors in an oscillation loop. The thermistor network is matched to the tight tolerance crystal causing the voltage to vary with temperature thereby compensating for the inherent frequency versus temperature characteristics.

3. A frequency/tuning adjustment allowing for manual trimming of the TCXO to compensate for aging and/or other Environmental influences. The method of frequency adjustment can also be electrical via an additional pin located on the package (utilizing an external potentiometer, for instance), or a TCVCXO utilizing both temperature compensation and external voltage control.

4. Buffering to minimize the effects of external changes in the circuit.

5. An internal regulator to minimize the effects of variation in voltage.

Specifying an TCXO:

Generally, TCXOs are manufactured according to industry standard guidelines pertaining to such variables as package type, output types, input voltages and currents, rise/fall times, and duty cycles. Although these parameters may vary slightly between manufacturers, and review of these parameters should be done when crossing between manufacturers, one can generally assume that these parameters will remain constant. When selecting a TCXO/VCTCXO product, careful consideration should be given to the following variables for which designations are made when selecting an Oscilent Part Number:

a. Package Type: Will the package be Through Hole or Surface Mount, and are there size constraints with either.

b. Output: Usually one of TTL, HCMOS, Clipped Sine Wave, or Compatible (simultaneous TTL and HCMOS compatible) while reviewing the characteristics of Load, Output Levels, and Input Currents associated with each Output type referenced in the Electrical Parameters portion of the Oscilent TCXO spec.

c. Frequency

d. Supply Voltage: Most TCXO products are designed to operate at either 5.0V, or 3.3V, although other options are available. The Oscilent Specification assumes this standard. For other Supply Voltage options, please contact Oscilent for assistance

Frequency Stability:

Over the Operating Temperature Range, an amount of total deviation acceptable for the application. Oscilent provides a Frequency Stability versus Temperature chart on most TCXO product Specifications. This chart should be used as a general guide only as, variations to this parameter are available. For a TCXO, industry standard selections range from 1.0ppm to 5.0ppm over temperature. For applications requiring <1ppm Frequency Stability, please contact Oscilent Engineering Support for assistance.

Operating Temperature:

Because of the more precise characteristics of a TCXO compared to standard Crystal Oscillators, an increased focus on Operating temperature is recommended. For TCXOs the Operating Temperature Ranges are smaller and offer many more options. Generally, a TCXO has a standard range of -40-+80 degrees Celsius, however the Operating Temperature is a significant influence on what is possible in production from Stability perspective, in addition to influences on production costs, so special consideration should be given in an attempt to avoid "over-specifying" the required Operating Temperature.

Frequency Deviation:

In TCXO product design, a package pin/pad is normally chosen to provide for initiating the VCTCXO (voltage control) option. For normal TCXO operation, this pin is designated as "No Connection." However, when the Voltage Control option is exercised in part number selection, a designation must be made that, not only specifies that this function is needed, but also specifies the amount of deviation required for this adjustment.

For more technical information, including Term Definitions, Suggested Reading Material, and Engineering Support, please visit the Oscilent Engineering Support page.