The growth of wireless communication and other devices has spawned a demand for small cells, sometimes referred to as femtocells, as a way to remove the load from cell towers and transfer directly to the Internet. Until recently, the frequency control needs in small cells have been met by OCXOs due to the need for highly precise, stable clocking devices.
Advances in IC technology and crystal manufacturing have led to enhancements in TCVCXOs that have blurred the historical differences between the TCXO and the OCXO. As technology has improved, the functionality of both types of oscillators has made it difficult for many designers to determine which technology is appropriate for a particular application. TCVCXO technology has been developed to a point where it is a viable option in applications where formerly only OCXOs were available.
TCXO technology
A TCXO is a voltage-controlled crystal oscillator with a correction voltage applied to the voltage-control pin. This voltage changes with temperature to bring the frequency back to nominal. This fluctuating voltage exercises the crystal over the entire frequency range of the specification. Any crystal-related issues such as coupled modes cannot be corrected and are superimposed on the resulting frequency-temperature curve. This makes the design and manufacturing of the crystal a difficult and critical part of the TCXO.
The majority of TCXOs also require a voltage-control function. This allows for exact setting of frequency, adjustments for long-term aging and the ability to phase-lock the device to other sources. A emperature-compensated voltage-controlled crystal oscillator (TCVCXO) works by adjusting the load capacitance presented to the crystal. Most small-cell requirements include a voltage-control option.
Small-cell TCXO technical requirements:
In general, small cells require ±0.1-ppm stability over a specified temperature range. One of the more common specifications is 0º to 80ºC with ±5 ppm of pullability for the voltage-control function. It is only in the last several years that crystal technology has improved to the point where they could be reliably compensated to this level. There are a number of technical issues to consider in the manufacture of these crystals. The crystals for the TCVCXO must be produced free of perturbations with very low aging characteristics as well as extremely low hysteresis characteristics.
Figure 1 shows a frequency-temperature and hysteresis characteristic for the small-cell-grade TCVCXO. Until recently, this performance level would have typically been delegated to the OCXO realm. Improved semiconductors and optimized crystal design and manufacturing processes make it possible for TCXOs to compete in the ±0.1-ppm stability realm. These devices are typically TCVCXOs. A correction voltage is required to adjust frequency for drift and long-term aging characteristics.
The frequency adjustment is usually generated by NTP, PTP (IEEE-1588) implementation. These TCXOs have very low aging rates and very linear characteristics with regards to the control voltage function. The precise compensation, low hysteresis, low aging rates and low power consumption make this device an ideal solution for small-cell applications. These devices use approximately 2 mA of current. An OCXO for this application would require approximately 100 mA of current for the application. The size, power requirements and warm-up time place traditional OCXOs at a significant disadvantage when compared to TCVCXOs designed for this application.
Fig. 1 Shown above is a frequency-temperature and hysteresis characteristic for the small cell grade TCVCXO.
OCXO technology
Ovenized crystal oscillators (OCXOs) are typically used for high-precision frequency-control applications. This technology heats the crystal and associated oscillator circuitry to the upper turning point of the crystal. Figure 2 shows the part of the upper turning point used in the OCXO application.
Fig. 2 Shown above is the upper turning point used in the OCXO application.
The crystals for these oscillators are manufactured so the upper turning point is above the highest specified temperature range. The crystal and associated circuitry is heated to and maintains a narrow temperature window around that point on the crystal, and the device is tuned to frequency at that temperature.
The OCXO has an advantage in only exercising the crystal over a very narrow temperature window, typically a couple degrees or less. This greatly reduces the chance of exciting unwanted modes in the crystal. By contrast, a TCVCXO electrically compensates for the crystal characteristic. This places a strong emphasis on the quality of the crystals being used in these applications.
The data in Table 1, taken from commercially available, off-the-shelf OCXOs and small-cell-grade TCVCXOs, outlines differences to be considered.
OCXOs historically had an advantage with a lower sensitivity to small stability changes when changing the voltage-control functions to the max or min value. The semiconductors used for small-cell-grade TCVCXOs have compensation circuitry for that function. In addition to the smalle-size, lower-current requirements, and faster stabilization time, TCVCXOs are a fraction of the cost of OCXOs, making them an ideal choice for small cells.
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