Understanding TC-OCXOs
TC-OCXOs offer OCXO stability at reduced cost, power, and package size
BY KARL WARD
C-MAC Frequency Products
Harlow, U.K.
http://cmac.com
Mobile, battery-powered and remote applications such as cellular base stations, satellite communications, secure radio and GPS have seen rapid growth over the last few years, with even broader deployment expected as advancing technology makes new devices and systems possible. However, as generally happens when technologies enter the mainstream, increasing competition is putting continuing pressure on both system and component prices, while demands for better performance grow louder.
In mobile communications, signal quality is a key issue. Cell phone users, for example, will readily testify that they would benefit from improved reception in low-signal-strength areas.
In GPS and other positioning systems such as distress beacons, a key demand is better positional accuracy, especially in the military sphere. While both requirements can be addressed by improving the stability of the frequency reference source, this has an impact on the size and power consumption of the component, as well as on its cost.
TCXO vs. OCXO
Temperature-compensated crystal oscillators (TCXOs) have conventionally been used to provide small, low-power, high-stability reference sources for mobile and remote equipment. The most stable TCXOs are based on a temperature compensation ASIC, a digitally programmed analog device (such as C-MAC's Pluto ASIC, see Fig. 1 ) that uses a fourth-order Chebychev polynomial to compensate the oscillator's frequency output for the effects of ambient temperature.
Fig. 1. The most stable TCXOs are based on a temperature compensation ASIC, such as C-MAC's Pluto.
These TCXOs are available in sizes down to 5 x 3.2 mm, with power consumption down to a few milliwatts, but their frequency stability over operating temperature range is limited to ±0.14 ppm. To improve frequency stability beyond that of a TCXO, a different class of device�an oven-controlled crystal oscillator (OCXO)�is required.
In an OCXO, the effect of ambient temperature is virtually eliminated by enclosing the entire oscillator within an 'oven' maintained at a constant temperature above the upper end of the specified operating temperature range. OCXOs can offer frequency stabilities up to three orders of magnitude better than the most stable TCXOs, but are bulkier, higher in price and, even after initial warm-up, generally consume at least 1 W at steady state.
Limitations of 'heating a TCXO'
Various parties have attempted to combine the benefits of temperature compensation and oven control by “heating a TCXO”�attaching the entire TCXO assembly to a heater that comes on whenever the ambient temperature falls below a certain threshold, say 0°C. This does improve power consumption over a conventional OCXO, and also offers some stability benefits due to the reduced temperature range seen by the crystal. However, overall frequency stability remains stubbornly inferior to that of existing devices.
The main limitation to the above approach is that each TCXO has to be programmed and its frequency correction signal optimized for stability versus temperature over its entire temperature range at the factory. Assuming this operation is performed with the heater off, the TCXO correction curve will be optimized while the assembly is in thermal equilibrium, albeit with a temperature offset between the crystal and the temperature sensor caused by heat from the oscillator and thermal flow across the TCXO assembly.
In actual use, applying heat to the TCXO assembly will then alter the thermal flow and therefore the temperature difference between the crystal and the temperature sensor so that the correction signal will no longer be optimized. This causes frequency deviations potentially exceeding those in the original TCXO. Careful thermal design and choice of materials can alleviate this problem, but would also increase the cost of the component.
TC-OCXO approach: Compensating an OCXO
The temperature-compensated oven-controlled crystal oscillator (TC-OCXO) combines OCXO equivalent stability with power consumption, size, weight, and cost closer to those of a TCXO.
The TC-OCXO approach takes the output of a crude OCXO and applies TCXO compensation to it rather than the other way around. A miniature oven keeps an uncompensated crystal oscillator at an approximately constant temperature slightly above the specified operating temperature range, for example, between 90° and 95°C for an operating temperature range of –40° to +85°C.
Because the oven does not have the strict temperature requirements demanded by a conventional OCXO, it can be implemented at much lower cost and in a smaller package. The whole assembly is then treated as a TCXO.
A temperature sweep is performed at the factory, and each device is programmed with a correction curve generated in the thermally stable environment it will be used in�for example, with the oven on. In effect, the TC-OCXO performs like a TCXO that experiences only a few degrees of temperature fluctuation.
In general, TC-OCXOs can offer an overall stability nearly an order of magnitude better than the most stable TCXOs. Early specifications can achieve ±0.05-ppm stability over a temperature range of –20° to 70°C, at standard frequencies from 5 to 20 MHz, with a highly linear ±5-ppm frequency adjustment for aging effects. Power consumption is less than 400 mW at –20°C steady state and only 1.0 W even during warm-up, from a 3.3-V supply. Compared with conventional OCXOs, size and power savings are complemented by significant cost savings.
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