Understanding TCXO Compensation
All uncompensated quartz crystal oscillators will experience drift according to the characteristics of their quartz crystal. The amount of drift in the frequency reference is usually in the range of ±10 to ±50ppm, depending upon individual crystal characteristics.
Temperature compensated crystal oscillators, or "TCXOs", are often selected for applications that require a more stable frequency reference and usually offer a resolution better than a standard, uncompensated quartz crystal oscillator.
TCXOs are compensated over the customer's required temperature range to a specific frequency stability needed for the particular application. TCXOs can, for example, be compensated to <1ppm over the industrial operating temperature range of -40°C to +85°C.
Various compensation methods have been used over the years, including:
Thermistor/Resistor Network Compensation - Thermistor/resistor compensation utilizes a correction voltage generated by a network of one or more thermistors to cancel the frequency vs. temperature variation. As early as 1961, compensation ratios of greater than 100-to-1 were being achieved, i.e., a crystal with a peak-to-peak deviation of 50ppm over temperature could be compensated to a level of 0.5ppm. Thermistor/resistor compensated TCXOs have been a mainstay in the industry for over 50 years.
Digital Temperature Compensation - By the late 1970s, advances in IC technology made it practical to devise compensation systems using analog-to-digital conversions and solid-state memory. Although crude by today's standards, digital TCXOs achieving better than 0.1ppm performance were produced by several manufacturers, including Rockwell Collins and Greenray Industries.
Over the years, other digital compensation implementations have been developed, many with embedded computing power as a means of facilitating calibration and system operation. Others utilized elaborate temperature measurement schemes, such as dual-mode crystal, self temp-sensing, to achieve temperature stabilities of 0.05ppm or better.
Analog Integration - The expanding capabilities of large-scale integration made it possible to include more of the functions required for temperature compensation into a single IC. This has led to development of the current generation of ASICs that allow construction of precision analog TCXOs with just two components - the ASIC and the quartz crystal. These ICs use analog multipliers to generate a smooth curve which fits a 5th order polynomial to the crystal to be compensated. Stabilities of better than ±0.2ppm can be achieved over temp ranges as wide as -40°C to +85°C.
Although first-generation fabrications often resulted in relatively large die, reductions in geometries and smaller ICs now enable precision TCXOs to be housed in packages as small as 2mm x 2.5mm.
While Greenray Industries manufactures TCXOs using each of these compensation methods, analog ASIC-type TCXOs outnumber all the other types, combined.