A temperature-compensated crystal oscillator (TCXO) is a crystal oscillator with a temperature-sensitive reactance circuit in its oscillation loop. The significance of having a temperature-sensitive reactance circuit in the oscillation loop enables a TCXO to compensate for frequency error over temperature characteristics that are inherent to the crystal. TCXOs are necessary when a level of temperature stability is required that cannot be achieved with standard oscillator (XO) or voltage-controlled oscillator (VCXO).
This whitepaper presents an in-depth examination of the evolution and differences between traditional discreetly built temperature-compensated crystal oscillators (TCXOs) and their modern application-specific integrated circuits (ASIC) based counterparts. As advancements in manufacturing processes and silicon technology continue to shape the landscape of electronic components, TCXOs have also undergone significant changes and improvements to meet the increasingly stringent demands of a fast-growing list of diverse applications.
Role of TCXOs in Electronic Systems
TCXOs are used in many electronic systems, from wireless communication systems, cell phones, base stations, telecommunications, GPS positioning, automotive navigation, metering and many other applications. TCXOs have a temperature-sensitive reactance circuit in their oscillation loop to compensate for the frequency-temperature characteristics of the crystal unit. This allows the oscillator component to provide clean stable output frequency that is very stable and accurate over the specified temperature range.
Features:
High frequency stability
Low phase noise + jitter
Voltage control
Low power
Small form factor
Applications:
RF Communications
Telecommunications
Networking
IoT
Smart meter
Industrial
Medical
Importance of Temperature Compensation in Crystal Oscillators
Temperature compensation is a method for calibrating a system’s performance to compensate for the effects of temperature change. It can significantly reduce the amount of frequency drift caused by temperature fluctuations on quartz crystals. It does this by applying a control voltage to adjust the output frequency. Temperature feedback is given by way of an NTC thermistor, which tells the ASIC how much compensation to add to the control voltage. By continuously monitoring and updating the output frequency TCXO’s can maintain very low PPM or even PPB stabilities.
Limitations of Old TCXOs
Early TCXO’s brought advantageous advantages to the circuitry world however, the early conception of TCXO’s also came with strict limitations. When building with discrete components you have more slop which is also known as “tolerance stacking”. Tolerance stacking is the cumulation of all individual component tolerances or their assembly in a circuit design. Each component has their own rated accuracy, the more parts you use, the higher the probability of having frequency errors.
Despite the difficulties around defining tolerances and accuracy, the introduction of an analog compensation network in an oscillator brought electrical design benefits. One of the unforeseen benefits of the TCXO was it’s ability to enable communication over great distances due to its discrete design.
The design offered frequency adjustments as analog compensation (which we will discuss more in-depth in the preceding paragraphs) but there were still areas for improvement.
An example of this is frequency stability, while much improved in stability performance, there was still a challenge when designs came under varying temperature conditions. Power consumption proved to be another area where improvements were needed. The increased number of components meant that there was going to be more power consumption, which proved difficult for early navigators as power was hard to produce when underway.
Modern TCXO Technologies
Engineers and inventors continued to look at ways to optimize and evolve frequency components. Today’s modern TCXO designs incorporate a breadth of new technologies that address many areas of performance optimization. The introduction of advanced materials, manufacturing techniques, and circuit architectures has vastly improved frequency stability, reduced phase noise, jitter and enhanced the overall performance of todays TCXO’s. Using modern manufacturing processes to produce high-quality AT-cut quartz crystal blanks has enabled modern TCXO’s to achieve Stratum level stabilities.
One of the unintended consequences of modern ASIC-based TCXO’s is the change from analog to digital compensation. ASIC-based TCXO’s are purely a digital processing circuit, which consists of a non-continuous electrical signal. Fundamentally, the functionality of the TCXO is the same, with the exception of how the compensation is applied.
When the temperature change reaches the threshold for compensation, digital compensation applies a sudden voltage to instantly move the frequency back to the center frequency. These sudden frequency corrections are known as micro-jumps. Sudden changes in frequency can make it difficult for circuits with sensitive frequency standards to maintain function.
One of the ways we can greatly minimize micro-jumps is by using a pseudo-analog compensation. With analog compensation, there is a continuous electrical signal which causes a continuous application of change over temperature. By slewing or dampening the compensation rate, you can minimize sudden changes in frequency. Micro-jumps can cause issues to sensitive processing and PLL circuits that are using the TCXO’s as their clock. The negative effects of micro-jumps can be loss of data or loss of lock with a PLL circuit.
Next, we will look at a plot graph that shows the effects of digital compensation compared to analog compensation over temperature. This will give a visual to the frequency jumps or micro-jumps in digital compensation that we discussed earlier. The visual is a great depiction of the positive effects of analog compensation in stabilizing frequency and mitigating micro-jumps.
The blue digital plot below clearly shows significant frequency jumps as digital correction is applied. The micro-jumps are significant enough to cause issues with sensitive electronics that depend on this clock. The green analog plot shows a much more linear compensation, with almost no micro-jumps.
Fig 2 Digital vs. Analog Compensation
Temperature Compensation Techniques
TCXOs have a temperature sensor built into the chip that monitors temperature and self-calibrates to correct the frequency output. This process helps to provide a stable output frequency ensuring a seamless user experience. TCXOs can be compensated over a specific temperature range to a specific frequency stability.
For example, a TCXO can be compensated to less than 1 parts per million (ppm) over the
industrial operating temperature range of -40°C to +85°C. TCXOs bring precision timing to designs that depend on stable frequency outputs. Temperature-compensated oscillators are ideal for use in stable clocks such as Stratum, GPS, RF, femtocell, and mobile connectivity applications.
Next, we will look at a diagram that will show a visual to how TCXOs work. We will look at uncompensated and uncompensated frequency plots. Included in the graph is the plotted data displaying the TCXO compensated frequency across temperatures.
If we look at the figure below, we see what temperature compensation looks like in practice. The blue line represents the desired base frequency. The red line shows standard oscillators’ frequency drift over temperature. We can see significant changes in frequency along the red line as we move along the temperature range. The black dotted line represents the compensation being applied to the control voltage input.
Fig 1 Temperature Compensation
As you can see, it applies an inverse signal that almost perfectly matches the error caused by changes in temperature. The green line represents the sum of the red and dotted lines. As you can see the green line has a much lower stability with only minor frequency drift. This same principle is used in various end applications where noise and stability precision are critical, such as noise-canceling headphones.
There are several different types of temperature compensation methods. In this paper, we will discuss more in-depth the various temperature compensation methods. Each one has advantages and disadvantages, which are dependent on a design’s end-use case and how it will be deployed in the field.
Types of Compensation:
Analog Digital Temperature Compensated Crystal Oscillator (ADTCXO)
An ADTCXO uses analog techniques to compensate the frequency of an oscillator as per temperature variation.
The output is compensated slowly to avoid micro-jumps or sudden changes in frequency.
This is widely used in cellular phones.
Digital Temperature Compensated Crystal Oscillator (DTCXO)
A DTCXO uses a temperature sensor and DAC along with logic functions to compensate for frequency drift caused by changes in temperature.
The circuit uses a temperature sensor that measures ambient temperature. This temperature input is sent to the logic circuitry, which uses data stored in a lookup table to select the appropriate digital value to be provided as compensation value. This digital value is converted to an analog value using DAC and provides correction to the crystal.
Fig 2 DTXO Compensation
Digitally Controlled Crystal Oscillator (DCXO)
A DCXO uses algorithms such as frequency estimation, correction and tracking algorithms.
It offers a very stable output frequency as compared to other VCXO and TCXO types.
This was widely used by GSM mobile phone manufacturers.
An MCXOuses microprocessors to provide highly efficient compensation processing.
ADC is used along with DAC in place of look-up tables.
Voltage Controlled Temperature Compensated Crystal Oscillator. (VCTCXO)
VCTCXOs have the same low power consumption, temperature characteristics, and fast warmup as TCXOs, but also include a voltage control function.
This function allows for precise tuning and calibration of the output frequency after PCB assembly and to compensate for any effects of aging in the field.
High-performance results make this a more expensive solution compared to other types of temperature-compensated oscillators.
Miniaturization and Integration
Smaller TCXOs are achieved by managing stress factors that contribute to aging in crystals.
Designing TCXOs into System-on-Chip (SoC) solutions and multifunctional modules can greatly impact design performance. Miniaturization and integration of TCXOs have been significant trends driven by the demand for smaller, more power-efficient, and feature-rich electronic systems.
Advancements in quartz crystal geometries and semiconductor manufacturing processes have enabled the development of miniaturized TCXOs. Applications that utilize reduced footprint TCXOs include mobile devices, smart devices and sensor-based applications. Miniaturized TCXOs are optimized to operate with lower power consumption, making them ideal for battery-powered and energy-conscious devices. Low-power TCXOs can help extend battery life while maintaining precise timing functions.
Depending on the design system, modern TCXOs are commonly integrated with other functions such as phase-locked loops (PLLS), frequency synthesizers, frequency dividers, and voltage-controlled oscillators (VCXOs). Combining multiple functions into a single chip or module can help designers reduce the number of components used, save board space, and reduce power consumption. Despite their compact form function TCXOs offer excellent frequency stability and accuracy over a wide temperature range. Advanced compensation techniques, temperature sensors, and digital calibration algorithms ensure that TCXOs deliver precise timing references even in harsh operating environments.
Future Trends and Outlook
With improvements in silicon design and quartz crystal technology. We can expect many further refinements in TCXO capabilities that will improve both overall performance and current consumption. Standard TCXO’s have a stability over temperature of about ±2.5 ppm with a stability potential of ±50 ppb. As technology advances, we can expect that higher levels of stability will increase in demand. With future technology needs in mind ECS Inc. released the Stratum 3E which offers a ±4.6 ppm free run stability, including ±0.012 ppm over 24-hour hold-over spec.
Summary
This whitepaper concludes with a comprehensive summary of the transformative journey from traditional TCXOs to their modern counterparts. The advancements in frequency stability, miniaturization, and energy efficiency underscore the pivotal role that modern TCXOs play in enhancing the performance and reliability of electronic systems across various industries. As technology continues to progress, staying informed about the latest developments in TCXOs becomes essential for engineers and designers seeking to optimize their applications.
ECS Inc. International’s TCXO’s offer excellent frequency stability with high precision, excellent phase noise, low aging & low power usage. They provide outstanding performance, exceptional consistency and stable long-term characteristics. ECS Inc. also offers TCXO’s with voltage-control options for frequency calibration. This type of precision makes them ideal for the demanding performance required from networking, communications, wireless and IoT applications.
