The Future of Temperature-Compensated Crystal Oscillators

Most modern digital and analog electronic devices require a time base to perform their intended function. Found in everything from cell phones to smart munitions, quartz crystal oscillators are widely used in many embedded applications. Quartz resonators’ high Q, excellent temperature performance, and superior long-term aging makes them the clear resonator of choice for many applications. The frequency versus temperature performance of a discrete LC oscillator will be on the order of several hundred parts per million (ppm) per °C, where a crystal oscillator (XO) will have roughly ±30 ppm over the entire industrial temperature range (–40 to +85°C). While being superior to a discrete oscillator, this temperature stability is not nearly sufficient for many modern applications.

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Source: John Esterline

The temperature-compensated crystal oscillator (TCXO) employs the use of an open loop compensation circuit to create a correction voltage to reduce the inherent frequency versus temperature characteristic of the crystal. The crystals used in TCXOs have frequency versus temperature characteristics that approximate a third-order polynomial, as seen in the nearby figure.

The early designs for TCXOs employed a network of thermistors and resistors to create a correction voltage. By using thermistors with different slopes and properly selecting the fixed value resistors, the correction voltage can be made to have a shape factor matched to the crystal’s frequency versus temperature performance. The correction voltage is applied to a varactor in the feedback path of the TCXO. This change in capacitance in the feedback path alters the tuning of the oscillator, thus changing the output frequency and compensating it for temperature effects. Thermistor/Resistor network TCXOs can achieve frequency versus temperature stabilities of around ±1 ppm over the industrial temperature range; however, they are limited in their curve-fitting capabilities because of the nature of using discrete thermistors and resistors.

Thermistor/resistor network TCXOs are still found in specialized environments including satellite and other space applications where modern solid-state devices do not have the radiation hardness to survive. Most TCXOs manufactured today utilize an ASIC which contains the oscillator circuit and a third- or fifth-order polynomial voltage generator. The polynomial generator is an analog output voltage but also has digital registers for setting the coefficients of the polynomial. The newest generations of TCXO ASICs can provide temperature performances of ±0.1 ppm over the industrial temperature range. This is a 10-fold improvement over what is obtainable with a traditional thermistor/resistor network TCXOs and also has the advantage of a much smaller footprint (5 mm × 3.2 mm).

Some high-precision applications require frequency versus temperature stabilities better than ±0.1 ppm. To meet these challenging specifications a different methodology is implemented. An oven-controlled crystal oscillator (OCXO) uses a heater circuit and thermal insulation to keep the crystal at an elevated temperature (≈15°C above the upper operating temperature limit). By controlling the crystal’s temperature and keeping it nearly constant, the frequency deviation due to ambient temperature changes is vastly reduced. OCXOs can achieve frequency versus temperature stabilities of ±0.005 ppm. This improved performance comes at the cost of a larger footprint and increased power consumption. The TCXO’s performance limit of ±0.1 ppm is due to several factors. First, the resonators are not perfect. Their frequency versus temperature stability approximates a third-order polynomial; however, higher order effects are present. Secondly, the polynomial generator is nonideal and induces some higher order artifacts, leaving the user with residuals of ±0.1 ppm. A new methodology which uses an artificial neural network (ANN) to create the correction voltage has recently been demonstrated. The ANN is superior in that the neural network is not inherently shape limited like a third-order polynomial. If enough data is presented to the ANN, it can “learn” the crystal’s temperature performance shape and correct for it. This new methodology has been shown to provide ±0.01 ppm frequency versus temperature stability over the industrial range. The ANN algorithm can achieve OCXO temperature performance in a much smaller footprint, and without the need for the power-hungry oven.

The evolution of quartz crystal time bases over the last 70 years has seen the frequency versus temperature stability improve by a factor of several thousand. As our need for more stable oscillators in smaller packages with less power consumption grows, the development of better compensation schemes is paramount. The ANN demonstrates a technology that has much potential. Its ability to adapt and change its shape factor makes it ideal for complex compensation problems.

EsterlinePhotoJohn Esterline is the CEO of Esterline Research and Design, LLC, a Pennsylvania based start-up company. John holds an MEngEE and a BSEE from Pennsylvania State University. His research interests focus on temperature compensation algorithms for the improvement of embedded time bases. John is the inventor on two US patents (US8188800 B2, US8525607 B2), and the inventor of one patent pending (US 13/570,563). Esterline Research and Design, LLC offers consulting services in frequency control, test and automation and other subject matter in addition to its RF testing products.

 

Circuit Cellar 291 (October 2014) is now available.

Traveling With a “Portable Workspace”

As a freelance engineer, Raul Alvarez spends a lot of time on the go. He says the last four or five years he has been traveling due to work and family reasons, therefore he never stays in one place long enough to set up a proper workspace. “Whenever I need to move again, I just pack whatever I can: boards, modules, components, cables, and so forth, and then I’m good to go,” he explains.

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Alvarez sits at his “current” workstation.

He continued by saying:

In my case, there’s not much of a workspace to show because my workspace is whichever desk I have at hand in a given location. My tools are all the tools that I can fit into my traveling backpack, along with my software tools that are installed in my laptop.

Because in my personal projects I mostly work with microcontroller boards, modular components, and firmware, until now I think it didn’t bother me not having more fancy (and useful) tools such as a bench oscilloscope, a logic analyzer, or a spectrum analyzer. I just try to work with whatever I have at hand because, well, I don’t have much choice.

Given my circumstances, probably the most useful tools I have for debugging embedded hardware and firmware are a good-old UART port, a multimeter, and a bunch of LEDs. For the UART interface I use a Future Technology Devices International FT232-based UART-to-USB interface board and Tera Term serial terminal software.

Currently, I’m working mostly with Microchip Technology PIC and ARM microcontrollers. So for my PIC projects my tiny Microchip Technology PICkit 3 Programmer/Debugger usually saves the day.

Regarding ARM, I generally use some of the new low-cost ARM development boards that include programming/debugging interfaces. I carry an LPC1769 LPCXpresso board, an mbed board, three STMicroelectronics Discovery boards (Cortex-M0, Cortex-M3, and Cortex-M4), my STMicroelectronics STM32 Primer2, three Texas Instruments LaunchPads (the MSP430, the Piccolo, and the Stellaris), and the following Linux boards: two BeagleBoard.org BeagleBones (the gray one and a BeagleBone Black), a Cubieboard, an Odroid-X2, and a Raspberry Pi Model B.

Additionally, I always carry an Arduino UNO, a Digilent chipKIT Max 32 Arduino-compatible board (which I mostly use with MPLAB X IDE and “regular” C language), and a self-made Parallax Propeller microcontroller board. I also have a Wi-Fi 3G TP-LINK TL-WR703N mini router flashed   with OpenWRT that enables me to experiment with Wi-Fi and Ethernet and to tinker with their embedded Linux environment. It also provides me Internet access with the use of a 3G modem.

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Not a bad set up for someone on the go. Alvarez’s “portable workstation” includes ICs, resistors, and capacitors, among other things. He says his most useful tools are a UART port, a multimeter, and some LEDs.

In three or four small boxes I carry a lot of sensors, modules, ICs, resistors, capacitors, crystals, jumper cables, breadboard strips, and some DC-DC converter/regulator boards for supplying power to my circuits. I also carry a small video camera for shooting my video tutorials, which I publish from time to time at my website (www.raulalvarez.net). I have installed in my laptop TechSmith’s Camtasia for screen capture and Sony Vegas for editing the final video and audio.

Some IDEs that I have currently installed in my laptop are: LPCXpresso, Texas Instruments’s Code Composer Studio, IAR EW for Renesas RL78 and 8051, Ride7, Keil uVision for ARM, MPLAB X, and the Arduino IDE, among others. For PC coding I have installed Eclipse, MS Visual Studio, GNAT Programming Studio (I like to tinker with Ada from time to time), QT Creator, Python IDLE, MATLAB, and Octave. For schematics and PCB design I mostly use CadSoft’s EAGLE, ExpressPCB, DesignSpark PCB, and sometimes KiCad.

Traveling with my portable rig isn’t particularly pleasant for me. I always get delayed at security and customs checkpoints in airports. I get questioned a lot especially about my circuit boards and prototypes and I almost always have to buy a new set of screwdrivers after arriving at my destination. Luckily for me, my nomad lifestyle is about to come to an end soon and finally I will be able to settle down in my hometown in Cochabamba, Bolivia. The first two things I’m planning to do are to buy a really big workbench and a decent digital oscilloscope.

Alvarez’s article “The Home Energy Gateway: Remotely Control and Monitor Household Devices” appeared in Circuit Cellar’s February issue. For more information about Alvarez, visit his website or follow him on Twitter @RaulAlvarezT.