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Digital TCXO Provides Extremely High Accuracy over a Wide Operating Temperature Range
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SUNNYVALE, CA—July 11, 2007—Maxim Integrated Products (NASDAQ: MXIM) introduces the DS4026, a digitally controlled Temperature Compensated Crystal Oscillator (DC-TCXO) with unprecedented accuracy among TCXOs. The DS4026 provides frequency accuracy and stability better than ±1.0ppm over an extremely wide operating temperature range (-40°C to +85°C). Over a 10-year operational life, the device is accurate to better than ±4.6ppm and provides excellent phase noise performance, with characteristics of -140dBc/Hz at a 1kHz offset. Other TCXOs can make neither this accuracy nor longevity claim.
This digitally tunable TCXO employs proprietary design and calibration techniques that negate the effects of electromechanical stress, thermal variations, and frequency perturbations (also known as "activity dips"), which can be excited in all crystal elements as they operate over temperature. To meet the expected accuracy performance specifications, the system also uses DSP-designed components, which are individually programmed to the specific crystal's element over temperature, voltage, and specific signal quality. The resulting digital design is highly robust and provides this unmatched ±1.0ppm over the entire temperature range for more than a decade.
Performance Limitations in Traditional TCXO Implementations
TCXOs often provide the primary reference oscillator for system timing in telecom, wireless, navigation and test systems. Many demanding applications, moreover, require reference oscillator stability of less than ±1.0ppm over a wide temperature range, typically -40°C to +85°C. Unfortunately, many TCXO devices are subject to frequency fluctuations induced by electro-mechanical stresses, thermal sensitivities, and/or activity dips. These undesired frequency fluctuations will noticeably degrade overall system performance. This performance degradation can be exhibited as a higher bit-error rate, lower transmit ranges, or higher susceptibility to adjacent channel interference.
Traditionally, systems that need extremely accurate performance rely on highly stable, but bulky and costly oven-controlled crystal oscillators (OCXOs). More accurate TCXO devices are available, but tend to exhibit less-than-desirable instabilities over temperature and time (i.e., the effects of aging). These latter devices are also generally specified over the reduced 0°C to +70°C operating temperature range to achieve the desired frequency stability of less than ±1.0ppm. Finally, TCXO aging specifications are generally not comprehended over a 10-year period, but provided on a per-year basis.
Using analog compensation techniques to mitigate the "natural" activity fluctuations in AT-cut quartz crystals is, in practical terms, virtually impossible. Designs using analog techniques generally require far more extensive screening of crystals to eliminate crystals that have activity "dips." This additional screening, in turn, adds cost to the final TCXO product solution. In contrast, using digital compensation has not always provided a robust approach nor the performance needed for temperature compensation of AT-cut quartz crystals. Typically, digital approaches introduce noise components onto the TCXO's output frequency.
Negating the Environmental Effects that Reduce TCXO Accuracy
Step size and step movement based on temperature were never controlled in digital TCXOs. This realization is key to the DS4026's novel design. The DS4026 overcomes the historical dilemma of noisy digital TCXO designs because of its finite step-compensation methods. The new approach features calibration routines and a proprietary temperature-controlled crystal loading design. In this design each DS4026 device is factory calibrated to compensate its embedded crystal's natural temperature characteristics. As a result, the digitally controlled DS4026 achieves the performance typically reserved for analog TCXO designs.
Besides providing smooth analog-compensation transition curves, the programmable DSP technique also addresses the industry issues and concerns about activity fluctuations in TCXO products. Paul Nunn, Business Manager for TCXO devices at Dallas Semiconductor, explains that, "The DS4026 detects any activity dip during the calibration of the device. It then drives the influence of this frequency anomaly to have zero affect on the system's overall performance."
Additional System Features
The DS4026 digital TCXO also integrates many unique capabilities and features not commonly found on TCXOs. These functions include a ±3°C accurate and readable temperature sensor and a 16-bit DSP-controlled DAC for frequency stability, trimming, and calibration. Frequency trimming and temperature reading are accomplished through an I2C control interface. The DS4026's frequency can be tuned with a resolution better than 1ppb and tuning ranges greater than ±10ppm. The device operates from a 3.3V ±5% supply voltage and is packaged in a standard 300-mil 16-pin SO. The device is fully RoHS compliant, and is available in commercial and industrial temperature versions.
This TCXO supports applications that require an extremely accurate and stable time base like wireless infrastructure applications, SONET/SDH systems, instrumentation and test systems, and GPS and navigation equipment.
The DS4026 is priced at $20.00 (10k-up, FOB USA). Samples are now available. For more information please visit: http://www.maxim-ic.com/DS4026-info
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