Seeking to eliminate the need for external synchronization circuitry (Figure 1), as well as to improve the holdover robustness of GPS-based precision timing applications, Spectratime (formerly Temex Time) has introduced its SXO-75, a synchronized crystal clock. The unit uses SmarTiming+ technology to synchronize, discipline or control any Stratum-1 timing reference, such as the master clock signals of the GPS, GLONASS, Galileo, or LORAN-C navigation systems, as well as E1 or T1 interfaces. It can also be used with Cesium clocks or hydrogen masers. The unit therefore enables precision timing functions in a wide range of defense and commercial applications, while also reducing the total size and cost of these systems.

Figure 1 The SXO-75 integrates synchronization functionality to eliminate the need for an external GPS sync circuit.

The SmarTiming+ technology in the unit provides a multi-vendor GPS interface, programmable outputs and phase offset adjustments, and auto-adaptive filtering of jitter, wander and noise at 1 ns GPS locking resolution for up to 100,000 s. In addition, the output-to-reference locking mode is user-selectable between either GPS phase synchronization or reference-frequency tracking.

The unit generates a 10 MHz output signal, maintaining accuracy to ±50 ns when locked to GPS, and provides a holdover accuracy of less than ±10 μs (or ±100 output cycles) if GPS is absent for 24 hours. According to Nino De Falcis, vice president of product management at Spectratime, this performance is achieved using an SC-cut crystal in a double oven-controlled crystal oscillator (OCXO) circuit, as well as digital modeling of the stability over the temperature range.

The aging of the crystal clock is less than 0.01 ppb per day, and the system has a long-term stability of less than 0.2 ppb per month. The SXO-75 also provides complex stability over temperature behavior modeling, and an internal EEPROM device for seamless frequency calibration and software upgrades.

The SXO-75 is more susceptible to physical shock than its rubidium-based counterpart, the SRO-100 (though the SXO-75 can survive a shock impulse of 40 g for 11 ms). It also requires a warm-up-time of about 15 minutes, whereas the SRO-100 requires about seven minutes. However, the crystal-based unit is available at lower cost, and both the packaging and functionality for the two units are identical and compatible. These features enable engineers to transparently provide either ultra-high-performance or reduced-cost versions of the same system using only a single design.

Figure 2 The SXO-75 uses an SiP approach, permitting the use of a compact ready-to-use package for integration into larger systems.

All the components for these clocks are contained in 3.25 in. x 2.25 in. x 0.75 in. packages (the SXO-75 package is shown in Figure 2). This system-in-package (SiP) approach gives the designer greater flexibility than if the functionality of these units were to be implemented with a discrete approach. However, according to De Falcis, Spectratime has no plans as of yet to implement the capabilities of these clocks in a single monolithic device with integrated resonant MEMS structures to replace the OCXO or rubidium as a frequency reference. Yet, while designers seeking that specific configuration are waiting for the necessary technologies to mature, the SXO-75 can provide a good holdover in the meantime.

Technical Editor’s note: Please contact Spectratime at SynClock@spectratime.com for more information about the SXO-75.

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