Smart Clocks Set Timing Standards

May 1, 2003
Each of these intelligent rubidium clocks integrates a wide range of timing and synchronization functions within a single compact housing.

Timing is everything in most telecommunications systems. Communications systems such as code-division-multiple-access (CDMA) cellular and synchronous-optical-network (SONET) systems rely on precise timing of transmitted data for proper operation. In a wireless environment, a lack of precise timing results in service-affecting dropped calls. In order to achieve that timing, these and other systems depend upon a precision atomic clock such as the iSync+™ Smart SRO-100 and SRO-75 rubidium (Rb) SynClock+™ standards from Temex Time. These smart synchronized-rubidium-oscillator (SROs) clocks can be seamlessly and auto-adaptively disciplined to a multi-vendor stratum 1 reference such as Global Positioning System (GPS), Cesium, LORAN-C, CDMA, and E1/T1 at industry's first 1-ns resolution thanks to its SmarTiming+™ technology, which controls parameters such as GPS's Time-RAIM and position hold and filters input noise such as jitter and wander dynamically up to 100,000 s for optimal output performance. The technology also provides a sync or track mode to phase or frequency align the output to the reference and to adjust the output offsets up to 1 s with a 1-s comparator. The SRO has also an integrated EEPROM to auto-calibrate the frequency in case of power failure or loss of reference and to measure time-interval-error (TIE) performance. In addition, it provides both sinewave and complementary-metal-oxide-semiconductor (CMOS)-level output signals with excellent short- and long-term stability, low current consumption, and fast warm-up times.

The SRO-100 and SRO-75 are actually complete miniature synchronization systems, rather than simple Rb oscillators, generating CMOS, sinewave, and 1 pulse per second (PPS) signals as well as time-of-day information. In addition to the Rb electronics, they incorporate locking, disciplining and synchronization circuitries, bus control, EEPROM, and direct-digital-synthesizer (DDS) circuitry. The SRO-100 is available with sinewave and CMOS outputs in a housing with volume of 11 in.3 while the SRO-75 provides CMOS output signals in a volume of 5.5 in.3

The clocks run in one of two modes: sync r or track. In sync mode, an SRO-100 or SRO-75 phase aligns the output to the reference. In case of loss of reference, the holdover feature for the SRO-100 is less than 2 µs over 48 hours, and less than 7 µs over 48 hours for the SRO-75. When locked to a stratum 1 reference, both clocks exceed the MTIE and TDEV masks defined by ITU-T G.811/823 and ANSI T1.101 standards. In track mode, an SRO-100 or SRO-75 frequency aligns the output to the reference. In either mode, an SRO-100 or SRO-75 can adjust the time or phase offset of the output to up to 1 s through a 1-ns resolution comparator, operating over a dynamic range of ±500 ns.

The SRO-100 (see figure) operates from a single voltage supply of +11 to +16 VDC or +18 to +32 VDC with standard sinewave output of 10 MHz (5 and 15 MHz are optional) and 60-MHz CMOS outputs. The frequency offset over temperature is ±3 × 1011 at temperatures from 20 to +60°C. The clock requires less than 1.2 A current during warm-up. The long-term stability is better than 5 × 1011/month and typically better than 3 × 1011/month. The short-term stability is 3 × 1011/s, 1 × 1011/10 s and 3 × 1012/100 s. The phase noise is 75 dBc/Hz offset 1 Hz from a 10-MHz carrier, 95 dBc/Hz offset 10 Hz from the same carrier, 125 dBc/Hz offset 100 Hz from the same carrier, 145 dBc/Hz offset 1 kHz from the same carrier, and 145 dBc/Hz offset 10 kHz from the same carrier. Harmonics are less than 25 dBc while spurious content is less than 80 dBc.

The SRO-75 provides 60-MHz CMOS output signals. It operates from a single voltage supply of +11 to +16 VDC or +18 to +26 VDC, and requires less than 0.8 A warm-up current at +24 VDC. Both the SRO-100 and SRO-75 feature user-programmable DDS circuitry capable of generating signals from DC to 20 MHz with 32-b digital resolution, as well as RS-232C ports for computer control.

The clocks are ideal for telecom synchronization applications in CDMA, synchronous-digital-hierarchy (SDH), and SONET, as well as tracking and guidance control, analog- or digital-television synchronization, military systems, and navigation systems. They are designed for reliable operation (the Rb lamp life expectancy is 20 years) and are equipped with an analog frequency-adjustment range (by means of a DC voltage of 0 to 5 V) of 5 × 109 ±20 percent. Jumpstart SRO-100 and Jumpstart SRO-75 Designer Kits are also available (with documentation and software) to help familiarize engineers with the operation of these clocks, while testing performance and validating their system design concept. Temex Time, Round Rock, TX; (512) 238-3127, FAX: (512) 238-3128, Internet:

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