SM3E

The SM3E timing module provides a clock output that meets or exceeds Stratum 3E specifications given in GR-1244-CORE (Issue 2), GR-253-CORE (Issue 3) and ITU-T G.812 (option 3). The SM3E features eight reference inputs. Each input will autodetect the following reference frequencies: 8 kHz, 1.544 MHz, 2.048 MHz, 12.96 MHz, 19.44 MHz, 25.92 MHz, 38.88 MHz, 51.84 MHz and 77.76 MHz.

The SM3E timing module can be configured during production to produce an output up to 77.76MHz. This output is derived from an onboard VCXO and must be specified when ordering. The second output is a BITS output selectable for either 1.544 or 2.048 MHz. The master/slave output is 8KHz. The user communicates with the SM3E module through a SPI port. The user controls the SM3E operation by writing to the appropriate registers. The user can also enable or disable SPI operation through a SPI_Enable pin.

The SM3E offers a wide range of options for the system designer. The bandwidth is SPI Port-selectable from 0.00084 Hz to 1.6 Hz. 0.0016 Hz is the recommended operational bandwidth for Stratum 3E applications. The 8 kHz output has an adjustable pulse width. The pull-in range is also adjustable to establish the desired reference frequency rejection limits. A Free Run frequency calibration value can be written to the module to provide a high degree of accuracy in the free run mode. The reference frequency for any given reference input is automatically detected. A wealth of status information is available through the SPI Port registers.

The user also has a choice between autonomous or full manual control operation.

In manual mode, the user controls the module operating modes Free Run, Hold Over or locked to a specific reference. If the chosen reference is unavailable or disqualified the module automatically enters Hold Over.
In autonomous control mode, operational mode selection occurs automatically based on reference priority and qualification status.

When the active reference becomes disqualified, the module will switch to another qualified reference. If none is available, it will switch to Hold Over. In the revertive mode the module will seek to acquire the highest priority qualified reference. In the nonrevertive mode the module will not return to the previous reference even after it is re-qualified unless there are no other qualified references.
Switching between references is hitless. Likewise, the output frequency slew rate is minimized during any change of operating mode, including entry into and return from Free Run or Hold Over to protect traffic from transient-induced bit errors.

Reference Status information and the operating mode information is accessed through status registers. The module will set the Interrupt pin (SPI_INT) low to indicate a status change.

Free Run operation guarantees an output within 4.6ppm of nominal frequency and Hold Over operation guarantees the output frequency will not change by more than 0.012ppm during the first 24 hours. Frequency accuracy is based on a precision oven to provide the stabilty required for Stratum 3E compliance.

The SM3E can be programmed to startup in any mode or bandwidth. The module may even be programmed to operate in a fully autonomous mode with no further configuration required.

The module operates on 3.3V ± 5% with a typical power drain of less than 3W at turn on, dropping to approximately 1W @ room temperature after warming up. The module operates over the 0° to 70 commercial temperature range.

Phase buildout can be enabled or disabled by means of the SPI port.