MSTM-S3-T2-FD

The Connor-Winfield Stratum 3 Miniature Simplified Control Timing Module MSTM-S3-T2-FD acts as a complete system clock module for general Stratum 3 timing applications. The MSTM is designed for external control functions. Full external control input of MSTM-S3-T2-FD allows for selections and monitoring of any of four possible operating states: 1) Holdover, 2) External Reference #1, 3) External Reference #2, and 4) Free Run. The table below illustrates the control signal inputs and corresponding operational states: In the absence of External Control Inputs (A,B), the MSTM enters the Free Run mode and signals an External Alarm. The MSTM will enter other operating modes upon application of a proper control signal. Mode 1 operation (A=1, B=0) results in an output signal that is phase locked to the External Reference Input #1. Mode 2 operation (A=0, B=1) results in an output signal that is phase locked to External Reference Input #2. Holdover mode operation (A=1, B=1) results in an output signal at or near the frequency as determined by the latest (last) locked-signal input values and the holdover performance of the MSTM.

The primary feature of this model MSTM-S3-T2-FD is the Reference Frequency Detector (RFD). This is an independent circuit that monitors both reference inputs simultaneously to determine that a signal is present and its frequency is within a valid range. A logical one on the outputs VALID_R1 and VALID_R2 indicates that the signals applied to EX_REF1 and EX_REF2 respectively have been detected and have a frequency that is within at least +/- 4.6 ppm of nominal. A range of +/-4.6 ppm is guaranteed for the life of the module. The actual range is somewhat more than twice that to account for normal drift and aging that will occur. When, for example, the reference applied to EX_REF1 disappears, VALID_R1 will go to a logical zero within 500 microseconds. When the signal returns at a frequency within +/- 4.6 ppm of nominal the VALID_R1 output of MSTM-S3-T2-FD  will return to a logical one after a 4-second delay. The delay is a validation period that requires the output of the frequency detector to remain stable for 4 seconds before confirming the status of the reference applied. This eliminates the incessant toggling of the frequency detector that occurs when the reference frequency is at the threshold frequency of the detector The function of the RFD of MSTM-S3-T2-FD is not related to the operational alarms, LOL and TVL, which monitor the operation of the PLL relative to the selected active reference.

In fact the operational alarms function only at the extreme edge of the PLL operating range. This means that it is quite possible for the active reference to drift out of range of the Reference Frequency Detector and still remain well within the capture range of the PLL and not activate the operational alarms. If the other reference of MSTM-S3-T2-FD was still marked valid by the RFD it might make sense to switch to the better reference before the selected reference drifts completely out of range. Alarm signals are generated at the Alarm Output during Holdover and Free Run operation. Alarm Signals of MSTM-S3-T2-FD are also generated by loss-of-lock, loss of Reference, and by a Tune-Limit indication from the PLL.

A Tune-Limit alarm signal of MSTM-S3-T2-FD  indicates that the OCXO tuning voltage is approaching within 10% the limits of its lock capability and that the External Reference Input may be erroneous. A high level indicates an alarm condition. Realtime indication of the operational mode is available from outputs S0 and S1, which are determined from internal mode registers. Control loop filters of MSTM-S3-T2-FD effectively attenuate any reference jitter and smooth out phase transients.