STC3800_15 Datasheet, PDF (9/48 Pages) Connor-Winfield Corporation – INTEGRATED - STRATUM 3E TIMING SOURCE

Detailed Description continued
Operating Modes: The STC3800 operates in either Free Run, locked, or Hold Over mode:
Free Run – In Free Run mode, Sync_Clk, Sync_8K, BITS_Clk, and Sync_2K, the output clocks, are determined
directly from and have the accuracy of the calibrated Free Running OCXO/TCXO. Reference inputs continue to be
monitored for signal presence and frequency offset, but are not used to synchronize the outputs.
Locked – The Sync_Clk, Sync_8K, BITS_Clk, and Sync_2K outputs are phase locked to and track the selected input
reference. Upon entering the Locked mode, the device begins an acquisition process that includes reference
qualification and frequency slew rate limiting, if needed. Once satisfactory lock is achieved, the “Locked” bit is set in the
DPLL_Status register, and a frequency history for the selected reference will begin to be compiled. When a usable
Hold Over history has been established, the Hold_Avail pin is set, and the “Hold Over Available” bit is set in the
DPLL_Status register.
Phase comparison and phase lock loop filtering operations in the STC3800 are completely digital. As a result, device
and loop behavior are entirely predictable, repeatable, and extremely accurate. Carefully designed and proven
algorithms and techniques ensure completely hit-less reference switches, operational mode changes, and master/slave
switches.
Basic loop bandwidth is programmable from .77 milliHertz to 1.6 Hertz, giving the user a wide range of control over the
system response. Recommended values are 0.098 Hz for stratum 3 and 0.77 mHz for stratum 3E.
When a new reference is acquired, maximum frequency slew limits ensure smooth frequency changes. Once lock is
achieved, (<100 seconds for stratum 3, <700 seconds for stratum 3E), the “Locked” bit is set. If the STC3800 is unable
to maintain lock, Loss of Lock (LOL) is asserted. All transitions between locked, Hold Over and Free Run modes are
performed with no phase hit and smooth frequency and phase transitions.
Reference phase differences encountered when switching references (or when entering locked mode) are nulled out
with an automatic phase build-out function, with a residual phase error of less than 1 nS. The optional Phase Build-out
feature can be disabled for phase hits on the selected reference, as required for Stratum 3.
Hold Over – Upon entering Hold Over mode, the Sync_Clk, Sync_8K, BITS_Clk, and Sync_2K outputs are
determined from the Hold Over history established for the last selected reference. Output frequency is determined by a
weighted average of the Hold Over history, and accuracy is determined by the OCXO/TCXO. Holdover mode may be
entered manually or automatically. Automatic entry into Hold Over mode occurs when operating in the automatic mode,
the reference is lost, and no other valid reference exists. The transfer into and out of Hold Over mode is designed to be
smooth and free of hits. The frequency slew is also limited to a maximum of ±2 ppm/ sec.
The NOVA kernel history accumulation algorithm uses a first order frequency difference filtering algorithm. Typical Hold
Over accumulation takes about 15 minutes. When a usable Hold Over history has been established, the Hold_Avail pin
is set, and the “Holdover Available” bit is set in the DPLL_Status register. The Hold Over history continues to be
updated after “Holdover Available” is declared.
The NOVA kernel accumulates the Hold Over history only when it has locked on either an external reference in Master
operation or the Xref clock in Slave operation. Tracking will be suspended automatically when acquiring a new
reference, while in the Holdover mode, and in the Free Run mode. A set of registers allows the application to control a
Hold Over history maintenance policy, enabling either a rebuild or continuance of the history when a reference switch occurs.
Furthermore, under register access control, a backup Hold Over history register is provided. It may be loaded from the
active Hold Over history or restored to the active Hold Over history. The active Hold Over history may also be flushed.
See Holdover History Accumulation and Maintenance in the Application Notes section for further details.
Holdover mode may be entered at any time. If the mode is forced and there is no Hold Over history available, the prior
output frequency will be maintained. When in Hold Over, the application may read (via register access) the time since
Hold Over was entered.
Preliminary Data Sheet #: TM061 Page 9 of 48 Rev: P06 Date: 11/22/04
© Copyright 2001 The Connor-Winfield Corp. All Rights Reserved Specifications subject to change without notice