5700MSC-IP
IP Network Grand Master Clock & Video Master Clock System
Page - 40 Revision 0.2
The GNSS receiver also provides highly accurate time. The 5700MSC-IP uses the time information to
set the phase of all outputs. This is done by choosing a moment in GNSS time when all standards
started at their initial phase. By working backwards to that moment, the instantaneous phase of any
standard can be determined for any point in time. This allows true deterministic cross-standard
phasing. See Figure 3-12 for the timing relationship between multiple standards when locked to
GNSS.
Because the 5700MSC-IP uses GNSS time to determine the phase of the outputs, the
time reference will be forced to use GNSS, regardless of the time Reference Src
menu item setting.
Using GNSS as a frequency reference is the only way to guarantee that all outputs across multiple
standards will be phased to each other across multiple units. Furthermore, when two 5700MSC-IP
units are separated by a large distance, using GNSS receivers can assure they will be phased to
within 100ns of each other. Note than when using multiple units the antenna cable lengths should be
matched as closely as possible to eliminate any cable length propagation delay effects that may cause
phase errors between them.
If the GNSS receiver is unplugged, or if the number of satellite signals being received drops to 0, the
5700MSC-IP will free-run on its high stability OCXO. The Lock type menu item controls how the unit
will respond when the GNSS signal is restored. When the unit first starts up, the phase of its outputs
are random until GNSS lock is acquired.
While GNSS provides a convenient way to lock the 5700MSC-IP in all standards simultaneously and
ensure extremely good frequency accuracy, there can be issues dealing with limited availability and
phase errors. A GNSS reference can be described as a “soft” lock. As more satellite signals are
discovered, the phase accuracy of the GNSS lock improves. However, there will always be some
“wander” associated with GNSS lock due to uncontrolled variables such as atmospheric disturbances
and terrestrial interference. This can cause problems when two different units are locked to separate
GNSS receivers.
Under ideal conditions, with 8 satellite signals being received, the maximum wander in phase between
two units locked to two separate GNSS antennas can be up to 15ns. This is greater than the duration
of a single sample of serial digital HD video which may cause problems for router installations that
require sample-accurate timing. During poor weather conditions, the satellite signal strength will drop
which will cause fewer satellite signals to be available for phase corrections. This is why mounting of
the GNSS receiver is extremely critical in order to get the best phase locking performance. If the
receiver does not have a clear view of the sky or is subject to multipath interference due to reflections
off buildings, the phase wander between units can quickly climb to unacceptable levels. The
unpredictability of such variations can be difficult to troubleshoot so these characteristics must be
taken into account during the design phase of any system.
3.3. TIMEKEEPING
The 5700MSC-IP contains a system clock to keep track of time and date. The stability of this clock is
equal to the selected frequency reference. The accuracy of this clock is determined by the selected
time reference and lock mode. The 5700MSC-IP can access time references through the GNSS
receiver, PTP from another 5700MSC-IP, SNTP, or VITC read from a black burst reference. The
system clock should always be set to Coordinated Universal Time (UTC). Time zone offsets can be
applied to the timecode outputs to provide local time.
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