
Preliminary Information
MT9071
33
In Holdover Mode, the MT9071 provides timing and synchronization signals, which are not locked to an
external reference signal, but are based on storage techniques. The storage value is determined while the
device is in Line or External Synchronization Mode and locked to an external reference signal.
When in Line or External Synchronization Mode, and locked to the input reference signal, a numerical value
corresponding to the MT9071 output reference frequency is stored alternately in two memory locations every
30ms. When the device is switched into Holdover Mode, the value in memory from between 30ms and 60ms is
used to set the output frequency of the device.
The frequency accuracy of Holdover Mode is
±
0.05ppm, which translates to a worst case 35 frame (125us)
slips in 24 hours.
Two factors affect the accuracy of Holdover Mode. One is drift on the Master Clock while in Holdover Mode, drift
on the Master Clock directly affects the Holdover Mode accuracy. Note that the absolute Master Clock (OSCi)
accuracy does not affect Holdover accuracy, only the change in OSCi accuracy while in Holdover.
example, a
±
32ppm master clock may have a temperature coefficient of
±
0.1ppm per degree C. So a 10 degree
change in temperature, while the MT9071 is in Holdover Mode may result in an additional offset (over the
±
0.05ppm) in frequency accuracy of
±
1ppm. Which is much greater than the
±
0.05ppm of the MT9071.
For
The other factor affecting accuracy is large jitter on the reference input prior (30ms to 60ms) to the mode
switch. For instance, jitter of 7.5UI at 700Hz may reduce the Holdover Mode accuracy from 0.05ppm to
0.10ppm.
3.2
Remote Loopback
In normal T1 mode (not remote loopback), the jitter attenuator should not be enabled because the transmit data
is synchronized with the transmit clock as it passes through the transmit slip buffer. However, when in T1 mode
and in remote loopback, the jitter attenuator must be enabled, (see Table 172 - T1 & E1 LIU Transmitter Control
- R/W Address YE2), since the transmit data is no longer passed through the slip buffer, and the transmit data
must be in sync with the transmit clock. In addition, when in E1 mode and in remote loopback, the jitter
attenuator must be enabled since the transmit data is no longer in sync with the transmit clock.