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IntroductionNetworks are transitioning from circuit switched to packet switched to become more efficient, flexible and cost effective.

With time critical services such as audio and real time video being transported through the inherently asynchronous

Packet Networks, Timing over Packet technologies such as IEEE1588 is used to support synchronization and is

becoming prominent. The requirements of Synchronization designs posing challenges to traditional methods.

Medium term stability challenges for oscillators used in IEEE 1588v2The local synchronised oscillator is moving from a physical layer based, phase or frequency locked loop, to a time lockedsystem via secondary layer protocols. The non stochastic nature of the packet delay variations (PDV) force the loop to

have smaller bandwidths and thus longer time constants. Oscillators present themselves as high pass filters in the control

loop, thus smaller loop bandwidths can mean that the medium term stability performance of an oscillator is important to

the overall system performance. Effectively the oscillators stability performance is now dominated by environmental

changes during the extended time periods. The oscillator stability over the medium term (minutes to hours) is either

poorly characterised or not been accounted for at all, since the stability under these conditions is covered indirectly by

other oscillator/system specifications.

Historically there have been only two oscillator / lock bandwidth combinations, namely Stratum 3 and Stratum 3E. For

Stratum 3 oscillators, the effect of variable temperature on TDEV and MTIE was likely to be minimal since the oscillator

has a lock bandwidth in the 0.1 to 10 Hz range (time constants of 1.6s to .02s) and any slow changes in frequency due to,

for example, temperature fluctuations were filtered out. Conversely, with the Stratum 3E oscillator, because of the very

high stability required for the variable temperature stability requirement (5 ppb), small changes in temperature would not

affect the TDEV and MTIE even with a 1 millihertz (mHz) lock bandwidth (time constants of 160s.)

Right oscillator for right application

The non - stationary nature of the PDVs impose new requirements for the local oscillator. Loops operate with lock

bandwidths in the 10 mHz to 0.1 mHz (time constant 1600s) region to filter the wander introduced by PDV. System

specifications of frequency stability, holdover requirements for frequency and if required phase will determine the

requirements for Oscillators. For systems that require frequency and time holdover will require Stratum 3E or higher

temperature stability performance. For systems that does not require holdover, oscillators that are compliant to G.8263

specifications may still provide the wander filtering necessary to meet the TDEV / MTIE requirements. The challenge for

the system designer is to assess which oscillator will work at which lock bandwidth and still meet the implied TDEV/MTIE

requirements in locked conditions and assess what oscillators may be required to provide adequate holdover for specified

duration of time required by the application. This is where Rakon can help.

Why Rakon for packet timing applications?

At Rakon we have the expertise to help determine the best oscillator solution for our customers systems requirements.

Oscillator solutions require knowledge of the differential in frequency versus the differential in temperature. Full

characterisation is determined by defining the temperature ramp and measurement rate and a detailed analysis over the

characterisation time period. Rakon has characterised various types of oscillators under changing environmental

conditions for the loop time periods relevant to IEEE 1588v2. Rakon presents a wide range of oscillators that

economically fit a wide range of application requirements in the IEEE 1588 space.