Synchronization System Performance Benefits of Precision MEMS TCXOs under Environmental Stress Conditions

This document cannot be viewed on a mobile device or with increased screen zoom functions. Please download the document.
Synchronization System Performance Benefits of  
Precision MEMS TCXOs under Environmental Stress Conditions  
The need for synchronization, one of the key mechanisms required by telecommunication systems,  
emerged with the introduction of digital communication systems. Synchronization requirements have  
been evolving with technology and adapting to the needs of networks. Networks were originally  
designed to primarily carry voice calls; whereas today most traffic is data. The tremendous increase of  
data traffic has triggered the migration from time division multiplexing (TDM) networks to packet  
networks, in particular Ethernet. This change has provided a cost efficient means for handling rapidly  
increasing data loads, but Ethernet is asynchronous in nature and some network services require some  
form of synchronization.  
New standards have been developed that enable synchronization in packet networks. One of these  
standards is Synchronous Ethernet (SyncE) that enables physical layer frequency synchronization for the  
Ethernet network. SyncE requires hardware support along the whole path of frequency synchronization  
transfer. Another standard is precision time protocol (PTP) defined by IEEE 1588 that enables frequency,  
phase and time synchronization through any packet network. Hardware support is not required from a  
packet network to carry PTP timing, however using PTP aware devices, such as transparent clocks and  
boundary clocks, may be necessary to achieve required synchronization accuracy.  
In both SyncE and PTP applications, the local oscillator is a key component that has a direct impact on  
the quality of the recovered clock or time. Network devices can be installed in different locations. Some  
may be in a stable indoor-temperature environment, and others might be mounted in outside boxes in  
harsh conditions. Local oscillators must deliver a high-quality, stable reference regardless of environ-  
mental factors. SiTime MEMS Super-TCXOs (temperature-compensated oscillators) offer significant  
benefits in this area compared to traditional quartz TCXO solutions.  
System Performance under Environmental Stressors  
Oscillator datasheets guarantee performance specifications under ideal operating conditions including  
controlled still air environment without any temperature transients of airflow, no vibration, and stable  
supply voltage. These ideal conditions do not exist in real applications and performance of a TCXO once  
subjected to these environmental stressors is unknown. A common performance risk mitigation strategy  
is to remove the stressors.  
November 2017  
Page 1 of 13  
Synchronization System Performance Benefits of Precision  
MEMS TCXOs under Environmental Stress Conditions  
Some common techniques include:  
Mounting a small plastic cover on the board over the TCXO to isolate it from external airflow  
Placing the TCXO in a section of the board that is far from high power ICs that generate thermal  
transients and are away from cooling fans  
Carefully designing the TCXO power supply which may include using a high quality dedicated LDO  
While considered good design practices for precision quartz TCXOs, these techniques make the design  
more difficult, restrictive, and expensive. In some cases applications impose additional restrictions that  
make it difficult or impossible to eliminate environmental stressors. For example, small form-factor  
pluggable (SFP) modules have size and power restrictions, which force the oscillator to be placed in a  
small and hot enclosure with no option for controlling temperature transients. Another example is  
equipment that must be located near vibration sources, like equipment mounted on poles next to  
railroad tracks.  
A better way to solve the problem is to use an oscillator that is not sensitive to environmental stressors  
and can maintain the same level of performance regardless of operating conditions. This reduces the  
risk of performance degradation, simplifies system design, and reduces cost.  
Architecture of MEMS Super-TCXO  
SiTime MEMS Super-TCXO products have been designed to be immune to common environmental  
stressors: air flow and temperature transients, shock and vibration, power supply voltage variation, and  
output load variation.  
Figure 1 shows a precision MEMS TCXO block diagram. At the heart of the device is the Dual-MEMS  
architecture. Two MEMS resonators with different temperature characteristics are located on the same  
silicon die, which ensures almost perfect thermal coupling between the two resonators. One of the  
resonators is used as a frequency reference to a fractional PLL which generates the output clock signal  
and the other resonator acts as a temperature sensor.  
The PLL has been engineered to provide excellent performance:  
Better than 0.1 ppb resolution (no frequency steps at the output)  
Low phase noise at high frequencies  
Excellent spur performance  
The device utilizes a complex multilevel voltage regulator architecture that serves multiple purposes:  
Dramatically reduces sensitivity to external supply variations and power supply noise  
Decouples internal power supply domains to eliminate output spurs  
November 2017  
Page 2 of 13  
Synchronization System Performance Benefits of Precision  
MEMS TCXOs under Environmental Stress Conditions  
Figure 1: Architecture of precision MEMS TCXO  
Reducing Sensitivity to Airflow and Temperature Transients  
SiTime MEMS precision TCXOs use a temperature sensor scheme that offers low noise, high  
compensation bandwidth, and best-in-class temperature measurement resolution of 30 µK (Figure 2).  
Figure 2: Temperature sensor architecture  
Two MEMS resonators reside on the same physical die. One of the resonators is a TempFlat resonator  
and is designed to have very low sensitivity to temperature variations, with less than 60 ppm frequency  
change over 200°C wide temperature range. The other resonator is engineered to have first order  
frequency over temperature response with 7 ppm/°C slope. The ratio of the two resonators provides  
a measure of die temperature.  
November 2017  
Page 3 of 13  
Synchronization System Performance Benefits of Precision  
MEMS TCXOs under Environmental Stress Conditions  
This approach offers tremendous benefits:  
No temperature gradient between the resonator and temperature sensor even in the case of fast  
thermal transitions  
No temperature measurement error due to the temperature difference between the sensor and  
Those benefits combined with an ultra-low-noise, high-bandwidth temperature-to-digital converter  
TDC) circuit result in a best-in-class semiconductor temperature sensor and make SiTime Super-TCXO  
devices insensitive to airflow and rapid temperature transients. This performance can be demonstrated  
using Allan deviation (ADEV) measurements that show statistical deviation of fractional frequency  
change over a time interval called averaging time (Figure 3). Under still air conditions, the SiTime MEMS  
Super-TCXO has slightly better ADEV performance at 1s to 100s averaging times and is 2.5 times better  
at 1000s. The difference in ADEV changes dramatically when the devices are exposed to light airflow  
(fan in TestEquity 115 Temperature Chamber). There is almost no impact on the SiTime MEMS TCXO,  
but up to 38 times performance degradation from the quartz TCXO!  
Figure 3: Allan deviation (ADEV) of MEMS and quartz TCXOs under airflow  
For additional details on the construction and elements of the DualMEMS architecture, and how they  
differ from quartz TCXOs, see SiTime technical paper: DualMEMS Temperature Sensing Technology.  
November 2017  
Page 4 of 13  
Synchronization System Performance Benefits of Precision  
MEMS TCXOs under Environmental Stress Conditions  
Synchronous Ethernet (SyncE)  
TDM networks, like SONET/SDH require frequency synchronization at the physical layer. Ethernet is  
asynchronous in nature and is not designed for synchronization transporting. TDM emulation is used to  
connect asynchronous and synchronous networks, but it requires a synchronized frequency reference.  
SyncE provides a way to synchronize Ethernet-based packet networks. The requirement of synchronization  
introduces additional restrictions to the equipment clock.  
Asynchronous Ethernet  
Figure 4: Timing distribution in SyncE  
Asynchronous Ethernet requires a ±100 ppm free running oscillator to clock the transmitter PLL  
Figure 4). The clock signal that is recovered through CDR is used only to receive the data and is isolated  
from transmitter. In SyncE, an Ethernet equipment slave clock (EEC) is used instead of the oscillator to  
transfer frequency synchronization from the RX and TX, so that transmitted data is clocked with the  
same frequency that is embedded in the received data. It creates a synchronization chain and all  
network devices downstream are synchronized to a common reference which is traceable to the PRC.  
The EEC is a low bandwidth PLL (0.1 Hz to 10 Hz), so it requires a good quality TCXO to limit slow  
frequency fluctuations called wander.  
November 2017  
Page 5 of 13