Introduction

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This document introduces the fundamental aspects of making valid timing and synchronisation measurements and includes the most common errors. If the recommendations contained within are not understood or ignored it is likely that the measurement data will be invalid. Some of the information in this document is general and applies to many synchronisation measurement scenarios however some information is specific to the SyncWatch platform.

Intended Audience

It is recommended reading before starting a SyncWatch measurement for the first time, and is intended as a first line troubleshooting guide if a measurement yields questionable data.

There are many different types of synchronisation measurement situations. This document contains principles that are common across all scenarios, including synchronisation-over-packet, and will assist in setting up correct measurements and identifying and addressing the root cause of issues.  Further research may be required to carry out the recommended actions or to fully understand the concepts introduced.

This document contains information regarding:

  1. Synchronisation and Timing Measurement Fundamentals
  2. Determining the Signal Type
  3. Controlling Signal Levels and Integrity
  4. Matching End-To-End Signal Impedance
  5. Using Correct Cable Infrastructure
  6. Compensating for Propagation Delay
  7. Understanding What is Being Measured
  8. Choosing a Valid Measurement Reference
  9. Validating the Internal Reference
  10. Identifying Invalid Measurements
  11. Identifying Valid Measurements

Further Documentation

This document does not contain step-by-step procedures for installation, set-up or troubleshooting.  This level of detail can be found in the SyncWatch User Guide or third party manufacturer documentation. 

Synchronisation and Timing Measurement Fundamentals

A synchronisation or timing measurement assesses the quality of a clock source, it may also include effects introduced by the delivery mechanism or network path from the clock itself to the test point.  The results are used to qualify, calibrate or troubleshoot clock sources and delivery methods.

Such measurements are performed by comparing the quality of the clock at the measurement point against a reference clock of known quality, the instantaneous ‘difference’ between the two clocks is sampled when required, consecutive samples show the performance of the measured clock over the measurement period.

The desired quality of a clock source is dependent on the application.  For some, being accurate to the nearest second is acceptable, for others accuracy is required to within trillionths of a second.

Synchronisation in Telecoms Networks

The quality requirements for clocks in telecom networks is dependent on the aspect of the network that the signal is being used for, these range from a few nanoseconds to around 30 microseconds. This level of accuracy, and subsequently the measurement accuracy, is finer than other ‘data’ or ‘packet jitter’ tests of telecom signals which typically have requirements in the millisecond range.

Determining the Signal Type

There are three major types of external signals that can be input into SyncWatch, and it is important to connect to ports of the correct impedance and configure the correct signal type.

If the signal shape is not known prior to measurement, it should be determined using an oscilloscope.

Controlling Signal Levels and Integrity

If the voltage levels of any input signal is out of the allowable range, incorrect or no sampling (triggering) will occur and the measurement will be invalid.  The allowable range varies depending on the SyncWatch port, the levels are detailed in the user documentation.

The signal level and integrity can be determined using an oscilloscope and, if required, subsequently modified by using 3rdparty amplification or attenuation hardware to bring the signal into the measurable range.

The measurement tolerances are tighter when measuring digital (E1/T1) signals due to the complex signal shape, this means there is an increased chance that signal modification will be required to enable correct measurement. 

A U-Link-Tap may be used to passively access a terminated data link, it presents a very low level copy of original signal.  If a U-Link-Tap is employed SyncWatch must be configured for ‘30dB Down’ to increase the port gain and allow visibility of the low level signal, however the signal may require further amplification after the U-Link-Tap to enable correct measurement.

Matching End-to-End Signal Impedance

A device, or port generating a signal for measurement by SyncWatch has ‘output impedance’ — the impedance value of its internal circuitry as 'seen' from the outside.  Each SyncWatch input port has ‘input impedance’ – the impedance value of that port as ‘seen’ from the outside.

The possible SyncWatch port impedance values are 50Ω/75Ω/100Ω/120Ω depending on the port.  Cabling, connectors and other hardware such as amplifiers and attenuators used in the chain also have impedance values.

The impedances of the equipment output port, SyncWatch input port, all cabling, and any other equipment in the signal chain must match to ensure an optimal measurement environment.  If impedance is not fully matched, not all of the signal power will be transferred to the SyncWatch input port and some may travel back up the coaxial line affecting the shape of subsequent signals or pulses and may cause the measurement results to be invalid.

Using Correct Cable Infrastructure

The characteristics and configuration of the cables and connectors used will affect measurements.  As well as ensuring the correct impedance, the type of cable used must also be suitable for the type of signal being measured. 

Telecoms grade cabling and connectors must be used to ensure the signal is transferred within the correct tolerances.

Compensating for Propagation Delay

A signal takes a certain amount of time to traverse a cable, the longer the cable the more time it will take the signal to travel from the source equipment to the destination equipment.  The speed that the signal will travel depends on the cable type so manufacturer documentation must be referenced to correctly calculate this however an average value is 4 nanoseconds per metre.

When using a pulse or digitally encoded timestamp data signal (including the GPS input) with SyncWatch it is important to know how long it takes the signal to traverse the cable link so that this can be factored out using the ‘Cable Delay’ settings in SyncWatch if required.

If cable delay is not compensated for when required, it is not possible to know if a measured offset is a function of the reference clock, the measured clock, or the cable infrastructure, rendering the measured data invalid.

Understanding what is being Measured

The quality of a timing signal is correctly measured by comparing it to a reference clock that has the same or better timing qualities than those being measured in the measured clock.  The relative timing differences between the two clocks as expressed by the output signals that are being measured are recorded as the absolute performance of the measured clock.

Choosing a Valid Measurement Reference

Making a measurement with SyncWatch requires at least one reference clock with which to compare the measured clock, correct reference selection is a critical aspect of ensuring a valid measurement.

The expected quality of a clock will be detailed in the manufacturer documentation or in published standards for clock or network interface types or location.  These should be referenced to ensure the reference clock is of a suitable quality compared to the measured clock.

Validating the Internal Reference

SyncWatch has two methods of generating a reference signal for a measurement - GPS or Rubidium.  For these references to be of suitable quality to use, care should be taken to ensure that the reference is allowed to properly stabilise before being used as a reference.

If the reference is not allowed to stabilise, artefacts such as drift, wander, and phase jumps will render the measured data invalid.

Identifying Invalid Measurements

It is important to be able recognise an invalid measurement within the shortest possible period of time after starting it, as this will limit wasted time and eliminate the need to redo long-term tests.

Before starting a measurement it is important to have an idea of what the results are expected to look like after certain periods of time, and if they don’t look as expected, to verify all elements of the test setup. 

It is unlikely that the exact characteristics of a measured signal are known prior to measurement, however some basic assumptions can be made based on information such as alarm state of the measured equipment, oscillator type, master reference, synchronisation trail length, delivery method and free-run drift rate.  These assumptions can be used place the expected results within a certain timing quality bracket and any deviations within an ‘order of magnitude’.  If test results are significantly different to those expected then there may be an issue with the measurement setup.

As the measurement is simply plotting the TIE between the measured signal(s) and the reference signal(s) invalid measurements can be caused as much by an issue with the reference signal as with the measured signal, particularly if an external reference signal is being used.

Identifying Valid Measurements

It is important to be able recognise a valid measurement within the shortest possible period of time after starting it as this will limit wasted time and eliminate the need to redo long-term tests.

Each synchronisation measurement situation is different and the characteristics of the measured data will vary but there are some common features that can be strong indicators of a valid measurement.