How to time-synchronise multiple devices running MetaWatch

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How to time-synchronise multiple devices running MetaWatch

Posted on July 26, 2017 by Matthew Knight 26 July 2017

MetaWatch is a powerful application designed for Metamako's high performance hardware which simplifies network data capture, monitoring and analytics. It combines several components of a traditional network monitoring solution into one powerful device:

 

As improvements and new features are added to MetaWatch regularly, such as improved time-synchronisation, it is definitely worth ensuring that the current version of MetaWatch is used.

Timestamping & Time Synchronisation

One of MetaWatch's features is its ability to timestamp incoming frames with 1 ns precision. To allow timestamped network events to be compared between multiple devices it is important that each device running MetaWatch's clock be synchronised to the most stable time reference possible. Any instability in the reference will cause MetaWatch's clock synchronisation algorithm to attempt to track the instability. As MetaWatch can only do so when it detects a divergence in clock phase from the time reference, any corrections to its clock will always lag the time reference. In short, the more stable the time reference, the better a device can maintain syncronisation to it.

MetaWatch supports both NTP and PTP for time synchonisation over the network. Both may optionally be coupled with a 1 PPS (Pulse-Per-Second) input. This input currently provides the most accurate method for MetaWatch clock synchronisation therefore Metamako recommends that clients looking for the most accurate timestamps use the PPS input option. 1 PPS is essentially a voltage pulse with the rising edge aligned with the start of each second typically delivered down a 50 Ω coax cable assembly. The midpoint of the rising edge allows the start of second to be determined extremely consistently. Most GNSS receivers designed to provide reference time offer 1 PPS output(s), as do stand-alone atomic clocks.

Key features to look for in a reference clock used to drive accurate timestamping

Here are the top three items to look out for:

  1. The short term stability (second) of its oscillator usually defined by its Allan Deviation (or Variance) which generally gives a good indication of how consistently the 1 PPS output will perform second-on-second
  2. The long term stability (hours/days) of its oscillator, that is mainly dictated by oscillator aging, and will dictate holdover performance
  3. GNSS
    1. Whether the reference clock is backed by one or all of the satellite navigation networks which provides it with a very stable (on average) frequency reference. It must be noted that second-on-second, received GNSS time can move by 100 ns (or more) mainly due to Tropospheric Delay (atmospheric effects on the speed of the electromagnetic waves from the GNSS satellites to Earth)
    2. When backed by GNSS, a very stable oscillator such as a Rubidium atomic clock is preferred as it is itself stable enough to continually "smooth out" the jitter on the GNSS timestamps over periods of hours, a day or even longer if required

If only relative timestamps are needed, a free-running Rubidium Reference Clock is generally more than adequate. They are typically exhibit a stability of around a 1 microsecond per day and remain frequency stable to around 50 picoseconds per month.

MetaWatch Clock Options

All Metamako devices contain a very high quality TCXO (Temperature-Compensated Quartz Oscillator) capable of driving MetaWatch to produce excellent timestamping results however TCXO's are somewhat susceptible to changes in temperature altering their frequency and are typically only stable to ppm (parts-per-million) i.e. when driving timestamping over the second between 1 PPS reference pulses, they are unlikely to be nanosecond-accurate and free-running, may drift by tens of milliseconds a day or more.

As an option for MetaWatch-capable devices, Metamako also offers an OCXO Clock Module (Oven-compensated Quartz Oscillator) and Atomic Clock Module (Rubidium Oscillator). Both increase stability to better than 1 ppb (part-per-billion) allowing them to drive true nanosecond precision timestamping. The key difference between the modules is free-running stability following loss of reference (holdover). In 24 hours, the OCXO Clock Module would maintain accuracy to several tens of microseconds whereas the Atomic Clock Module would maintain accuracy to around a microsecond.

Pulse Distribution Units

To synchronise multiple devices running MetaWatch from the same 1 PPS source, a Pulse Distribution Unit is the recommended solution. Pulse Distribution Units typically take in a single 50 Ω 1 PPS output from the time reference and redistribute it to multiple devices while maintaining the correct 50 Ω load on the time reference and presenting the same 50 Ω source to the devices connected to it. To ensure that individual device clocks are optimally synchronised, it is key that the 1 PPS pulses each arrive from the Pulse Distribution Unit to the device absolutely concurrently. To achieve this, the cable assemblies connecting the Pulse Distribution Unit to each device need to be as similar as possible both in terms of cable material and length as a longer cable has a greater propagation delay and will delay the pulse arrival compared to a shorter cable of the same material.

The key features to look for in a Pulse Distribution Unit are:

  1. Port-to-port output skew and jitter (ideally the outgoing pulses will all leave the unit within picoseconds of each other)
  2. Number of output ports
  3. Ability to potentially drive long cables (>100m)

Assembling the above components and multiple MetaWatch instances

As part of a forthcoming timing benchmark, Metamako assembled:

  • A free-running SRS FS725 Rubidium Frequency Standard providing an extremely stable 1 PPS output with quoted specifications:
    • Allan Variance: < 2 × 10-11 (1s)
    • Aging: < 2.5 × 10-11 (monthly)
  • A TimeTech 10535 16-port Pulse Distribution Unit redistributing the pulse from the SRS FS725 to a pair of MetaApp 32's running MetaWatch with quoted specifications:
    • Port-to-port output skew: < 20ps
  • Pasternack PE341 SMA Male to SMA Male Low Loss Test Cables provided the physical PPS connectivity

The following Oscilloscope trace, run as part of the calibration for the benchmark, shows the difference in arrival time of two of the outputs from the TimeTech Pulse Distribution Unit of pulses generated by the SRS Rubidium Frequency Standard. Points of note:

  • There is a skew of ~4.2 ns between them as there was an additional 1 m of cable on the output connected to C2 (required as part of the benchmark calibration measurements)
  • The measurement statistics show that over 2,000 samples (seconds), all pulses were within a range of 18.4 ps of each other
  • The derived histogram channel also shows that though the distribution is probably not Gaussian, the distribution of the majority of samples fell within ~5 ps of the mean

Oscilloscope trace for time synchronisation

Synchronising multiple devices running MetaWatch using the above reference frequency standard and pulse distribution unit

  • The 1 PPS output from the SRS Rubidium Frequency Standard connects to the DC Input of the TimeTech Pulse Distribution Unit
  • Each MetaWatch device has its 1 PPS input connected to one of the 1 PPS outputs on the TimeTech Pulse Distribution Unit
  • All PPS connections use 50 Ω coax - each cable assembly should be of the same product type from the same manufacturer and ideally identical in length to ensure that the pulses reach each MetaWatch instance concurrently
  • This Pulse Distribution Unit would support up to 16 MetaWatch devices though this number could be extended via cascading Pulse Distribution Units
    • Cable lengths would need to be adjusted to ensure equal propagation delays from the SRS Rubidium Frequency Standard's 1 PPS output to each MetaWatch instance
    • Alternatively, MetaWatch does allow a cable length correction parameter to be entered in nanoseconds to compensate for scenarios where cables are of different lengths or Pulse Distribution Units are cascaded
Synchronising multiple devices running MetaWatch using the above reference frequency standard and pulse distribution unit

 *Image Components sourced from www.timetech.de and www.thinkSRS.com

Wrapping it up - our 5 Top Tips

  1. For optimal MetaWatch clock-synchronisation to a time reference, use NTP or PTP coupled with 1 PPS
  2. The more stable the 1 PPS time reference, the better MetaWatch will maintain clock synchronisation
  3. Optimally synchronising multiple MetaWatch instances to the same reference requires that each MetaWatch instance receives 1 PPS pulses consistently within a few picoseconds of each other
  4. The choice of Pulse Distribution Unit will dictate how well synchronised MetaWatch instances are to each other along with ensuring that cable propagation delays from the Pulse Distribution Unit to the MetaWatch instances are identical or compensated for in the MetaWatch configuration
  5. When synchronising more than 16 MetaWatch instances to a single 1 PPS reference, Pulse Distribution Units may be cascaded

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