You Received Your Enhanced Primary Reference Time Clock (ePRTC). Now What?
Mobile operators wish to lower their dependency on GPS and protect 4G or 5G networks against outages. Enhanced PRTC, a critical building block, is very accurate and resilient. Understanding how ePRTC reaches its promise is critical.
You just received your long-awaited enhanced primary reference time clock (ePRTC). You want to unpack it, open the box, start the device – and finally verify the very best accuracy available and compliance to the standard. You’d like to test the ePRTC in tough conditions and disconnect GPS and see how long it continues to operate at the required accuracy levels.
Yet, this isn’t a smartphone. You can’t simply start it up and in a matter of an hour have a device that is sophisticated, completely functional and 100 percent ready, in just a blink.
Your expectations are for 30ns or better time accuracy in compliance to ITU-T G.8272.1, 14 days holdover maintaining 100ns to the International Telecommunication Union (ITU) standard and you’re right – that’s what you should get. But when you wish to eat a chocolate souffle, you need to know it takes 20 minutes and the waiter will tell you to order as soon as you get your appetizers.
In the case of your ePRTC, you need to expect a total of six weeks until you can truly verify that your unit performs as expected and even better. Let’s review the three phases you need to take into account.
Planning the Environment
Before you even think of the ePRTC, you need to plan the environment in which it will operate. The ITU standard actually describes commissioning verifications that need to take place. These include, but are not limited to:
• The ePRTC is fully locked to the incoming reference time signal and is not operating in warm-up
• There are no failures or facility errors in the reference path including but not limited to antenna failures
• Environmental conditions are within the operating limits specified for the equipment
• The equipment is properly commissioned and calibrated for fixed offsets such as antenna cable length, cable amplifiers and receiver delays
• The reference time signal (e.g., the global navigation satellite system – GNSS – signal) is operating within limits, as determined by the relevant operating authorities.
And further, if the reference time signal is operated over a radio system such as GNSS, multipath reflections and interference from other local transmissions such as jamming must be minimized to an acceptable level. There are no extreme propagation anomalies, such as severe thunderstorms or solar flares.
Have you reviewed all ITU requirements and completed all critical steps?
Connecting the ePRTC
Now you need to think about what you connect this ePRTC to, so it acts as a fully protected timescale and not just a fancy oscillator. You need to consider which type of atomic clock you will connect as source, not just for holdover backup but also for protection against real world GNSS daily events.
Hopefully, you’ll consider leveraging two Cesium clock systems that the ePRTC will lock to and will properly use as a weighted timescale ensemble.
In that process, remember that your ePRTC holdover capabilities will be only as good as the Cesium clock quality you decide upon. Microchip’s TimeProvider 4100, when equipped with dual Cesium, supports a new adaptive weight protection security feature that will de-weight a degrading or noisy clock and protect the output performance of the ePRTC system.
Setting up the ePRTC
Now, let the fun begin! First, please avoid a common mistake. Time reference is GNSS. Frequency reference is 1pps/10Mhz. The common mistake is to set GNSS highest priority for both time and frequency. Doing so puts the atomic clocks in a classic backup role and you will see none of the ePRTC operational advantages.
Next, consider the six week period touched on earlier. Why six weeks? You will need a 21-day “learning” period, then a 14-day “holdover” period and finally a seven-day “recovery” period – for a total of six weeks.
The learning period determines with ultra-high precision the UTC calibration correction parameters for the ePRTC timescale – the frequency offset estimate of the local Cesium. The GNSS subsystem reports the continuous stream of time error measurements of the local timescale with respect to UTC so the local timescale rate is slowly being adjusted. This first period helps verify the ePRTC indeed meets the time accuracy specifications by the ITU Telecommunication Standardization Sector (ITU-T).
The second period is the holdover test. You disconnect the GNSS signal and need to verify that the ePRTC can hold 100ns for 14 days. The better your Cesium clock, the more performant this test will be. For instance, with Microchip’s TimeProvider 4100 we maintained normal time error performance limits within the 100ns and also maintained 25ns clock class for almost the entire duration of the outage, which is four times better than required by the standard. For this we used Microchip’s high-performance 5071 cesium atomic clock.
Finally, the last test over several days and probably ideally over one week is to verify when reconnecting GNSS to the ePRTC unit, everything returns to normal. The goal is to verify successful re-converge and reestablishment of normal 100-percent timescale protection operation.
At the end of the six-week verification period, your chocolate souffle is finally ready. You are enjoying the utmost delicacy in timing and synchronization and are not disappointed. You knew from the beginning that to enjoy the best, you need the right planning.
From this process you will understand that not all ePRTC systems are created equal, not all Cesium products are created equal and not all suppliers know how to guide operators through the important steps of deploying timing and synchronization systems to obtain the best operational result.
Deploy all your precise time and phase requirements with the same TimeProvider 4100 technology – to ensure performance, accuracy and precision using proven timing technology in operation around the world.
Read more about TimeProvider® 4100. Microchip is a market leader in timing and synchronization serving diverse industries worldwide. Contact us directly for more information or contact our channel partner.
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