The Use of Atomic Clock Holdover for Precision Time Applications in the Field - Quick Guide

Although using PRTC in Holdover mode isn't recommended for Precision Timing test applications, Time Error, Wander, One-Way Delay measurements are needed in places with no access to traceable reference clock. So, it's often seen as a fair alternative.

Although the use of primary reference time clocks (PRTC) in Holdover mode is not recommended for Precision Timing test applications, Time Error, Wander, One-Way Delay measurements are still needed in places with no access to any other traceable or accurate reference clock. So, Holdover is often proposed as an alternative workaround. However, one must be fully aware of the compromises being made, the uncertainties, the process and precautions required, and their effects in the measurement results.

Should you decide to use one pulse-per-second (1PPS) Atomic Clock holdover for timing and delay measurements, you must adjust your expectations accordingly. As well as develop a test procedure defined to minimize the effects of external conditions in the Time or Timing drift (Time Error accumulation over time). Have a clear goal of what is the minimum accuracy required for the type of measurements being performed and the expected resolution. For example, if you plan to measure one-way Ethernet packet delay with ±1 μs resolution, then a well-trained clock holdover may provide that level of uncertainty for up to six hours. If the expected resolution is ±0.01 ms or less, then holdover certainly starts making a much more sense, since the required accuracies (and expected) uncertainties are more attainable, and in such scenario the reference time clock could last a full working day in holdover mode. Know the application and make the necessary informed decisions.

1. General Application Workflow

Proposed Procedure

1. Connect the test set's GNSS receiver to the roof antenna at the central location, turn the test set ON and let it's GNSS receiver perform the Location Survey (find its most accurate position) and Discipline the internal Atomic Clock (calibrate its frequency and align its timing to the standard second) for 24 hours or longer.
2. Launch the desired test application to let the test set warm up while the clock disciplining process is taking place. Some examples of time-based test application are: 1PPS Clock Phase (Time) Error, One-Way Delay (Ethernet), Recovered Clock Wander Measurements, SyncE, etc.)
3. Keeping the test set ON, disconnect the antenna, to enter holdover mode, and transport the test set to the test location. Avoid sudden changes in temperature during the trip.
4. At the location, connect the test ports to the device or link under test, select Atomic_1PPS as the Reference Clock for time-oriented measurements or the Atomic_10MHz for clock frequency and wander applications.  Note: The use of Clock Holdover is not recommended for high-precision timing applications, like PRTC or PTP Time Error verification, requiring nanoseconds-level (ns) accuracy.
5. Since the holdover time accuracy slowly degrades over time, for most test applications, it is recommended to keep the total test time under four (4) hours, including the transit time. If the measurement doesn't require high accuracy, like measuring One-Way Delay with 1 μs resolution, the total time could be extended to six (6) hours. 
6. When finished, it is recommended to keep the test set ON during the return to the central location. Then go back to step 1.

2. Recommended Configuration for VeEX Test Sets

Use a flexible coaxial (adapter) cable to connect the GNSS antenna feed to the test set's GNSS receiver SMA connector . Turn the test set ON.

2.1 GNSS Receiver Configuration

Go to > >Utilities >Settings >More >High Precision Clock Source and select the GNSS Receiver tab.

The test set may be configured with one of the two Precision Timing GNSS Receivers offered by VeEX and their matching portable antennas:

• Precision-timing Single-band Dual-constellation GNSS Receiver (P/N: Z88-00-009P), supporting up to two constellations at a time, including GPS (L1C/A), GLONASS (L1OF), Galileo (E1B/C) and BeiDou (B1I). It can be identified by the GNSS Receiver label on the configuration and status page.
• High-precision Multi-band Quad-constellation GNSS Receiver (P/N: Z88-00-010P), supporting up to four simultaneous constellations and two bands, including GPS (L1C/A, L2C), GLONASS (L1OF, L2OF), Galileo (E1B/C, E5b) and BeiDou (B1I, B2I). It can be identified by the GNSS Receiver label on the configuration and status page. 

1. Select all the GNSS Systems and Frequency Bands supported by the GNSS receiver and antenna being used, for best performance. You may exclude GNSS systems or bands that are not trustworthy or have problems in your region (e.g., jamming, spoofing, etc.). The use of multi-constellation and multi-band receivers is recommended for best performance.
2. If the GNSS antenna is not already powered, turn the Antenna 5V ON.
3. Location Survey - The Site Survey is a fine-tuning process in which the GNSS receiver reduces the position uncertainty to a desired level of accuracy, in order to lock the coordinates (no longer updated) to produce more accurate and stable timing signal (1PPS, One pulse per second). Enable the survey for precision timing applications and Atomic Clock disciplining. 
4. Window (s) - Is the amount of time the position must remain below the target accuracy, before the GNSS receiver locks the coordinates and enters the precision time mode. The recommended value is 600s.
5. Accuracy (m) - Is the target position accuracy for the survey to lock the coordinate and enter precision time mode. Use 1m for roof antennas with excellent signal quality (multiple satellites with >42 dB-Hz C/No) or 3m for portable antennas, with unobstructed view to the sky. Note: If the GNSS signal reception is poor, the position may never reach the target accuracy (3D Deviation).
6. Antenna Cable Delay Compensation (ns) - Enter the coaxial cable delay measured from the antenna to the connector, including any additional patch cord cables. If the exact cable delay is not known, users may approximate it by using the total Length (L) and multiply it by the Velocity of Propagation (VP) stated on the cable datasheet or use the generic 5 ns/m. Refer to this article for more details on How to Measure Antenna Cable Delay.
7. Time Base - Select the satellite constellation to be used as the time reference. GPS or Galileo are recommended.
8. UTC Offset - Select the proper time zone offset for the region.
9. Connect the antenna cable to the test set's GNSS receiver SMA port, using a flexible coaxial cable. Turn the GNSS Receiver ON, to start the location survey (site's coordinates) and time alignment process.

The 3D Deviation value should start to decrease until it reaches the desired Accuracy (m) target. The Location Survey process will take some time to complete. The total time will depend on the targeted Window (s) and Accuracy (m) thresholds, as well as the quality of the satellite view and reception. Poor RF signal quality will make the process take longer or not complete at all, if the signals' quality (C/No) are poor.

A.  GNSS Status shows Locked when at least four satellites have been acquired and the GNSS receiver is cable of calculating the local coordinates and can perform an initial rough alignment of the internal GNSS 1PPS timing reference pulse. 

B.  Satellites displays the total number of GNSS satellite vehicles that the antenna can currently "see".

C.  Antenna Status - If the test set is the one powering the antenna (Antenna 5V = ON), this field will give an indication of the antenna and cable status. OK indicates that an antenna is connected at the far end of the cable. Short indicates that a short circuit or high current draw has been detected. Open indicates that there is no antenna connected.

D.  Reset GNSS - Use this button to make the GNSS receiver forget all the parameters that has learned from the satellites and start the satellite acquisition process again (cold start).

E.  Site Survey - Status of the Location Survey process. Active indicates that the survey is still in progress. Locked indicates that the survey has finished and that the coordinated have been locked (no longer updated).

F.  3D Dev. - Tridimensional position deviation, in meters (m). If the deviation stops decreasing and doesn't reach the target Accuracy (m), check the GNSS antenna installation and the Signals (C/No), to make sure that there are at least four satellites with >38 dB-Hz at all times.

G.  Antenna's 3D Coordinates displayed in Degrees, Minutes and Seconds (DMS) and in Decimal Degree (DD) formats, as well as the altitude.

H.  Save - Once the Location Survey has finished (Site Survey = Locked), users can save the coordinates with a descriptive name. The saved location can be used in future tests to save time, by avoiding the survey process.

I.  ToD - Displays the current universal (UTC) Time of Day (date and time) in YYYY/MM/DD hh:mm:ss format.

J.  Sync ToD - Applies the current universal date and time (ToD) obtained from GNSS to the test set, with the appropriate local time zone offset correction.

Once the Site Survey = Lock, the 3D coordinates stop updating and the GNSS clock goes into Precision Timing mode. Users can tap on the Save button to store the coordinates for commonly used locations, with a name, then save time next time by selecting Location Survey = Manual and selecting the saved location. In Manual mode, the test set doesn't have to go through the Site Survey process, saving up to an hour of preparation time.

2.2 Atomic Clock Configuration

Go to  >Utilities >Settings >More >High Precision Clock Source and select the Atomic Clock tab. 

1. Disciplining Profile - Users can select between multiple pre-loaded settings or Manual mode. Each profile has different a Time Constant and a hint of the minimum time required to achieve Disciple Lock, with a good reference signal. 
2. Time Constant (s) - This is the time the Atomic Clock's disciplining circuitry (servo) will require the relative Time Error to be below the configured Discipline Threshold (ns), before declaring 1PPS Discipline Status = Locked. For a multi-band quad-constellation GNSS receiver, with good roof antenna installation, the recommended Time Constant is 1800s.
3. Discipline Threshold (ns) - This is the maximum relative Time Error variation allowed by the Atomic Clock's disciplining circuitry (servo), required to enter 1PPS Discipline Status = Locked status. For a multi-band quad-constellation GNSS receiver, with good roof antenna installation, the recommended Discipline Threshold is 20ns. 
4. Discipline Source - Selecting a 1PPS clock reference starts the disciplining process. For this application, select GNSS 1PPS. 
5. 1PPS Signal Health - Verify that the Atomic Clock is receiving a valid 1PPS clock reference signal. If it shows invalid, check the GNSS Receiver configuration and that the GNSS Status = Locked.
6. 10MHz Oscillator Status - Indicates the status of the internal Atomic Clock oscillator frequency. It should display Locked after it has finished its quick warm up and internal calibration sequence, which should happen within 180 seconds after the test set is turned ON. 
7. 1PPS Disciplining Status - Once the Discipline Source is selected, the status turns into Acquiring, to indicate that the Atomic Clock is being trained by the 1PPS reference clock. Once the Atomic 1PPS output meets the relative Time Error threshold (Discipline Threshold) for at least the amount of time defined by the Time Constant, the discipline process is declared Locked and ready to use.
8. Lock Time - Time that has passed since the internal Atomic Oscillator achieved Locked status. It can also serve as an indicator of how long the test set has been ON.
9. Total Holdover - This is the indication of how long the test set has been in Atomic Clock Holdover mode. 
10. Phase Graph - Displays the relative Time Error between the 1PPS Reference (from GNSS) and the Atomic Clock 1PPS output (Atomic 1PPS). The Atomic Clock time correction (disciplining) is a slow process that can take several hours. As the Atomic 1PPS timing aligns with the standard second the relative time error should converge towards zero. The phase monitor graph shows the last 10 minutes (600s).
11. Atomic Clock 1PPS Timing status Indicator (always visible).
12. GNSS Receiver Satellite Lock status indicator (always visible). Green indicates that satellites are being tracked and a valid GNSS 1PPS is available.

Once the Disciplining Status = Locked the test set's Atomic Clock 1PPS and 10MHz signals have achieved their most accurate state.

2.3 Entering 1PPS Holdover

Once the test set's Atomic Clock is discipled and locked , it will automatically enter Holdover mode any time it loses the reference 1PPS clock being used for disciplining (GNSS 1PPS).

To force the test set's Atomic Clock into Holdover, users can quicky disconnect the GNSS antenna cable from the SMA port of the GNSS receiver. The icons at the bottom of the screen will show Holdover and loss of satellite signal .

3. Field Testing

Time and timing-based measurements are performed in the same way, whether the reference time clock is being actively synchronized by GNSS or in holdover mode. Follow the procedure for the required test and use Atomic 1PPS or Atomic 10MHz as the measurement reference clock.

It is highly recommended to load the test application right after the disciplining process has started at the central location, so the disciplining process takes the internally generated heat into account, while disciplining the Atomic Clock.

Do not turn the test set OFF or it will lose its frequency and time synchronization.

Avoid sudden or significant ambient temperature changes during holdover (ΔT=<10ºC).

Tests that Use 1PPS as a Reference for Measurements

• One-Way Delay (OWD): Uses two test sets, at two different locations, both synchronized to the standard second (1PPS), so they can measure the payload (data) delay (or latency) in each direction independently. This type of test is available for different types of links, such Ethernet, G.703 Codirectional, IEEE C37.94, etc. Refer to this Ethernet One-Way Delay Measurement article for more details.
• Clock Wander & Phase Measurements: Uses 1PPS reference to measure network elements' physical clock output Time Error or 10 MHz to measure frequency wander, stability and offset. 
• PTP Slave Emulation with Recovered Clock Time Error Measurements: Uses the 1PPS clock reference to measure Time Error (2WayTE). Reference clocks in holdover mode may only be advisable for quick clock checks at the edge of the link, where the maximum allowance is large enough (e.g., 1,100ns).

4. Important Considerations

Can the 1PPS Pulse be Quickly Re-Aligned, at the Location, Before Testing?

Reconnecting the portable antenna in the field to re-sync the 1PPS (e.g., outside of the customer's building) is NOT recommended. There are several reasons why this could increase the Reference Time Error:

• The antenna cable delay compensation for the 5m portable antenna is typically 25ns (5m), while the compensation used for the longer (roof) antenna cable, at the central site, will be much larger. The difference would be reflected in the measurements as added Time Error.
• After the Location Survey at the central office, the GNSS receiver goes into Precision Time mode, fixing the coordinates used for time calculations. If the portable antenna is connected at a different location (different coordinates), the time calculations will be wrong (more added Time Error). In certain cases, if the distance exceeds certain GNSS receiver thresholds, it may trigger to restart the process again. One option to reduce the effects of different coordinates is to set Location Survey = Disabled, to allow the GNSS receiver to get the new coordinates, but this would also reduce the timing accuracy and stability.
• These issues not only could introduce the difference as an extra error to the TE or OWD measurements, but if the difference (in ns) is large enough, it could trigger the atomic clock to restart the disciplining process.
• The RF signal quality (C/No) and multi-path conditions at street level are mainly unpredictable. Bad GNSS signal could lead to bad performance and increased Time Error. 

What About Temperature Changes?

Temperature changes, especially temperature shocks, must be avoided! An example of temperature shock would be if in a cold winter or hot summer, the user takes the time to discipline the test set outside and then quickly brings it inside to an air-conditioned building at 22ºC. That could mean an instant change in ambient temperature >±10ºC. That will definitely affect the characteristic of the Holdover's time drift behavior.

During the disciplining process the atomic clock's tuning system learns about the effects of slow temperature changes and it will use that information during the Holdover state to try compensating for it.

In general, try to keep the temperature changes slow and no more than 10ºC within one hour.

One should also keep into account the heat generated by the instrument itself, which is many cases helps maintain a somewhat constant temperature. So, it is important to keep the test set ON all the time and with the required test application loaded and at their operational temperature. (Loading test applications just before the start of the test may create extra heat, that was not accounted for.)

DO NOT operate the test set below 10ºC or above 40ºC, while in Holdover. Operate it inside an air-conditioned van/truck if necessary and keep it between 18~24ºC, for best results.

DO NOT place a powered-ON test set inside the carrying case, as it can overheat. Keep the fans airflow from being obstructed. 

Keep the Test Set Running at All Times

In general, Atomic Clocks are designed to be running continuously, to stay in top condition, but that can be a challenge for portable test equipment. For that reason, it is recommended to keep test sets used for precision timing applications always ON and disciplining against a good GNSS antenna, even when not in use. 

Why the Uncertainty? Shouldn't Atomic Clocks be Highly Predictable?

Actively disciplined Atomic Clocks are highly predictable, accurate and stable, in their steady-state (long-term) operation. However, Holdover is not considered normal operation and their performance in this condition will depend on the clock quality, among other parameters.

While in normal operation (locked mode) the disciplining circuitry continuously changes the Atomic Clock's frequency, to adjust the 1PPS rising edge aligned with the start of the standard second. Although those variations (frequency offsets) can be just a few parts-per-trillion up and down, the atomic oscillator will try to retain the last correction, in the event of a reference loss. That means, the atomic oscillator's frequency is never 100% accurate, because the priority is on keeping the 1PPS timing aligned. However, in the field, it is not easy to quantify, predict or estimate what the last frequency offset was. Any frequency error will cause the Time Error to accumulate over time.

The graph below shows a theoretical worst-case scenario and the possibilities for a Time Clock in holdover, in absence of any other environmental factors. The datasheet of the chip-scale Atomic Clock, used in VeEX products, specifies a frequency accuracy of ±5E-11 (0.05ppb) at shipment. Of course, disciplining helps correct any intrinsic error from the atomic oscillator. The dark-red lines would be the expected Time Error drift if not disciplined (free running) and it only takes about 33 minutes to drift to 100ns. The dark-green lines show an example of a disciplined (corrected) clock, drifting to 100ns in about three hours, because its last frequency offset for the last correction was 10 parts-per-trillion (1E-11). But there are thousands of possible last corrections in between.

That is to show that, even if the frequency error (slope) doesn't change over time, the Time Error is cumulative and adds up very quickly, no matter how "good" a portable atomic clock is.

For example, if we wanted to maintain a reference time clock error (in holdover) <±100ns at the end of the day (let's say 10 hours), to measure values around 1,100ns, then the frequency error of the Atomic Clock must be better than 2.8E-12 at all times. Although this could be possible, it is certainly not repeatable. Every time you start a new holdover, there are multiple paths that cumulative Time Error function could take.  

That is what makes Time Holdover unpredictable. Then we can add temperature changes, GNSS short-term stability issues and vibration to the mix.

In VeEX products, the Atomic Clock's Phase Graph can be used in the field to get an idea of the current state of the corrections, to minimize the frequency error and its effect on Holdover Time Error. First, wait until the graph is "flat" and completely horizontal (theoretically, the lowest frequency offset possible). Then Zoom in to 10ns/Div and you will notice that the yellow line is not actually flat, but it is changing all the time with small amounts of short-term phase noise (mainly coming from the GNSS). You may wait until the average trend is zero (flat) to disconnect the antenna, to try getting the lowest frequency offset possible. However, there is a long-term wander component to the GNSS that is difficult to characterize and take into account in the field.

Related Articles

For more detailed information on disciplining and holdover, refer to: The Use of Atomic Clock Disciplining with GNSS/GPS & Holdover for Synchronization and Time Error Testing on the Field.

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• TX340s Advanced Multi-Service 16G Test Set (single module)
• RXT-6200 Advanced Multi-service 100G Test Module