Disciplined Oscillator

Info

This section describes a possible setup for a 10 MHz disciplined clock using the mosaic-T as a precision time source, which is particularly suitable in combination with Fugro AtomiChron^® service. For more details, please refer to Appendix D of the hardware manual and firmware manual.

Warning

If the reference clock switch is set to EXT, users must provide a 10MHz input signal. Otherwise, the mosaic-T module will cease to operate, without a clock signal. Additionally, switching between external and internal frequency references must occur when the module is powered off, or the module must be reset after switching.

Hardware Setup

The mosaic-T utilizes the 10-MHz signal from the steered oscillator as frequency reference, instead of its own internal crystal oscillator. An external controller, then reads the clock bias reported from the mosaic-T module in its SBF messages and steers it to zero by adjusting the frequency of the oscillator. When the clock bias is zero, the receiver time scale is aligned with GNSS time. Unlike traditional approaches, this architecture does not require any hardware to measure time delays.

Disciplined clock setup — The setup for a disciplined clock with a mosaic-T module, controller, and a tunable oscillator.

Software Configuration

mosaic-T

Users will configure the mosaic-T module to disable the PPS pulses and perform an initial synchronization to GNSS time on boot. These configurations are set by entering the SBF commands below. The commands only have to be entered once, as the last exeCopyConfigFile command makes them persistent in the boot configuration.

Serial

These configurations are set by entering the following SBF commands:

setPPSParameters, off
setClockSyncThreshold, usec500, on

To save the configurations to the module, enter the following SBF command:

exeCopyConfigFile, Current, Boot

Info

Refer to the firmware manual for a more detailed description of these commands.

Controller

At each subsequent (re)start of the system, the controller will need to execute the following algorithm:

Enable the output of the PVTCartesian (or PVTGeodetic) and of the ReceiverTime SBF blocks to the controller at a desired rate (1Hz, in the example below).
Serial

These configurations are set by entering the following SBF commands:
```
setSBFGroups, Group1, PVTGeodetic+ReceiverTime
setSBFOutput, Stream1, COM1, Group1, sec1
```
To save the configurations to the module, enter the following SBF command:
```
exeCopyConfigFile, Current, Boot
```
Wait until the FINETIME bit is set in the SyncLevel field of the ReceiverTime SBF block. This indicates that the receiver's time initialization is complete.
Each time the PVTCartesian block is received by the controller, read the clock bias from its RxClkBias field.
The initial clock bias should be small (typically <200ns)
- If the value is positive, adjust the tuning level to slightly decrease the oscillator frequency.
- If the value is negative, adjust the tuning level to slightly increase the oscillator frequency.
Continue this process above, until the clock bias has converged to zero.
- While steering the oscillator frequency, it is recommended to keep the rate of frequency change smaller than 3 ppb per second (i.e. not to change the 10-MHz frequency by more than 0.03 Hz per second).
Once a lock is achieved, the receiver time will be synchronized with the GNSS time.
- Users can then, enable the PPS signal from the mosaic-T in RxClock mode using the setPPSParameters command.
  Serial
  
  These configurations are set by entering the following SBF command:
```
setPPSParameters, <Interval>, <Polarity>, <Delay>, <TimeScale>, <MaxSyncAge>, <PulseWidth>
```
  To save the configurations to the module, enter the following SBF command:
```
exeCopyConfigFile, Current, Boot
```
  Info
  
  In RxClock mode, the PPS pulses will be aligned with the receiver's time and locked to the 10-MHz reference signal from the oscillator. (i.e. Assuming a 1Hz PPS rate, the pulses will be generated exactly every 10 million cycles of the oscillator, in a constant phase with respect to the oscillator cycles.)
Continue steering the clock bias to zero to keep the oscillator and the PPS aligned with the GNSS time.
- During GNSS signal outages, the PPS pulses from the module will remain phase locked to the oscillator. The stability of the hold-over period, will depend on the quality of the oscillator.

Tip

When using Fugro's AtomiChron^® service, the reported clock will initially refer to the GNSS time scale during the first two to three minutes after start up, before switching to the AtomiChron^® time scale. The change between reference time scales, may lead to a clock bias shift by a couple of nanoseconds. The reference time scale is reported in the TimeSystem field of the PVTCartesian SBF block and the jump can be avoided by waiting until that field is set to Fugro AtomiChron (100).

Timing Delay

The PPS aligned signal, as described above, will typically be late by a few tens of nanoseconds. This is due to delays from the antenna, cables and RF frontend of the receiver. - In the mosaic modules, the frontend RF delay is about 10 ns. - Typical values for the antenna delay range from 10 to 20 ns, and coax cable delay amounts to about 5 ns per meter. - If the total delay is known, it can be compensated with the setCalibCommonDelay SBF command.