The receiver synchronizes its own internal time reference (i.e. its measurement latching) with the low-to-high transitions of the reference time signal at the PPS IN connector. This synchronization occurs with some small delay, often referred to as ‘X0’ in the literature (see How to set up the PolaRx5TR for timing applications). In the PolaRx5TR, X0 can take any value from 24ns to 59ns depending on the phase relationship between the 10 MHz frequency reference and the PPS input signal at the REF IN and PPS IN connectors respectively. X0 remains constant when the receiver is reset or power-cycled, but changes when the cables feeding the REF IN and/or the PPS IN connectors are changed.
X0 can either be left uncompensated or can be measured and compensated for by the receiver itself. For legacy reasons, the compensation is disabled by default. In new installations that have not been calibrated yet, it is easier to enable X0 compensation. Both options are described below.
X0 uncompensated
This is the default behaviour. To verify that the X0 compensation is disabled, check that ‘off’ has been selected in the ‘PPS In Parameters’ field on the Timing>PPS window of the Web Interface as shown in Figure 1.
Figure 1: Configuration to disable automatic PPS IN internal delay compensation and to bring the measurement latching strobe to the PPS OUT connector
The X0 delay is the delay from the PPS IN connector to the actual measurement latching (see How to set up the PolaRx5TR for timing applications ). The measurement latching is an internal signal, but it can be brought to the PPS OUT connector so that it can be monitored externally. This is done by selecting ‘RxClock’ in the ‘PPS OUT Parameters’ field as shown in Figure 1. In this configuration, the PPS OUT materializes the internal receiver time reference.
An example of a setup which can be used to manually determine the delay between the PPS IN and the internal time reference (X0) is shown in figure 2. For this calibration, it is not necessary to connect an antenna to the receiver. For the most straightforward interpretation of the results, cables C1 and C2 should have the same length.
Figure 2: Manual calibration of the PPS IN to internal reference delay
X0 is the time difference between the leading edges of the pulses on the Input 1 and Input 2 ports of the oscilloscope. The oscilloscope trigger level should be set to 1V. The measured X0 should be a value between 24ns and 59ns. The value depends on the relative length of the REF IN and PPS IN cables, as shown in figure 3.
Figure 3: Variation of X0 as a function of the relative cable length
It is recommended to avoid operating the receiver in the gray zones of figure3. Therefore, if X0 is found to be lower than 26ns or larger than 57ns, it is recommended to change the length of the REF IN or of the PPS IN cable. A change of 1 meter in the cable length corresponds to a change of X0 by about 5ns. Adding length to the REF IN cable increases the X0 value. Adding length to the PPS IN cable decreases the X0 value.
Auto-calibration and compensation of X0
The PolaRx5TR incorporates a circuit to calibrate the PPS IN internal delay and to compensate for it. Enabling X0 compensation can be done on the Timing>PPS window of the Web Interface, as shown in figure 4.
Figure 4: Select 'auto" in the PPS IN Parameters field to enable automatic PPS IN internal delay compensation
To ensure that the receiver continues to compensate for the X0 delay following a reboot or reset, click ‘Save’ on the pop-up that appears in the lower-right corner in the web page.
From now on, if a PPS signal is fed into the PPS IN connector, the PolaRx5TR calibrates the X0 delay and adjusts its measurements accordingly. This calibration process is repeated after each reboot or reset. The result of this process is that X0 can be considered zero in time transfer applications, for example when generating a CGGTTS file.
The calibration takes up to 20 minutes (this time being required to ensure that most of the temperature transients have vanished) during which time the X0 estimation gradually improves to reach a final accuracy of 100 ps. During calibration, the xPPS output signal is unavailable, but all other receiver functions work normally. The PPS_IN_CAL bit of the RxState field of the ReceiverStatus SBF block is set to indicate that calibration is ongoing. When calibration is complete, the receiver outputs a message with the result of the calibration. It can be viewed in the ‘Receiver Messages’ window such as is shown in figure 5.
Figure 5: Calibration complete message in the web-interface
