Sentinel: Cable Delays
Introduction
While frequency measurements using the Calnex Sentinel can be performed with a fairly straight forward test setup, time error measurements require consideration of the effects of delays introduced by differing cable lengths.
This article identifies the sources of measurement error and provides procedures to ensure that all measurement uncertainties introduced by the test environment are compensated for in the Sentinel settings, ensuring maximum accuracy in the test results.
Time error measurements are performed by comparing the edges of the 1PPS output from the device under test to that of a reference 1PPS signal. The 1PPS reference can be a precision clock source or the 1PPS generated by Sentinel. The Sentinel 1PPS output is generated from an internal GNSS disciplined precision Rubidium oscillator.
Time error delays can be introduced into the Sentinel measurement from the following sources:
GNSS cable delay
External reference cable delay.
1PPS cable delay from the device under test
Cable Delays
Electrical cables will introduce a delay of 5ns per metre. Obviously using short cables will minimise this delay but this may not be possible when, for example, the GNSS antenna is on the roof of a building or the Sentinel cannot be located close to the device under test.
GNSS Cable Delay
The antenna supplied with Sentinel has a 20m cable, which equates to a delay of 100ns.
GNSS Antenna Group Delay
In addition to cable delay customers may wish to add 34ns antenna delay if GPS or Galileo are used and 45ns for Glonass. See Sentinel: GNSS Cable and Antenna Group Delays for more details.
Setting GNSS Antenna Delay
From the Settings tab select the GNSS screen. Enter the Antenna Delay and click Apply.
For Sentinel releases prior to R22, changing the antenna delay while GNSS is connected could cause a step change in the 1PPS phase input to the internal Rubidium oscillator and will require the Sentinel to be re-disciplined for a period of 6 hours. To avoid this use the procedure at Sentinel: How to set the GNSS antenna delay without requiring to re-discipline .
External Disciplining Reference Cable Delay
To select an external 1PPS reference as the Disciplining source, from the Settings tab navigate to the Measurement > Timebase screen, and select the appropriate Internal Reference Disciplining Source. Then enter the appropriate value in the Cable Delay (External 1PPS) field.
Reference Selection
From the Settings tab, navigate to the Measurement > Common screen as shown below .
Ensure that TIE Mode on the is set to TIE + 1PPS TE
Set the Diff TIE Reference to 1 PPS Ref to select the internal 1 PPS as the measurement reference.
Clock Channel Cable Delay
Go to the Settings > Clock Channels page and select the relevant channel.
Enter the cable delay associated with measured channel in the Cable Compensation field.
Press Apply