Offset between the WRS PPS and TR PPS output signals

Measure the offset between the WRS PPS and TR PPS output signals with respect to the WRS software versions. Examined WRS software versions are: v7.0 and v8.0.

1. Used components

• Oscilloscope: Tektronix DPO 2024B (200MHz, 1GS/s)
• WR Switch (WRS): Creotech WRS-3/18 (sw v7.0 & 8.0)
• Timing Receiver (TR): pexaria5 and pexp (both with GW version fallout v6.1.2)
• SFP: BlueOptics BO15C4931620 (purple) and BO15C3149620D (blue)
• Linux host: EtherBone and Saft tools (v3.x)
• Cables: 2x 2 m cable with the LEMO and SMA connectors, 2x 2m cable with the LEMO connector
• Terminator: 4x 50 Ohm feed through terminations

2. Measurement setup

Figure: Measurement setup (WRS manamegement network is not shown)

2.1. WRS

All switches have the GSI TN2 compatible roles: grandmaster, localmaster and access. Therefore, configure each switch with the designated dot-config file and connect them in cascade. Check the PPS output signals at the localmaster and access switches. Normally, the PPS output signal is generated in the BC devices if they are synchronized.

2.2. TR

TRs are installed in a Linux host with necessary EtherBone and Saft tools. They are programmed with the gateware version 'fallout v6.1.2'. Connect the WR port of each TR to the wri ports of the access WRS and check the WR synchronization using 'eb-console'.

2.3. Oscilloscope

Connect the PPS output of the localmaster and access WRS to the oscilloscope's channel 1 & 2, respectively. Use the terminator and suitable cable. PPS output signal from the localmaster WRS is chosen as a trigger signal.

Connect the IO1 ports of each TR to the oscilloscope's channel 3 and 4, respectively.

Channel	Signals
1	PPS (localmaster WRS)
2	PPS (access WRS)
3	PPS (pexaria5)
4	PPS (pexp)

Table 2.1. Oscilloscope channel connection

3. Measurement

The reference (or triggering) PPS signal is the PPS output of the localmaster WRS. If WRSs are synchronized, then the PPS is generated automatically at the PPS output. For the TRs, the PPS signal is generated by using the 'saft-pps-gen' tool. Note: if TR PPS cannot be generated, then enable the output mode of the chosen IO port by using the 'saft-io-ctl' tool.

All measurements are done with the synchronized devices, that means WR is locked in all WRSs and TRs.

The measurement results show that the offset between WRS PPS and TR PPS signals stay in range of 107,7-111,7 ns. The offset remains constantly at 111,7 ns for the pexp TR in both WRS software versions. Unlike to it, the offset varies between 107,7 ns and 110 ns for the pexaria5 TR.

Table 1 shows the results of the all measurements.

WRS SW	LM-AC, ns	LM-pexaria5, ns	LM-pexp, ns
v7.0	< 0,2	-107,7	-111,7
v8.0	< 0,2	-110	-111,7

Table 3.1. Offset between PPS signals, nanoseconds

Note: minus offset means that PPS signal is ahead of the reference PPS signal

4. Used saft tools

• start saftbusd

$ export SAFTBUS_SOCKET_PATH=/tmp/saftbus
$ saftbusd libsaft-service.so tr0:dev/wbm0 tr1:dev/wbm1

• generate the PPS signal

$ saft-pps-gen tr0

• (optional) enable IO output, if PPS is not output

 
$ saft-io-ctl tr0 -n IO1 -o 1

• enable timing event from the IO input port IO1

 
$ saft-io-ctl tr0 -n IO1 -b 0xffff100000000000

• snoop IO events

 
$ saft-ctl tr0 -xv snoop 0 0 0

-- EnkhboldOchirsuren - 2025-05-19