Minutes of the 17^th Timing Working Group Meeting (TimWG)
Held on Friday the 20^th April 2001

Present:

Agenda

• Location of TTC transmitters
• New BI requirements
• UTC saga
• AOB

Approval of previous minutes

There were no corrections on the previous minutes.

Location of TTC transmitters

Philippe BAUDRENGHIEN reminded clearly the position of the SL/HRF group regarding the distribution of the different fast timing signals (40MHz and frev) to the different users. SL/HRF would like to avoid to manage the distribution of the fast timing signals directly from the RF faraday cage located in SR4 . They request to have only to provide one single source for each type of signal from the RF faraday cage.

Etienne CARLIER explained that in order to keep high reliability and high availability on the frev signals between the RF faraday in point 4 and the beam dump system in point 6, he would like to keep individual transmission system. Philippe BAUDRENGHIEN accepted that for this special case, both frev signals (one for each ring) will be issued directly from the RF faraday cage to the beam dump system on the basis of a point to point optical connection.

Jean-Jacques SAVIOZ confirmed that for SL/BI, the distribution of the two revolution and RF frequencies are requested. Gary BEETHAM confirmed that three independent distribution system will be installed: one for ring 1, one for ring 2 and one for transfer lines and SPS interconnection.

Philippe BAUDRENGHIEN raised-up the question of availability of RF and revolution frequencies when there is no beam in the machine (access, shutdown period,...). SL/HRF can not guaranteed the availability and the consistency of these signals when there is no beam in the machine. Commutation between an operation mode and a simulation mode appears to be one possible solution . Location where the frequency simulators will be located (SL/HRF or TTC) has still to be identified if this approach is selected.

Bruce TAYLOR reviewed the different possibilities for the implementation of the TTC system around the machine regarding the available TTC components and fibre optics cables already available between the different LHC points and the PCR (see slide attached). He reminded also that there is some interest from the experiments to receive the BST signals through the TTC (Channel B).

Luit de JONGE presented the infrastructure of fibre optics actually available around the LHC. He explained that two different types of fibre optics have been installed for LEP between the different points and PCR (5 thermally compensated single mode fibre  and 7 standard single mode fibres). Thermally compensated fibre optics guaranteed no longitudinal modification of the optical signal. Philippe BAUDRENGHIEN suggested to try to find a solution for phase compensation of the 40Mhz signal instead of trying to use/install thermally compensated cables. Luit de JONGE reminded that PCR appears to be a special point. It will not be possible to install easily active optical equipment without taking into account safety rules related to laser classification. This has to be take into account when the location of the TTC transmitter will be decided.

Gary BEETHAM concluded that a point to point connection will be implemented for each signal between LHC point 4 RF faraday cage and PCR, that the TTC transmitters will be installed in the PCR and that the signals will be fan out from the PCR to the different clients. Type of fibres used for the different signal has still to be decided. The link with CMS (LHC point 5) has to be studied more in details (direct from LHC point 4 or through the PCR).

After the meeting, Philippe BAUDRENGHIEN has summarized the situation with the optical fibre installed in LEP and used last year by SL/LRF as follow:

• Over one year, the delay variation from PCR to SR4 (9.5 km) is 8 ns (1000 degrees at 352 MHz) for the normal fibres and 0.32 ns (40 degrees at 352 MHz) for the compensated ones.
• This is in good agreement with a seasonal temperature change around 20 degrees Celsius (9.5 km at 0.66 c gives 48 microsec total delay. Normal fibres have a thermal expansion coeff. around 7 ppm/degree C -> 24 degree C gives 8 ns. Compensated fibres have thermal expansion coeff of 0.4 ppm/degree C -> 17 degree C gives 0.32 ns.) Considering that these fibres are laid in (relatively) deep trenches, the 20 degrees temperature variation is UNEXPECTED. The SL/HRF group is operating a fibre optic link from the SPS to the CPS (4.6 km, monomode fibre). The total delay is 23.2 microsec. The thermal expansion coeff is 7 ppm/degree C. We have tracked the phase variation from 1st May to 31st May: the daily variation was 3.6 degrees at 200 MHz corresponding to 0.05 ns. The yearly variation was of the same order (an additional 0.05 ns). This measurement suggests a variation of the temperature ACTUALLY SENSED BY THE FIBRE AROUND 1 DEGREE CELSIUS ONLY. We explained this low figure by the fact that the fibre is resting in an underground trench where the temperature stays very constant over the year. Before 1998 the link was ending in a 269 meters long fibre sitting on the surface, in the sun, and this small portion alone was introducing 3 times more phase variation than the underground 4.4 km ... This has been corrected since.

He suggested that  the working group should:

• Try to specify for each client (Experiments, BI, RF, ...)  the required stability of the 40 MHz (200 MHz, 400 MHz) transmitted on the fibres, both short term (daily) and long term (seasonal),
• Explain why the LEP fibres suffer this large temperature variations. J.Sladen told me that the variations were not identical for two equal length fibres going to different buildings. Maybe a small portion of the link is responsible for most of the phase change.
• Measure the daily/seasonal phase variation from PCR to SR4 round trip (and eventually other buildings).

New BI requirements

Jean-Jacques SAVIOZ presented the modification in the layout of the LHC BPM & BLM systems (see slides attached). The major difference with the previous layout is the displacement of the BPM / BLM electronics from the tunnel to the surface building SR. Signal transmissions between the equipment located in the tunnel and their electronics in the surface building will be done through optical fibre links. This modification has consequences on the fast timing distribution layout with a significant simplification as distribution of fast timing signals is not required anymore in the tunnel (RA and arc). Jean-Jacques presented the updated requirements from SL/BI for different timing signals with the related installation planning. (see slides attached)

UTC saga

Gary BEETHAM will discuss with Quentin KING the different open questions regarding the use of  UTC as time reference. Conclusions on this subject will be presented at the next meeting.

AOB

John PETT raised up the question of how the distribution of the slow timing will be done. Gary BEETHAM agreed that this subject has to be put on the agenda of the next meeting.

Next meeting

Date of the next meeting to be annouced.

Agenda

1. Review of minutes
2. Accuracy requirements of the 40MHz and frev signals for the experiments
3. Slow timing transmission and distribution
4. Functional specification