Production, measurement and simulation of a low mass flex cable for multi gigabit/s readout for the LHCb VELO upgrade

The goal of this project is to examine the feasibility of data transmission up to ~ 5 Gbit/s on a short ( ~ 60 cm) low mass flex cable, for the readout of the upgraded vertex detector (VELO) of the LHCb experiment. They will be in a vacuum and very high radiation environment and also partly in the particle acceptance. For the full system 1600 readout links will be required. A set of single-ended and differential (edge-coupled) striplines, with a variety of line parameters have been prototyped using a material specifically tailored for this type of application (Dupont Pyralux AP-plus polyimide). To reduce mass, the total thickness of the cable is kept to 0.7 mm. We will present measurements of the characteristic impedance, insertion and return loss, obtained both from time and frequency domain, as well as a comparison with simulations and expectations. Also the effectiveness of grounded guard traces and the use of ground via holes to reduce crosstalk will be reported. From the measurements we were also able to extract the material properties such as the dielectric constant and loss factor up to several GHz. The measurements were done with a Vector Network Analyzer (VNA), TDR/TDT Digital Sampling Oscilloscope, serial PRBS generator and analyzer for eye diagram and CAD tools such as Agilent ADS and ANSYS HFSS simulators.