Speaker
Description
Summary
For the upgraded VELO detector of LHCb, 1600 multi gigabit/s readout links will be required. The first 60cm of these links will be in a vacuum and very high radiation environment and also partly in the particle acceptance. 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 0.7mm. 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. As well, results on crosstalk, the effectiveness of grounded guard traces and the use of ground via holes 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.
To characterize the lines several equipment have been used, such as a Vector Network Analyzer (VNA), TDR/TDT Digital Sampling Oscilloscope, serial PRBS generator and analyzer for eye diagramand CAD tools such as Agilent ADS and ANSYS HFSS simulators.
Several effects were studied in the frequency domain:
• Guard traces are very effective for suppressing crosstalk.
• Via holes on the ground traces: they do not contribute significantly to further suppress crosstalk.
• Surface mount right angle SMA Connectors were used to connect all the lines.Using a special layout we could avoid the use of routing via’s on the signal path. Nevertheless a sharp insertion loss appeared around 6GHz. By using a 3D EM-field simulation we could explain it as a resonance in a parasitic cavity and eliminate it by an improved layout.
• Physical line parameters: variation of trace width, inter-trace gap and length in addition to single and differential line approaches were studied to see the effect on transmission line parameters. The trace width was varied from 250 to 310 µm, the inter-trace gap from 150 to 750 um and the length from 50 to 100cm.
• Micro miniature connectors (MOLEX SlimStack 0.4mm): these very low mass connectors turn out to have very low insertion losses at high frequencies.
In the time domain, we studied:
• The characteristic impedance profile along the line. The values correspond very well with the mwi2010 calculator from Rogers corp.
• Eye diagrams: they were measured at 4 and 8GHz. We compare them with simulated eyediagrams obtained with the Lecroy SI-studio software.
The measurements prove that transmission is very well possible up to at least 5GHz. A second version will be designed soon which will include several enhancements, such as a new SMA launch pad layout, an improved race-track routing resulting in lower intra-pair skew and a reduced dielectric thickness to improve cable flexibility and total mass.
