Minimizing crosstalk in a high-speed cable-connector assembly

This paper presents the detailed signal-integrity analysis results of the connector and cable linking the ALICE Time Projection Chamber (TPC) to its Front-End Electronics. The goal was to find a cable design that minimizes the crosstalk (electromagnetic coupling) between the signal lines. Other considerations taken into account were the signal line capacitances and the imposed mechanical constraints (cable flexibility, thickness, and physical dimensions). Crosstalk effects in the connector pins were also analysed. The design was tackled using different software tools. For the cable, a Finite Element Method was used to extract an equivalent distributed circuit model which was then exported to PSpice. The resulting simulations will be presented. For the connector, an electromagnetic fullwave solver was used to simulate completely all highspeed effects. We will show how these programs helped us to quickly investigate different cable configurations. I. PROBLEM DESCRIPTION The ALICE TPC detector has to be linked to the readout electronics by some electrical means. The most convenient way of doing this is by a flexible flat cable. However, the nature of the signals to be transmitted (lowamplitude, analogue, very fast rise-times) requires a thorough study of the characteristics of this transmission line, particularly regarding signal integrity issues such as crosstalk and reflections. The main objective of this design is to minimize crosstalk induced by a signal pulse on the neighbouring traces to avoid false triggering. Several factors influence crosstalk, but the most important are signal rise-time, coupling length, track separation (i.e. signal pitch), stackup and the form of the propagating electromagnetic fields. Given the problem constraints, we will focus on how to bound the electromagnetic fields so as to reduce coupling between channels. The only method practically available for achieving this is by proper ground and signal placement in the cable and connector. There are some other effects to consider. First, the capacitance to ground has to be kept low so as to reduce the parasitic effect to the charge detector input. Also, the cable has to remain flexible, so that a proper mechanical connection and maintenance is ensured. This implies that we need a minimum thickness cable with no solid planes. The cable is built on a flexible PCB with one layer of polyamide (εr=3.5) of 70 μm thickness between two 55 μm thick layers of coverlay (εr=4.4). The copper signal tracks are 160 μm wide and are separated by 640 μm. The cable will be mated to a PCB using a connector. This will play an important role in the system behaviour due to the fast (~50 ps) edge rates and so has to be properly characterised. II. CROSSTALK DEFINITIONS Crosstalk occurs in any system with two or more conductors. Each wire segment acts individually as an inductor and capacitor, and also as an antenna. Together, they act as coupled antennae due to their mutual coupling. This coupling can be expressed in terms of capacitive and inductive components. Figure 1 shows a simple crosstalk scenario with one active (aggressor) and one passive (victim) line. The nodes of the transmission lines are labelled: 1. Near-end (driver-end) of the aggressor line 2. Far-end of the aggressor line 3. Near-end of the victim line (backward crosstalk) 4. Far-end of the victim line (forward crosstalk)