Printed Circuit Design & Fab - June 2008 - (Page 26)

SIGNAL intEGrity modeling Common ground noise in High-speed multi-board PCB systems The amount of noise between reference planes in two adjoining boards depends on the inductance produced by the inter-board connector. by MOSIN MONDAL, BHyRAV MuTNuR, PRAVIN PATEL, SAMuEL CONNOR, BRuCE ARCHAMBEAuLT, and MOISES CASES The high-speed link paths in today’s computer systems frequently span across multiple circuit boards. These signal paths on multiple boards are connected through connectors and cables, which often introduce impedance discontinuities. The challenges presented by multi-board signaling have been addressed in most high-speed interfaces by using differential signaling, which is less severely impacted by connector discontinuities. However, one pitfall of differential signaling is the tendency to ignore the accompanying common mode signal and mode conversion effects. Signal integrity tools often consider only the differential-to-differential parameters (SDD11 and SDD21), ignore other mixed mode S-parameters, and assume that the reference planes in each circuit board are tied perfectly to a common ground. Therefore, this kind of methodology fails to capture the common mode effects on signals. The true scenario, however, is that at high frequencies the reference planes in multi-board systems are independent, and therefore significant noise voltages can develop between reference planes due to common mode currents flowing through the connectors. This noise voltage is a function of the common mode current and the loop inductance of the 26 connector as seen by the signal line. In this paper, the impact of the nonideal common ground on the electrical behavior of multi-board differential signaling is studied. In particular, the amount of common mode signal present in a realistic design, the amount of inductance created by a signal line due to inter-board connectors, the amount of noise between reference planes in two adjoining boards, and the overall impact on the high speed differential signals are described. Common Mode Signal Due to Non-Ideal Common Ground In a multi-board system, signals are transmitted throughout the boards using either microstrip or stripline transmission lines. FiGurE 1 shows an example of an interface where two boards with microstrip lines are connected through connector pins, bent at an angle that properly connects one plane to the other. Some of the pins in the connector are designated as the ground pins that connect to the ground planes. The actual configuration of the ground pins will impact inductance (as discussed in a later section) and is therefore important for the performance of the design. In this section, the existence of common mode noise due to non-ideal common ground connection for the example shown in Figure 1 is demonstrated. Measurement data proves the importance of considering non-ideal !FiGurE 2. eye diagram for signal mea- ! FiGurE 1. schematic of two perpendicular boards connected by a connector. sured at a Locust board with 800mV p-p at 2.125 Gbps input fed into a Cricket board. JUNE 2008 printEd circuit dESign & fAB

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Printed Circuit Design & Fab - June 2008

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