Design of a 20 GHz Bandwidth Dual-stage Dual-FIR

A 20 GHz channel emulator for broadband differential data links has been developed using a trace overlaid with lossy materials and a two stage programmable Finite Impulse Response (FIR) filter. It allows to emulate the main channel and the cross-talk in the data links. The hardware design and emulation procedure are described. The emulated channel matches the target channel, validating the design methodology. A serial data link channel between an active driver and a receiver can only be tested if backplanes or interconnects in the channel have already been manufactured. Full channel simulation may also be restricted due to proprietary information, legacy devices, or the complexity of the adaptive protocol. A channel emulation instrument can be a solution in these situations. Channel emulators are widely used nowadays in hardware test setups for high speed interconnect interfaces such as PCI express, USB 3.0, 28 Gbit and 40 Gbit links, etc. Channel emulation concepts can also be extended to other applications that need rapid modifications to achieve specialized channel properties. Current techniques for channel emulation includes using multiple lengths of cables (1)(2), traces overlaid with lossy or dielectric materials (3)-(6), digital filter (7)-(9), and digital signal processing implementations (10). The Finite Impulse Response (FIR) filter technique has an advantage over other methods in channel emulation because of well-developed filter approximation algorithms (11)(12) and its programmable filter response. However, this method is often limited by the bandwidth of the FIR filter chips. Furthermore, the commercial filter-based emulators, such as Anritsu MP1825B and Tektronix LE320 suffer from fixed tap coefficients, limited tap numbers, and/or limited total delay between the taps. These limitations restrict the impulse response duration and shape that a single FIR filter can reproduce. Therefore a dual-stage dual-FIR channel emulator has been used in conjunction with a trace that has adjustable frequency dependent loss. The channel emulator described uses mechanically adjusted lossy material above a low los trace to emulate the smooth roll- off of the channel frequency response and two FIR filters to emulate small deviations from the smooth roll off such as nulling which can be a result of reflections (13)(14). The emulator is also able to reproduce the effect of cross-talk. I. SYSTEM CONCEPT AND ARCHITECTURE