Electro-mechanical structures for channel emulation

Channel emulators are used to evaluate communication system performance either in absence of the real channel or to test the system´s response for varying channel characteristics. For high speed differential digital channels bandwidths in excess of 20 GHz are common making it difficult to recreate the channel performance by electronic means such as FIR filters. An alternative solution is using a low loss short transmission line and having its properties modified by mechanical means. Passive structures are robust, have a frequency range only limited by the low loss trace, do not add noise, cannot be damaged by ESD and are very economical. This paper describes two electro-mechanical structures for introducing loss and nulling into the frequency response of a channel. The first part describes the design of a mechanically tuned quarter-wavelength stub filter that can be used to emulate the resonances of a channel. In the second part, an electromechanical structure, consisting of Bragg grating and lossy materials, is constructed to emulate the loss behaviour and the resonances of a channel.