c blocks within the reconfigurable network inside an FPGA based on elementary switches and SRAM controllers. (After Actel, Technical Reference for Actel Antifuse FPGAS, 2010, www.Actel.com [accessed: October 18, , 2010].) The other type of programmable switch used in FPGAs is the antifuse. Antifuses are originally open circuits and take on low resistance only when programmed. Antifuses are suitable for FPGAs because they can be built using modified CMOS technology [62]. Generally antifuses rely on saturable insulators, that is, materials that have a very high resis- tance without an applied voltage or if the voltage is below a threshold, but if the voltage is brought above the threshold abruptly, the resistance goes down to values typical of a conductor. Amorphous silicon has a similar behavior and is effectively used in a few antifuses, but much more specialized materials exist that are compatible with the CMOS technology. 5.7.3.2 Logic Block Logic blocks of an FPGA can be constituted by a large variety of sub-circuits, depending on the use for which the FPGA is designed. Logic blocks simply as a couple of transistors or a flip-flop are used in some FPGAs, like in other complex structures using a look-up table and a set of logic ports are adopted. An example of a complex-structure logic block is shown in Figure 5.47 [61]. Such a logic block includes look-up tables and two flip-flops, besides a certain number of logical gates. Logic blocks of this type are also called configurable, since their functionality can be configured through the content of the look-up tables. 5.7.3.3 FPGA Performances Since FPGAs are modular elements whose inner connectivity depends on the task to be realized; the performances of these elements depends both on the particular FPGA inner architecture and on the type of function to be carried out. In telecommunication applications FPGAs are used, for example, to implement digital signal processing in equalization engines, to manage header information in simple com- munication systems or to c