September 29, 2005 -- A simple categorization assigns most signal-processing blocks to one of two groups. On the one hand are generic functions, such as op amps and ADCs, which perform one task and may serve in many disparate applications. On the other hand are application-specific blocks, such as GSM transceivers and 802.11 basebands, that have narrow definitions at a much higher level of functional abstraction and, therefore, perform a more complex set of tasks for only one application. An interesting and growing number of signal-processing blocks lie somewhere in the middle. Further scrutiny reveals that the two categories actually lie on a continuum characterized by a dominantly bimodal distribution with a small but decidedly nonzero population between the groups.

The way designers interact with instances from the two large groups is likewise distinct. Tables of parametric performance characterize single-function generic parts under operating conditions that their manufacturers specify. Part of the design task, then, includes deriving expected circuit-level behaviors from topological analysis and components' spec-table performance. On the other end of the spectrum, conformance to industry standards defines many application-specific blocks. Individual parametric measures may be difficult to extract and impossible to control beyond simply choosing one part over another. In the sparsely populated middle, however, lie parts not given to such easy description. These flexible bits of silicon can take on behavioral attributes that depend on an OEM designer's programming or configuration decisions. But, unlike general-purpose programmable devices, these devices operate at a higher level of abstraction than bytes and words, instantaneous voltages, or individual samples. Instead, these devices operate at a level in which the signal is itself parametric, defined by concepts such as, for example, spectral shape or dynamic behavior. In other words, the functions that these parts offer are not fully defined until you decide what you want them to be, and, unlike with general-purpose signal processors, with these devices, you express those decisions in application-relevant parametric terms.

By Joshua Israelsohn, EDN Technical Editor