By that I mean a circuit that can't be broken into separate blocks that... err... do something by themselves. For example, one can get a pretty lengthy circuit consisting of an RF amplifier / mixer / oscillator / IF amplifier / FM demodulator / audio amplifier / power amplifier / etc... but these are really individual blocks joined in together, which many times can be used for other unrelated applications (as that same power amplifier can amplify a PC audio output). So, while a FM radio may be a complicated little thingie, it is built out of smaller blocks that, once identified, can be analyzed one by one. I'm talking about those building blocks themselves... what's the most complicated one you know of? For that I nominate the cubic-function generator by Cipriani and Takeshian from Conexant: Link to patent Anything else can beat that?