All nukes produce an EMP, but some more than others.
Suppose that you put a nuke in a coil of superconducting wire carrying a huge current and consequently a huge magnetic field. The nuke detonates, the magnetic field, now locked in a plasma where the superconductor used to be, expands violently, converting some small fraction of the energy in the explosion into extremely low frequency EM waves. The extremely low frequency pulse causes induced currents in virtually all electric circuits in range, from overland electric lines to computer wiring, frying stuff right and left.