All you need is a small fission device: it is the gamma ray flux which causes Compton scattering (electrons knocked out of the atoms / molecules in the upper atmosphere) which travel at relativistic speeds, and interact with the Earth's magnetic field.
The design of the bomb, and the casing, can enhance the proportion of the weapon's energy which is released as gamma radiation: and by designing the case properly one can focus the gamma rays "downward" to the ground.
A fission weapon, according to these sources, will still be quite a good source of EMP: it is not directly a function of notional yield.
Cheers!
Now that is scary. From your comments, it sounds more like a laser then a light bulb and focusing throws "inverse squared" out the window.
Thanks for clearing up my misconception Re the EMP device. My comments on the Iranian/Pakistani/N Korean device design remain plausible. The pursuit of enriched uranium indicates their intention to use gun type designs for the core assembly method. Our first efforts with gun assembly gave us devices which required no testing before going to war, had yields in the 12 to 15 kiloton range, weighed about 10,000 lb and were ten feet long. Well beyond the throw weight of a Scud or anything else they currently posses. My guess for a likely delivery system would be a fishing trawler up the Hudson or Potomac in a suicide run with a dirty ground burst to follow. Another possibility would be sea/land delivery using ISO shipping container(s) through multiple points of entry.
Regards,
GtG