Check this out....
Effects of an EMP
Although the electric field created from an EMP lasts for only a short time, its effects can be devastating. It is predicted that a single high altitude burst 200 miles above Kansas could propagate an EMP enveloping the entire United States. Electrical systems connected to things that can conduct current like wires, antennas, and metal objects will suffer significant damage. EMP effects on electronics include interference of radio frequency links, irreparable damage to microcircuits, and even the disabling of satellites. Fortunately, electronic equipment that is turned off is less likely to be damaged.
Yes, that would be a very large thermonuclear device. It's not something that can be launched by a Scud. It would take a dedicated large ICBM.