You should care about the amplitude. If you don't, then 0 volts is OK, right? But seriously, if it is going into an EM receiver (I take that to mean a threat receiver, something useful for sensing radar and other forms of electromagnetic warfare threats) through a wire connection, you will want the ability to fine tune the amplitude. That is of course, unless your intention is to simply jam it, in which case a lot of ampitude is needed.
In any case, your search will not turn up much of anything, since variable oscillators do not exist that cover the frequency range you want in only one box. It will be necessary to have several oscillators, one for 3KHz to about 1 MHz, another for 1 MHz to about 3 GHz, another for about 3 GHz to about 20 GHz and perhaps one more for the rest of the range. If you intend to generate one steady signal at a time, these different oscillators can be joined together via a control bus and put under the command of a PC that manages which one is turned on and when. In this way, you can create a single system that does what you want. But it will cost a few dollars thats for sure. Still interested?
Perhaps its time to ask our favorite question which is... what exactly do you want to do? Please explain in detail.