1. A system for generating a very low phase noise signal, comprising: at least one first low phase noise oscillator each providing an output signal; at least one first multiplier chain that receives the output signal from a respective one of said at least one first oscillator, each of said at least one first multiplier chain including a multiplier that multiplies the signal frequency by integer multiplication factor M 1 , an amplifier that amplifies the multiplied signal and a filter that filters the amplified signal; a frequency mixer having a local oscillator (LO) input, an intermediate frequency (IF) input and a low phase noise radio frequency (RF) output, said at least one first multiplier chain providing an output signal that is continuously input from said at least one multiplier chain to said LO input of said frequency mixer; a plurality of second low phase noise oscillators each providing output signal; a first, multi-pole switch coupled to said second oscillators and said IF input of said frequency mixer and arranged to provide an output signal derived from a signal from at least one of said second oscillators to said IF input of said frequency mixer; a plurality of second multiplier chains, each of said second multiplier chains continuously receiving an output signal only from a respective one of said second oscillators, modifying the received output signal and providing the modified output signal to said first switch, each of said second multiplier chains including a multiplier that multiplies the signal frequency of the output signal of said respective second oscillator by an integer multiplication factor, an amplifier that amplifies the multiplied signal and a filter that filters the amplified signal, the filtered, amplified signal being directed to said first switch and then through said first switch to said IF input of said frequency mixer; and a control unit that determines a frequency being input to said IF input of said frequency mixer to enable variation of a range and number of output frequencies at said RF output of said frequency mixer, said control unit being coupled to said second multiplier chains and controlling via said second multiplier chains, modification of the output signals of said second oscillators received by said second multiplier chains, said control unit being further coupled to said first switch and controlling said first switch to select one of said second oscillators to provide an input signal to said IF input of said frequency mixer such that the output signal from said selected one of said oscillators, after modification by the respective one of said second multiplier chains under control of said control unit, is a source of the input signal to said IF input of said frequency mixer.