A frequency tunable signal source (100) with first (105) and a second (115, 315) oscillators, each of which outputs a signal at a fundamental frequency (f1, f2) and at least one signal at a harmonic frequency (f1′, f2′) and a mixer (120) with first (121) and second (122) input ports and an output port (124), and a control unit (110) which controls switches (S1, S2, S3, S4), by means of which two of said signals (f1, f2, f1′, f2′) are switchably connected to the first input port. The other two signals are switchably to the other input port, with one switch (S1, S2, S3, S4) for each signal (f1, f2, f1′, f2′). There is also comprised a third oscillator (125), with an output signal connected to a third input port (123) of the mixer (120). At least one of the oscillators (105, 115, 315, 125) is a VCO, a Voltage Controlled Oscillator.