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This paper presents a low phase noise 20 GHz signal source based on a voltage control oscillator(VCO) and a mixer integrated on a single MMIC. The mixer generates a third harmonic output by mixing the fundamental and the second harmonic signals of the VCO. Using a relatively low frequency VCO has the advantage of better Q factor and thus better phase noise. The MMIC-signal source is designed in a GaAs-InGaP HBT process. The VCO uses balanced Colpitts topology and the mixer is a high conversion-gain single-balanced mixer. The MMIC delivers signals in the frequency range 19.4-21.2 GHz with an excellent phase noise of -100 to -93 dBc/Hz at 100 kHz offset frequency. The output power ranges between -8 and -1 dBm.