Ultra-Low Noise Voltage Supply Using CMC Filter for GaN Oscillator Phase Noise Measurements

An ultra-low noise voltage supply is proposed for precise phase noise measurements of high-power GaN MMIC oscillators. The design incorporates a filtering circuit utilizing a common mode choke (CMC) and discrete capacitors. The CMC effectively suppresses common mode noise, while the differential capacitors combined with the CMC leakage inductance create a lowpass filter that mitigates differential mode noise. In comparison to the built-in power supplies from the signal source analyzers (SSAs), the proposed method exhibits a comparable baseband noise level while offering the advantage of more independent voltage outputs at significantly higher voltage levels with a very low cost. Baseband noise measurements indicate that the proposed design can provide an average of ~ 20 dBV/Hz noise suppression compared to a typical power supply from 1 kHz to 10 MHz. The proposed method was employed to characterize a GaN oscillator operating at 28 V (exceeding the highest voltage offered by the SSA). The measured phase noise is ~ 11 dBc/Hz (1 MHz offset) lower than that of using the power supply directly.