Frequency flicker limitation in dual oscillator phase noise measurement instruments

Dual oscillator phase noise test sets are based on a PLL. In order to pull the slave oscillator in the dynamic range of the control, a DC voltage must be added. In an automatic test set this voltage comes from a DAC. We demonstrate that the flicker noise of the DAC yields an additional frequency flicker of the instrument that is not detected with the usual calibration procedures. In some circumstances the DAC noise turns out to be the main factor limiting the instrument sensitivity at low Fourier frequencies.