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The measurement of close-to-carrier phase modulation (PM) noise of state-of-the-art oscillators is always challenging. Quite often the residual noise of the phase detector used in these measurements is higher than the noise of the source at Fourier offset frequencies between 5 and 100 Hz. A conventional double balanced mixer using 2N2222A transistors as the nonlinear components of a diode ring was constructed for use as a phase detector. Residual single-sideband PM noise measurements at 5 MHz for this device have shown a low flicker noise floor of L(10 Hz) = -163 dBc/Hz. When this mixer design is implemented in a dual-channel measurement system, a cross-correlated PM noise floor of better than L(10 Hz) = -170 dBc/Hz is expected.