Residual PM noise evaluation of radio frequency mixers

Direct observation of phase-modulation (PM) noise is often difficult due to the high dynamic range that exists between the carrier and the modulated sidebands. A common tool used to reduce the dynamic range is the phase detector, which removes the carrier and down-converts its noise sidebands to baseband. The double balanced mixer (DBM) is the most widely used phase detector for high-resolution PM noise detection at most carrier frequencies. For Fourier offset frequencies close to the carrier, the residual flicker phase noise of the DBM is often the limiting factor of a PM noise measurement system. Careful evaluation of the phase detector under various operating conditions can lead to the optimization of a PM noise measurement system´s sensitivity. This paper describes a survey of residual PM noise measurements for a variety of DBMs at 5 MHz. In order to attain quality measurements, careful attention is devoted to the reduction of ground loops during PM noise measurements. The input powers to the local oscillator (LO) and reference frequency (RF) ports of the mixers are varied to determine the optimal operating point of these devices.