While trying to measure the frequency response levels of a mixer, I was having problems accounting for the observed mixer gain. I do not have a high quality wide ranging frequency source. I use synthesizers built from previous efforts. These work fine with respect to frequency accuracy and phase noise. I knew they were not designed as signal generators and had some level variance. For my most purposes this was fine. With this activity I needed a better estimate. Below is the output power level at the fundamental of a ADF4351 based synthesizer.