There is a frequent need to measure the frequency stability and phase noise levels of very high performance signal sources that are required for certain spacecraft missions. These measurements need to be done at different locations as the spacecraft subsystems progress through the various stages of development, assembly, test and integration. Allan Deviation and Phase Noise of high performance sources are generally measured by comparing the unit under test to a reference standard. Five basic requirements are associated with making these kind of measurements: (1) The reference standard performance needs to be equal or better than the unit under test. (2) The measurement system needs to accomodde odd, non-standard measurement frequencies that can range from 4 MHz to 35 GHz. (3) Warm-up frequency drip and aging can corrupt a measurement and must be dealt with. (4) Test equipment generated noise must be understood and prevented from limiting the measurements. (5) Test equipment noise performance mud be verijkble in the field as needed. This paper describes a portable measurement system that has been built by JPL and used in the field. The methods of addressing the above requirements are outlined and some measurement noise floor values are given. This test set has recently been used to measure state of the art crystal oscillator frequency standards on the TOPEX and MARS OBSERVER spacecraft during several stages of acceptance tests.