A frequency source with improved phase noise. In one embodiment a phase-locked loop is used as a component of a frequency source and a signal to noise enhancer (SNE) is used to suppress phase noise produced by the phase-locked loop. The signal to noise enhancer is a nonlinear passive device that attenuates low-power signals while transmitting high power signals with little loss. The signal to noise enhancer may be fabricated as a thin film of yttrium iron garnet (YIG) epitaxially grown on a gadolinium gallium garnet (GGG) substrate, the GGG substrate being secured to a microwave transmission line from the input to the output of the signal to noise enhancer, such that the thin film of yttrium iron garnet is close to the transmission line.