A frequency conversion system with improved performance. In one embodiment an image reject mixer is used to perform frequency conversion providing an initial degree of suppression of the image and local oscillator leakage signals, and a signal to noise enhancer (SNE) is used to further suppress the image and local oscillator signals, the signal to noise enhancer being 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 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.