A highly stable microwave exciter system has been developed for ⁸⁷Sr⁺ ion microwave frequency standards. The controller was built to optimize the transfer function of the phase-locked loop. The upper limit of the frequency tracking error achieved was 7.7×10^-15 at τ=1s. A phase frequency discriminator using an FPGA was also made and applied to a phase-locked loop. This paper reports on the design of and results obtained from the microwave exciter for Sr⁺ ion microwave frequency standards.