In this paper, a new 7-bit frequency discriminator for instantaneous frequency measurement applications is presented for the first time. The proposed discriminator is designed using microstrip band-stop spiral filters to achieve a compact size structure with high reject values between pass and reject bands. As these passive filters are fabricated on available ordinary substrates, the proposed structure is ultralow cost. Rectangular and circular spiral resonators are used to reject the specific frequency bands in each channel. The designed filters for each channel divide the frequency bandwidth into several pass/reject bands to produce the desired binary code. The proposed structure works in S-band with the maximum frequency error of 16 MHz. The proposed discriminator was simulated by a momentum simulator of an advanced design system (ADS) and fabricated on a 20-mil Rogers RO4003 substrate with tan( $\delta$ ) and a conductor thickness about 0.002 and $18~\mu \text{m}$ , respectively. An excellent agreement was achieved between the simulation results obtained by ADS and those of measurements; the average frequency shift is less than 10 MHz.