We propose and demonstrate a novel method to generate multispectrum frequency and hopping from one frequency spectrum to another using a single external cavity based Fabry-Perot laser diode (FP-LD) subjected to a single optical injection with negative wavelength detuning. The external cavity based FP-LD has a single dominant mode, single-mode FP-LD (SMFP-LD), and hence the beating of the dominant mode and the injected beam, and the beating of the injected beam and the corresponding side mode of SMFP-LD are possible with a single optical injection. The generated frequency can be tuned to a wide range of GHz to several THz. Individual or simultaneous RF signals generation and hopping between microwave and millimeter wave spectrum are reported. The effect of the injected beam power on generating and hopping of RF frequency is analyzed in the case of negative wavelength detuning with the input beam injected on different modes of SMFP-LD. Also, the stability of beating wavelengths, power variations of beating wavelengths for an interval of 1 hour, and linewidth of RF signals are observed.