Ring interferometers containing both an amplifier and a frequency-shifter have been proposed in different contexts, either for enhancing the performance of self-heterodyne laser line-width measurements [1], or for frequency-shifted feedback lasers and mode-locked operation [2]. It has received considerable interest recently, leading to accurate frequency-to-time mapping [3], rapid wavelength scanning [4], or pulsed laser high-frequency modulation [5] for instance. These schemes rely on the multiple interference of frequency-shifted and time delayed laser waves. Here we investigate a fully fibered frequency-shifting loop containing an optical amplifier in the context of microwave photonics. The aim of this research is to show that the recirculating frequency-shifting ring interferometer is a convenient method for radio-frequency up-conversion and waveform generation. Compared with previous works based on similar setups , we focus on (i) loop lengths shorter than the coherence length of the seed laser in order to provide high-purity RF harmonic beats, and (ii) sub-threshold operation in order to control the RF phase and amplitudes of the recirculating components. We provide a theoretical model to derive a time-delayed interference equation, in order to obtain the output intensity of the loop, and we compare it with an all-fibered experiment.