Abstract In this study, a novel type of Fourier transform radio spectrometer (termed as all-digital radio spectrometer; ADRS) has been developed in which all functionalities comprising a radio spectrometer including a sampler and Fourier computing unit were implemented as a soft-core on a field-programmable gate array (FPGA). A delay-line-based ramp-compare analog-to-digital converter (ADC), which was completely digital, was used, and two primary elements of the ADC, an analog-to-time converter (ATC) and a time-to-digital converter (TDC), were implemented on the FPGA. The sampling rate of the ADRS f and the quantization bit rate n are limited by the relation τ = 1/(2nf), where τ is the latency of the delay element of the delay-line. Given that the typical latency of the delay element implemented on FPGAs is ∼10 ps, the adoption of a low-quantization bit rate, which satisfies the requirements for radio astronomy, facilitates the realization of a high sampling rate up to ∼100 GSa s−1. In addition, as the proposed ADRS does not require a discrete ADC and can be implemented on mass-produced evaluation boards, its fabrication cost is much lower than that of conventional spectrometers. The ADRS prototype was fabricated with values of f = 600 MSa s−1 and n = 6.6 using a PYNQ-Z1 evaluation board, with a τ of 16.7 ps. The performance of the prototype, including its linearity and stability, was measured, and a test observation was conducted using the Osaka Prefecture University 1.85−m mm–submm telescope; this confirmed the potential application of the prototype in authentic radio observations. With a cost performance 10 times better (∼800 USD GHz−1) than conventional radio spectrometers, the prototype facilitates cost-effective coverage of intermediate frequency bandwidths of ∼100 GHz in modern receiver systems.