This paper presents a detection system for high-accuracy amplitude and phase measurements on RF signals. It is typically used to set up the low-level RF system for particle accelerators. The proposed detector employs the principle of a heterodyne receiver and uses the digital IF architecture with a high oversampling rate to increase the SNR. A discrete Fourier transform-based quadrature demodulator is used to suppress the analog-to-digital converter (ADC) quantization noise and IF harmonics. The system has been evaluated at an IF of 1 MHz and in an RF range of 500-3000 MHz. A graphical user interface is developed to display and record the measured values. The detection accuracy achieved on 500-MHz signals in a 250-kHz bandwidth is better than +/- 0.0051% for the amplitude and better than +/- 0.0032 degrees for the phase. Experimental results at different frequencies prove that the detection system can meet the demands of state-of-the-art RF control systems for modern particle accelerators.